1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3<html> 4<head> 5 <meta http-equiv="Content-type" content="text/html;charset=UTF-8"> 6 <title>LLVM Programmer's Manual</title> 7 <link rel="stylesheet" href="llvm.css" type="text/css"> 8</head> 9<body> 10 11<h1> 12 LLVM Programmer's Manual 13</h1> 14 15<ol> 16 <li><a href="#introduction">Introduction</a></li> 17 <li><a href="#general">General Information</a> 18 <ul> 19 <li><a href="#stl">The C++ Standard Template Library</a></li> 20<!-- 21 <li>The <tt>-time-passes</tt> option</li> 22 <li>How to use the LLVM Makefile system</li> 23 <li>How to write a regression test</li> 24 25--> 26 </ul> 27 </li> 28 <li><a href="#apis">Important and useful LLVM APIs</a> 29 <ul> 30 <li><a href="#isa">The <tt>isa<></tt>, <tt>cast<></tt> 31and <tt>dyn_cast<></tt> templates</a> </li> 32 <li><a href="#string_apis">Passing strings (the <tt>StringRef</tt> 33and <tt>Twine</tt> classes)</a> 34 <ul> 35 <li><a href="#StringRef">The <tt>StringRef</tt> class</a> </li> 36 <li><a href="#Twine">The <tt>Twine</tt> class</a> </li> 37 </ul> 38 </li> 39 <li><a href="#DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt> 40option</a> 41 <ul> 42 <li><a href="#DEBUG_TYPE">Fine grained debug info with <tt>DEBUG_TYPE</tt> 43and the <tt>-debug-only</tt> option</a> </li> 44 </ul> 45 </li> 46 <li><a href="#Statistic">The <tt>Statistic</tt> class & <tt>-stats</tt> 47option</a></li> 48<!-- 49 <li>The <tt>InstVisitor</tt> template 50 <li>The general graph API 51--> 52 <li><a href="#ViewGraph">Viewing graphs while debugging code</a></li> 53 </ul> 54 </li> 55 <li><a href="#datastructure">Picking the Right Data Structure for a Task</a> 56 <ul> 57 <li><a href="#ds_sequential">Sequential Containers (std::vector, std::list, etc)</a> 58 <ul> 59 <li><a href="#dss_arrayref">llvm/ADT/ArrayRef.h</a></li> 60 <li><a href="#dss_fixedarrays">Fixed Size Arrays</a></li> 61 <li><a href="#dss_heaparrays">Heap Allocated Arrays</a></li> 62 <li><a href="#dss_tinyptrvector">"llvm/ADT/TinyPtrVector.h"</a></li> 63 <li><a href="#dss_smallvector">"llvm/ADT/SmallVector.h"</a></li> 64 <li><a href="#dss_vector"><vector></a></li> 65 <li><a href="#dss_deque"><deque></a></li> 66 <li><a href="#dss_list"><list></a></li> 67 <li><a href="#dss_ilist">llvm/ADT/ilist.h</a></li> 68 <li><a href="#dss_packedvector">llvm/ADT/PackedVector.h</a></li> 69 <li><a href="#dss_other">Other Sequential Container Options</a></li> 70 </ul></li> 71 <li><a href="#ds_string">String-like containers</a> 72 <ul> 73 <li><a href="#dss_stringref">llvm/ADT/StringRef.h</a></li> 74 <li><a href="#dss_twine">llvm/ADT/Twine.h</a></li> 75 <li><a href="#dss_smallstring">llvm/ADT/SmallString.h</a></li> 76 <li><a href="#dss_stdstring">std::string</a></li> 77 </ul></li> 78 <li><a href="#ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a> 79 <ul> 80 <li><a href="#dss_sortedvectorset">A sorted 'vector'</a></li> 81 <li><a href="#dss_smallset">"llvm/ADT/SmallSet.h"</a></li> 82 <li><a href="#dss_smallptrset">"llvm/ADT/SmallPtrSet.h"</a></li> 83 <li><a href="#dss_denseset">"llvm/ADT/DenseSet.h"</a></li> 84 <li><a href="#dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a></li> 85 <li><a href="#dss_set"><set></a></li> 86 <li><a href="#dss_setvector">"llvm/ADT/SetVector.h"</a></li> 87 <li><a href="#dss_uniquevector">"llvm/ADT/UniqueVector.h"</a></li> 88 <li><a href="#dss_otherset">Other Set-Like ContainerOptions</a></li> 89 </ul></li> 90 <li><a href="#ds_map">Map-Like Containers (std::map, DenseMap, etc)</a> 91 <ul> 92 <li><a href="#dss_sortedvectormap">A sorted 'vector'</a></li> 93 <li><a href="#dss_stringmap">"llvm/ADT/StringMap.h"</a></li> 94 <li><a href="#dss_indexedmap">"llvm/ADT/IndexedMap.h"</a></li> 95 <li><a href="#dss_densemap">"llvm/ADT/DenseMap.h"</a></li> 96 <li><a href="#dss_valuemap">"llvm/ADT/ValueMap.h"</a></li> 97 <li><a href="#dss_intervalmap">"llvm/ADT/IntervalMap.h"</a></li> 98 <li><a href="#dss_map"><map></a></li> 99 <li><a href="#dss_inteqclasses">"llvm/ADT/IntEqClasses.h"</a></li> 100 <li><a href="#dss_othermap">Other Map-Like Container Options</a></li> 101 </ul></li> 102 <li><a href="#ds_bit">BitVector-like containers</a> 103 <ul> 104 <li><a href="#dss_bitvector">A dense bitvector</a></li> 105 <li><a href="#dss_smallbitvector">A "small" dense bitvector</a></li> 106 <li><a href="#dss_sparsebitvector">A sparse bitvector</a></li> 107 </ul></li> 108 </ul> 109 </li> 110 <li><a href="#common">Helpful Hints for Common Operations</a> 111 <ul> 112 <li><a href="#inspection">Basic Inspection and Traversal Routines</a> 113 <ul> 114 <li><a href="#iterate_function">Iterating over the <tt>BasicBlock</tt>s 115in a <tt>Function</tt></a> </li> 116 <li><a href="#iterate_basicblock">Iterating over the <tt>Instruction</tt>s 117in a <tt>BasicBlock</tt></a> </li> 118 <li><a href="#iterate_institer">Iterating over the <tt>Instruction</tt>s 119in a <tt>Function</tt></a> </li> 120 <li><a href="#iterate_convert">Turning an iterator into a 121class pointer</a> </li> 122 <li><a href="#iterate_complex">Finding call sites: a more 123complex example</a> </li> 124 <li><a href="#calls_and_invokes">Treating calls and invokes 125the same way</a> </li> 126 <li><a href="#iterate_chains">Iterating over def-use & 127use-def chains</a> </li> 128 <li><a href="#iterate_preds">Iterating over predecessors & 129successors of blocks</a></li> 130 </ul> 131 </li> 132 <li><a href="#simplechanges">Making simple changes</a> 133 <ul> 134 <li><a href="#schanges_creating">Creating and inserting new 135 <tt>Instruction</tt>s</a> </li> 136 <li><a href="#schanges_deleting">Deleting <tt>Instruction</tt>s</a> </li> 137 <li><a href="#schanges_replacing">Replacing an <tt>Instruction</tt> 138with another <tt>Value</tt></a> </li> 139 <li><a href="#schanges_deletingGV">Deleting <tt>GlobalVariable</tt>s</a> </li> 140 </ul> 141 </li> 142 <li><a href="#create_types">How to Create Types</a></li> 143<!-- 144 <li>Working with the Control Flow Graph 145 <ul> 146 <li>Accessing predecessors and successors of a <tt>BasicBlock</tt> 147 <li> 148 <li> 149 </ul> 150--> 151 </ul> 152 </li> 153 154 <li><a href="#threading">Threads and LLVM</a> 155 <ul> 156 <li><a href="#startmultithreaded">Entering and Exiting Multithreaded Mode 157 </a></li> 158 <li><a href="#shutdown">Ending execution with <tt>llvm_shutdown()</tt></a></li> 159 <li><a href="#managedstatic">Lazy initialization with <tt>ManagedStatic</tt></a></li> 160 <li><a href="#llvmcontext">Achieving Isolation with <tt>LLVMContext</tt></a></li> 161 <li><a href="#jitthreading">Threads and the JIT</a></li> 162 </ul> 163 </li> 164 165 <li><a href="#advanced">Advanced Topics</a> 166 <ul> 167 168 <li><a href="#SymbolTable">The <tt>ValueSymbolTable</tt> class</a></li> 169 <li><a href="#UserLayout">The <tt>User</tt> and owned <tt>Use</tt> classes' memory layout</a></li> 170 </ul></li> 171 172 <li><a href="#coreclasses">The Core LLVM Class Hierarchy Reference</a> 173 <ul> 174 <li><a href="#Type">The <tt>Type</tt> class</a> </li> 175 <li><a href="#Module">The <tt>Module</tt> class</a></li> 176 <li><a href="#Value">The <tt>Value</tt> class</a> 177 <ul> 178 <li><a href="#User">The <tt>User</tt> class</a> 179 <ul> 180 <li><a href="#Instruction">The <tt>Instruction</tt> class</a></li> 181 <li><a href="#Constant">The <tt>Constant</tt> class</a> 182 <ul> 183 <li><a href="#GlobalValue">The <tt>GlobalValue</tt> class</a> 184 <ul> 185 <li><a href="#Function">The <tt>Function</tt> class</a></li> 186 <li><a href="#GlobalVariable">The <tt>GlobalVariable</tt> class</a></li> 187 </ul> 188 </li> 189 </ul> 190 </li> 191 </ul> 192 </li> 193 <li><a href="#BasicBlock">The <tt>BasicBlock</tt> class</a></li> 194 <li><a href="#Argument">The <tt>Argument</tt> class</a></li> 195 </ul> 196 </li> 197 </ul> 198 </li> 199</ol> 200 201<div class="doc_author"> 202 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>, 203 <a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a>, 204 <a href="mailto:ggreif@gmail.com">Gabor Greif</a>, 205 <a href="mailto:jstanley@cs.uiuc.edu">Joel Stanley</a>, 206 <a href="mailto:rspencer@x10sys.com">Reid Spencer</a> and 207 <a href="mailto:owen@apple.com">Owen Anderson</a></p> 208</div> 209 210<!-- *********************************************************************** --> 211<h2> 212 <a name="introduction">Introduction </a> 213</h2> 214<!-- *********************************************************************** --> 215 216<div> 217 218<p>This document is meant to highlight some of the important classes and 219interfaces available in the LLVM source-base. This manual is not 220intended to explain what LLVM is, how it works, and what LLVM code looks 221like. It assumes that you know the basics of LLVM and are interested 222in writing transformations or otherwise analyzing or manipulating the 223code.</p> 224 225<p>This document should get you oriented so that you can find your 226way in the continuously growing source code that makes up the LLVM 227infrastructure. Note that this manual is not intended to serve as a 228replacement for reading the source code, so if you think there should be 229a method in one of these classes to do something, but it's not listed, 230check the source. Links to the <a href="/doxygen/">doxygen</a> sources 231are provided to make this as easy as possible.</p> 232 233<p>The first section of this document describes general information that is 234useful to know when working in the LLVM infrastructure, and the second describes 235the Core LLVM classes. In the future this manual will be extended with 236information describing how to use extension libraries, such as dominator 237information, CFG traversal routines, and useful utilities like the <tt><a 238href="/doxygen/InstVisitor_8h-source.html">InstVisitor</a></tt> template.</p> 239 240</div> 241 242<!-- *********************************************************************** --> 243<h2> 244 <a name="general">General Information</a> 245</h2> 246<!-- *********************************************************************** --> 247 248<div> 249 250<p>This section contains general information that is useful if you are working 251in the LLVM source-base, but that isn't specific to any particular API.</p> 252 253<!-- ======================================================================= --> 254<h3> 255 <a name="stl">The C++ Standard Template Library</a> 256</h3> 257 258<div> 259 260<p>LLVM makes heavy use of the C++ Standard Template Library (STL), 261perhaps much more than you are used to, or have seen before. Because of 262this, you might want to do a little background reading in the 263techniques used and capabilities of the library. There are many good 264pages that discuss the STL, and several books on the subject that you 265can get, so it will not be discussed in this document.</p> 266 267<p>Here are some useful links:</p> 268 269<ol> 270 271<li><a href="http://www.dinkumware.com/manuals/#Standard C++ Library">Dinkumware 272C++ Library reference</a> - an excellent reference for the STL and other parts 273of the standard C++ library.</li> 274 275<li><a href="http://www.tempest-sw.com/cpp/">C++ In a Nutshell</a> - This is an 276O'Reilly book in the making. It has a decent Standard Library 277Reference that rivals Dinkumware's, and is unfortunately no longer free since the 278book has been published.</li> 279 280<li><a href="http://www.parashift.com/c++-faq-lite/">C++ Frequently Asked 281Questions</a></li> 282 283<li><a href="http://www.sgi.com/tech/stl/">SGI's STL Programmer's Guide</a> - 284Contains a useful <a 285href="http://www.sgi.com/tech/stl/stl_introduction.html">Introduction to the 286STL</a>.</li> 287 288<li><a href="http://www.research.att.com/%7Ebs/C++.html">Bjarne Stroustrup's C++ 289Page</a></li> 290 291<li><a href="http://64.78.49.204/"> 292Bruce Eckel's Thinking in C++, 2nd ed. Volume 2 Revision 4.0 (even better, get 293the book).</a></li> 294 295</ol> 296 297<p>You are also encouraged to take a look at the <a 298href="CodingStandards.html">LLVM Coding Standards</a> guide which focuses on how 299to write maintainable code more than where to put your curly braces.</p> 300 301</div> 302 303<!-- ======================================================================= --> 304<h3> 305 <a name="stl">Other useful references</a> 306</h3> 307 308<div> 309 310<ol> 311<li><a href="http://www.fortran-2000.com/ArnaudRecipes/sharedlib.html">Using 312static and shared libraries across platforms</a></li> 313</ol> 314 315</div> 316 317</div> 318 319<!-- *********************************************************************** --> 320<h2> 321 <a name="apis">Important and useful LLVM APIs</a> 322</h2> 323<!-- *********************************************************************** --> 324 325<div> 326 327<p>Here we highlight some LLVM APIs that are generally useful and good to 328know about when writing transformations.</p> 329 330<!-- ======================================================================= --> 331<h3> 332 <a name="isa">The <tt>isa<></tt>, <tt>cast<></tt> and 333 <tt>dyn_cast<></tt> templates</a> 334</h3> 335 336<div> 337 338<p>The LLVM source-base makes extensive use of a custom form of RTTI. 339These templates have many similarities to the C++ <tt>dynamic_cast<></tt> 340operator, but they don't have some drawbacks (primarily stemming from 341the fact that <tt>dynamic_cast<></tt> only works on classes that 342have a v-table). Because they are used so often, you must know what they 343do and how they work. All of these templates are defined in the <a 344 href="/doxygen/Casting_8h-source.html"><tt>llvm/Support/Casting.h</tt></a> 345file (note that you very rarely have to include this file directly).</p> 346 347<dl> 348 <dt><tt>isa<></tt>: </dt> 349 350 <dd><p>The <tt>isa<></tt> operator works exactly like the Java 351 "<tt>instanceof</tt>" operator. It returns true or false depending on whether 352 a reference or pointer points to an instance of the specified class. This can 353 be very useful for constraint checking of various sorts (example below).</p> 354 </dd> 355 356 <dt><tt>cast<></tt>: </dt> 357 358 <dd><p>The <tt>cast<></tt> operator is a "checked cast" operation. It 359 converts a pointer or reference from a base class to a derived class, causing 360 an assertion failure if it is not really an instance of the right type. This 361 should be used in cases where you have some information that makes you believe 362 that something is of the right type. An example of the <tt>isa<></tt> 363 and <tt>cast<></tt> template is:</p> 364 365<div class="doc_code"> 366<pre> 367static bool isLoopInvariant(const <a href="#Value">Value</a> *V, const Loop *L) { 368 if (isa<<a href="#Constant">Constant</a>>(V) || isa<<a href="#Argument">Argument</a>>(V) || isa<<a href="#GlobalValue">GlobalValue</a>>(V)) 369 return true; 370 371 // <i>Otherwise, it must be an instruction...</i> 372 return !L->contains(cast<<a href="#Instruction">Instruction</a>>(V)->getParent()); 373} 374</pre> 375</div> 376 377 <p>Note that you should <b>not</b> use an <tt>isa<></tt> test followed 378 by a <tt>cast<></tt>, for that use the <tt>dyn_cast<></tt> 379 operator.</p> 380 381 </dd> 382 383 <dt><tt>dyn_cast<></tt>:</dt> 384 385 <dd><p>The <tt>dyn_cast<></tt> operator is a "checking cast" operation. 386 It checks to see if the operand is of the specified type, and if so, returns a 387 pointer to it (this operator does not work with references). If the operand is 388 not of the correct type, a null pointer is returned. Thus, this works very 389 much like the <tt>dynamic_cast<></tt> operator in C++, and should be 390 used in the same circumstances. Typically, the <tt>dyn_cast<></tt> 391 operator is used in an <tt>if</tt> statement or some other flow control 392 statement like this:</p> 393 394<div class="doc_code"> 395<pre> 396if (<a href="#AllocationInst">AllocationInst</a> *AI = dyn_cast<<a href="#AllocationInst">AllocationInst</a>>(Val)) { 397 // <i>...</i> 398} 399</pre> 400</div> 401 402 <p>This form of the <tt>if</tt> statement effectively combines together a call 403 to <tt>isa<></tt> and a call to <tt>cast<></tt> into one 404 statement, which is very convenient.</p> 405 406 <p>Note that the <tt>dyn_cast<></tt> operator, like C++'s 407 <tt>dynamic_cast<></tt> or Java's <tt>instanceof</tt> operator, can be 408 abused. In particular, you should not use big chained <tt>if/then/else</tt> 409 blocks to check for lots of different variants of classes. If you find 410 yourself wanting to do this, it is much cleaner and more efficient to use the 411 <tt>InstVisitor</tt> class to dispatch over the instruction type directly.</p> 412 413 </dd> 414 415 <dt><tt>cast_or_null<></tt>: </dt> 416 417 <dd><p>The <tt>cast_or_null<></tt> operator works just like the 418 <tt>cast<></tt> operator, except that it allows for a null pointer as an 419 argument (which it then propagates). This can sometimes be useful, allowing 420 you to combine several null checks into one.</p></dd> 421 422 <dt><tt>dyn_cast_or_null<></tt>: </dt> 423 424 <dd><p>The <tt>dyn_cast_or_null<></tt> operator works just like the 425 <tt>dyn_cast<></tt> operator, except that it allows for a null pointer 426 as an argument (which it then propagates). This can sometimes be useful, 427 allowing you to combine several null checks into one.</p></dd> 428 429</dl> 430 431<p>These five templates can be used with any classes, whether they have a 432v-table or not. To add support for these templates, you simply need to add 433<tt>classof</tt> static methods to the class you are interested casting 434to. Describing this is currently outside the scope of this document, but there 435are lots of examples in the LLVM source base.</p> 436 437</div> 438 439 440<!-- ======================================================================= --> 441<h3> 442 <a name="string_apis">Passing strings (the <tt>StringRef</tt> 443and <tt>Twine</tt> classes)</a> 444</h3> 445 446<div> 447 448<p>Although LLVM generally does not do much string manipulation, we do have 449several important APIs which take strings. Two important examples are the 450Value class -- which has names for instructions, functions, etc. -- and the 451StringMap class which is used extensively in LLVM and Clang.</p> 452 453<p>These are generic classes, and they need to be able to accept strings which 454may have embedded null characters. Therefore, they cannot simply take 455a <tt>const char *</tt>, and taking a <tt>const std::string&</tt> requires 456clients to perform a heap allocation which is usually unnecessary. Instead, 457many LLVM APIs use a <tt>StringRef</tt> or a <tt>const Twine&</tt> for 458passing strings efficiently.</p> 459 460<!-- _______________________________________________________________________ --> 461<h4> 462 <a name="StringRef">The <tt>StringRef</tt> class</a> 463</h4> 464 465<div> 466 467<p>The <tt>StringRef</tt> data type represents a reference to a constant string 468(a character array and a length) and supports the common operations available 469on <tt>std:string</tt>, but does not require heap allocation.</p> 470 471<p>It can be implicitly constructed using a C style null-terminated string, 472an <tt>std::string</tt>, or explicitly with a character pointer and length. 473For example, the <tt>StringRef</tt> find function is declared as:</p> 474 475<pre class="doc_code"> 476 iterator find(StringRef Key); 477</pre> 478 479<p>and clients can call it using any one of:</p> 480 481<pre class="doc_code"> 482 Map.find("foo"); <i>// Lookup "foo"</i> 483 Map.find(std::string("bar")); <i>// Lookup "bar"</i> 484 Map.find(StringRef("\0baz", 4)); <i>// Lookup "\0baz"</i> 485</pre> 486 487<p>Similarly, APIs which need to return a string may return a <tt>StringRef</tt> 488instance, which can be used directly or converted to an <tt>std::string</tt> 489using the <tt>str</tt> member function. See 490"<tt><a href="/doxygen/classllvm_1_1StringRef_8h-source.html">llvm/ADT/StringRef.h</a></tt>" 491for more information.</p> 492 493<p>You should rarely use the <tt>StringRef</tt> class directly, because it contains 494pointers to external memory it is not generally safe to store an instance of the 495class (unless you know that the external storage will not be freed). StringRef is 496small and pervasive enough in LLVM that it should always be passed by value.</p> 497 498</div> 499 500<!-- _______________________________________________________________________ --> 501<h4> 502 <a name="Twine">The <tt>Twine</tt> class</a> 503</h4> 504 505<div> 506 507<p>The <tt>Twine</tt> class is an efficient way for APIs to accept concatenated 508strings. For example, a common LLVM paradigm is to name one instruction based on 509the name of another instruction with a suffix, for example:</p> 510 511<div class="doc_code"> 512<pre> 513 New = CmpInst::Create(<i>...</i>, SO->getName() + ".cmp"); 514</pre> 515</div> 516 517<p>The <tt>Twine</tt> class is effectively a 518lightweight <a href="http://en.wikipedia.org/wiki/Rope_(computer_science)">rope</a> 519which points to temporary (stack allocated) objects. Twines can be implicitly 520constructed as the result of the plus operator applied to strings (i.e., a C 521strings, an <tt>std::string</tt>, or a <tt>StringRef</tt>). The twine delays the 522actual concatenation of strings until it is actually required, at which point 523it can be efficiently rendered directly into a character array. This avoids 524unnecessary heap allocation involved in constructing the temporary results of 525string concatenation. See 526"<tt><a href="/doxygen/classllvm_1_1Twine_8h-source.html">llvm/ADT/Twine.h</a></tt>" 527for more information.</p> 528 529<p>As with a <tt>StringRef</tt>, <tt>Twine</tt> objects point to external memory 530and should almost never be stored or mentioned directly. They are intended 531solely for use when defining a function which should be able to efficiently 532accept concatenated strings.</p> 533 534</div> 535 536</div> 537 538<!-- ======================================================================= --> 539<h3> 540 <a name="DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt> option</a> 541</h3> 542 543<div> 544 545<p>Often when working on your pass you will put a bunch of debugging printouts 546and other code into your pass. After you get it working, you want to remove 547it, but you may need it again in the future (to work out new bugs that you run 548across).</p> 549 550<p> Naturally, because of this, you don't want to delete the debug printouts, 551but you don't want them to always be noisy. A standard compromise is to comment 552them out, allowing you to enable them if you need them in the future.</p> 553 554<p>The "<tt><a href="/doxygen/Debug_8h-source.html">llvm/Support/Debug.h</a></tt>" 555file provides a macro named <tt>DEBUG()</tt> that is a much nicer solution to 556this problem. Basically, you can put arbitrary code into the argument of the 557<tt>DEBUG</tt> macro, and it is only executed if '<tt>opt</tt>' (or any other 558tool) is run with the '<tt>-debug</tt>' command line argument:</p> 559 560<div class="doc_code"> 561<pre> 562DEBUG(errs() << "I am here!\n"); 563</pre> 564</div> 565 566<p>Then you can run your pass like this:</p> 567 568<div class="doc_code"> 569<pre> 570$ opt < a.bc > /dev/null -mypass 571<i><no output></i> 572$ opt < a.bc > /dev/null -mypass -debug 573I am here! 574</pre> 575</div> 576 577<p>Using the <tt>DEBUG()</tt> macro instead of a home-brewed solution allows you 578to not have to create "yet another" command line option for the debug output for 579your pass. Note that <tt>DEBUG()</tt> macros are disabled for optimized builds, 580so they do not cause a performance impact at all (for the same reason, they 581should also not contain side-effects!).</p> 582 583<p>One additional nice thing about the <tt>DEBUG()</tt> macro is that you can 584enable or disable it directly in gdb. Just use "<tt>set DebugFlag=0</tt>" or 585"<tt>set DebugFlag=1</tt>" from the gdb if the program is running. If the 586program hasn't been started yet, you can always just run it with 587<tt>-debug</tt>.</p> 588 589<!-- _______________________________________________________________________ --> 590<h4> 591 <a name="DEBUG_TYPE">Fine grained debug info with <tt>DEBUG_TYPE</tt> and 592 the <tt>-debug-only</tt> option</a> 593</h4> 594 595<div> 596 597<p>Sometimes you may find yourself in a situation where enabling <tt>-debug</tt> 598just turns on <b>too much</b> information (such as when working on the code 599generator). If you want to enable debug information with more fine-grained 600control, you define the <tt>DEBUG_TYPE</tt> macro and the <tt>-debug</tt> only 601option as follows:</p> 602 603<div class="doc_code"> 604<pre> 605#undef DEBUG_TYPE 606DEBUG(errs() << "No debug type\n"); 607#define DEBUG_TYPE "foo" 608DEBUG(errs() << "'foo' debug type\n"); 609#undef DEBUG_TYPE 610#define DEBUG_TYPE "bar" 611DEBUG(errs() << "'bar' debug type\n")); 612#undef DEBUG_TYPE 613#define DEBUG_TYPE "" 614DEBUG(errs() << "No debug type (2)\n"); 615</pre> 616</div> 617 618<p>Then you can run your pass like this:</p> 619 620<div class="doc_code"> 621<pre> 622$ opt < a.bc > /dev/null -mypass 623<i><no output></i> 624$ opt < a.bc > /dev/null -mypass -debug 625No debug type 626'foo' debug type 627'bar' debug type 628No debug type (2) 629$ opt < a.bc > /dev/null -mypass -debug-only=foo 630'foo' debug type 631$ opt < a.bc > /dev/null -mypass -debug-only=bar 632'bar' debug type 633</pre> 634</div> 635 636<p>Of course, in practice, you should only set <tt>DEBUG_TYPE</tt> at the top of 637a file, to specify the debug type for the entire module (if you do this before 638you <tt>#include "llvm/Support/Debug.h"</tt>, you don't have to insert the ugly 639<tt>#undef</tt>'s). Also, you should use names more meaningful than "foo" and 640"bar", because there is no system in place to ensure that names do not 641conflict. If two different modules use the same string, they will all be turned 642on when the name is specified. This allows, for example, all debug information 643for instruction scheduling to be enabled with <tt>-debug-type=InstrSched</tt>, 644even if the source lives in multiple files.</p> 645 646<p>The <tt>DEBUG_WITH_TYPE</tt> macro is also available for situations where you 647would like to set <tt>DEBUG_TYPE</tt>, but only for one specific <tt>DEBUG</tt> 648statement. It takes an additional first parameter, which is the type to use. For 649example, the preceding example could be written as:</p> 650 651 652<div class="doc_code"> 653<pre> 654DEBUG_WITH_TYPE("", errs() << "No debug type\n"); 655DEBUG_WITH_TYPE("foo", errs() << "'foo' debug type\n"); 656DEBUG_WITH_TYPE("bar", errs() << "'bar' debug type\n")); 657DEBUG_WITH_TYPE("", errs() << "No debug type (2)\n"); 658</pre> 659</div> 660 661</div> 662 663</div> 664 665<!-- ======================================================================= --> 666<h3> 667 <a name="Statistic">The <tt>Statistic</tt> class & <tt>-stats</tt> 668 option</a> 669</h3> 670 671<div> 672 673<p>The "<tt><a 674href="/doxygen/Statistic_8h-source.html">llvm/ADT/Statistic.h</a></tt>" file 675provides a class named <tt>Statistic</tt> that is used as a unified way to 676keep track of what the LLVM compiler is doing and how effective various 677optimizations are. It is useful to see what optimizations are contributing to 678making a particular program run faster.</p> 679 680<p>Often you may run your pass on some big program, and you're interested to see 681how many times it makes a certain transformation. Although you can do this with 682hand inspection, or some ad-hoc method, this is a real pain and not very useful 683for big programs. Using the <tt>Statistic</tt> class makes it very easy to 684keep track of this information, and the calculated information is presented in a 685uniform manner with the rest of the passes being executed.</p> 686 687<p>There are many examples of <tt>Statistic</tt> uses, but the basics of using 688it are as follows:</p> 689 690<ol> 691 <li><p>Define your statistic like this:</p> 692 693<div class="doc_code"> 694<pre> 695#define <a href="#DEBUG_TYPE">DEBUG_TYPE</a> "mypassname" <i>// This goes before any #includes.</i> 696STATISTIC(NumXForms, "The # of times I did stuff"); 697</pre> 698</div> 699 700 <p>The <tt>STATISTIC</tt> macro defines a static variable, whose name is 701 specified by the first argument. The pass name is taken from the DEBUG_TYPE 702 macro, and the description is taken from the second argument. The variable 703 defined ("NumXForms" in this case) acts like an unsigned integer.</p></li> 704 705 <li><p>Whenever you make a transformation, bump the counter:</p> 706 707<div class="doc_code"> 708<pre> 709++NumXForms; // <i>I did stuff!</i> 710</pre> 711</div> 712 713 </li> 714 </ol> 715 716 <p>That's all you have to do. To get '<tt>opt</tt>' to print out the 717 statistics gathered, use the '<tt>-stats</tt>' option:</p> 718 719<div class="doc_code"> 720<pre> 721$ opt -stats -mypassname < program.bc > /dev/null 722<i>... statistics output ...</i> 723</pre> 724</div> 725 726 <p> When running <tt>opt</tt> on a C file from the SPEC benchmark 727suite, it gives a report that looks like this:</p> 728 729<div class="doc_code"> 730<pre> 731 7646 bitcodewriter - Number of normal instructions 732 725 bitcodewriter - Number of oversized instructions 733 129996 bitcodewriter - Number of bitcode bytes written 734 2817 raise - Number of insts DCEd or constprop'd 735 3213 raise - Number of cast-of-self removed 736 5046 raise - Number of expression trees converted 737 75 raise - Number of other getelementptr's formed 738 138 raise - Number of load/store peepholes 739 42 deadtypeelim - Number of unused typenames removed from symtab 740 392 funcresolve - Number of varargs functions resolved 741 27 globaldce - Number of global variables removed 742 2 adce - Number of basic blocks removed 743 134 cee - Number of branches revectored 744 49 cee - Number of setcc instruction eliminated 745 532 gcse - Number of loads removed 746 2919 gcse - Number of instructions removed 747 86 indvars - Number of canonical indvars added 748 87 indvars - Number of aux indvars removed 749 25 instcombine - Number of dead inst eliminate 750 434 instcombine - Number of insts combined 751 248 licm - Number of load insts hoisted 752 1298 licm - Number of insts hoisted to a loop pre-header 753 3 licm - Number of insts hoisted to multiple loop preds (bad, no loop pre-header) 754 75 mem2reg - Number of alloca's promoted 755 1444 cfgsimplify - Number of blocks simplified 756</pre> 757</div> 758 759<p>Obviously, with so many optimizations, having a unified framework for this 760stuff is very nice. Making your pass fit well into the framework makes it more 761maintainable and useful.</p> 762 763</div> 764 765<!-- ======================================================================= --> 766<h3> 767 <a name="ViewGraph">Viewing graphs while debugging code</a> 768</h3> 769 770<div> 771 772<p>Several of the important data structures in LLVM are graphs: for example 773CFGs made out of LLVM <a href="#BasicBlock">BasicBlock</a>s, CFGs made out of 774LLVM <a href="CodeGenerator.html#machinebasicblock">MachineBasicBlock</a>s, and 775<a href="CodeGenerator.html#selectiondag_intro">Instruction Selection 776DAGs</a>. In many cases, while debugging various parts of the compiler, it is 777nice to instantly visualize these graphs.</p> 778 779<p>LLVM provides several callbacks that are available in a debug build to do 780exactly that. If you call the <tt>Function::viewCFG()</tt> method, for example, 781the current LLVM tool will pop up a window containing the CFG for the function 782where each basic block is a node in the graph, and each node contains the 783instructions in the block. Similarly, there also exists 784<tt>Function::viewCFGOnly()</tt> (does not include the instructions), the 785<tt>MachineFunction::viewCFG()</tt> and <tt>MachineFunction::viewCFGOnly()</tt>, 786and the <tt>SelectionDAG::viewGraph()</tt> methods. Within GDB, for example, 787you can usually use something like <tt>call DAG.viewGraph()</tt> to pop 788up a window. Alternatively, you can sprinkle calls to these functions in your 789code in places you want to debug.</p> 790 791<p>Getting this to work requires a small amount of configuration. On Unix 792systems with X11, install the <a href="http://www.graphviz.org">graphviz</a> 793toolkit, and make sure 'dot' and 'gv' are in your path. If you are running on 794Mac OS/X, download and install the Mac OS/X <a 795href="http://www.pixelglow.com/graphviz/">Graphviz program</a>, and add 796<tt>/Applications/Graphviz.app/Contents/MacOS/</tt> (or wherever you install 797it) to your path. Once in your system and path are set up, rerun the LLVM 798configure script and rebuild LLVM to enable this functionality.</p> 799 800<p><tt>SelectionDAG</tt> has been extended to make it easier to locate 801<i>interesting</i> nodes in large complex graphs. From gdb, if you 802<tt>call DAG.setGraphColor(<i>node</i>, "<i>color</i>")</tt>, then the 803next <tt>call DAG.viewGraph()</tt> would highlight the node in the 804specified color (choices of colors can be found at <a 805href="http://www.graphviz.org/doc/info/colors.html">colors</a>.) More 806complex node attributes can be provided with <tt>call 807DAG.setGraphAttrs(<i>node</i>, "<i>attributes</i>")</tt> (choices can be 808found at <a href="http://www.graphviz.org/doc/info/attrs.html">Graph 809Attributes</a>.) If you want to restart and clear all the current graph 810attributes, then you can <tt>call DAG.clearGraphAttrs()</tt>. </p> 811 812<p>Note that graph visualization features are compiled out of Release builds 813to reduce file size. This means that you need a Debug+Asserts or 814Release+Asserts build to use these features.</p> 815 816</div> 817 818</div> 819 820<!-- *********************************************************************** --> 821<h2> 822 <a name="datastructure">Picking the Right Data Structure for a Task</a> 823</h2> 824<!-- *********************************************************************** --> 825 826<div> 827 828<p>LLVM has a plethora of data structures in the <tt>llvm/ADT/</tt> directory, 829 and we commonly use STL data structures. This section describes the trade-offs 830 you should consider when you pick one.</p> 831 832<p> 833The first step is a choose your own adventure: do you want a sequential 834container, a set-like container, or a map-like container? The most important 835thing when choosing a container is the algorithmic properties of how you plan to 836access the container. Based on that, you should use:</p> 837 838<ul> 839<li>a <a href="#ds_map">map-like</a> container if you need efficient look-up 840 of an value based on another value. Map-like containers also support 841 efficient queries for containment (whether a key is in the map). Map-like 842 containers generally do not support efficient reverse mapping (values to 843 keys). If you need that, use two maps. Some map-like containers also 844 support efficient iteration through the keys in sorted order. Map-like 845 containers are the most expensive sort, only use them if you need one of 846 these capabilities.</li> 847 848<li>a <a href="#ds_set">set-like</a> container if you need to put a bunch of 849 stuff into a container that automatically eliminates duplicates. Some 850 set-like containers support efficient iteration through the elements in 851 sorted order. Set-like containers are more expensive than sequential 852 containers. 853</li> 854 855<li>a <a href="#ds_sequential">sequential</a> container provides 856 the most efficient way to add elements and keeps track of the order they are 857 added to the collection. They permit duplicates and support efficient 858 iteration, but do not support efficient look-up based on a key. 859</li> 860 861<li>a <a href="#ds_string">string</a> container is a specialized sequential 862 container or reference structure that is used for character or byte 863 arrays.</li> 864 865<li>a <a href="#ds_bit">bit</a> container provides an efficient way to store and 866 perform set operations on sets of numeric id's, while automatically 867 eliminating duplicates. Bit containers require a maximum of 1 bit for each 868 identifier you want to store. 869</li> 870</ul> 871 872<p> 873Once the proper category of container is determined, you can fine tune the 874memory use, constant factors, and cache behaviors of access by intelligently 875picking a member of the category. Note that constant factors and cache behavior 876can be a big deal. If you have a vector that usually only contains a few 877elements (but could contain many), for example, it's much better to use 878<a href="#dss_smallvector">SmallVector</a> than <a href="#dss_vector">vector</a> 879. Doing so avoids (relatively) expensive malloc/free calls, which dwarf the 880cost of adding the elements to the container. </p> 881 882</div> 883 884 885<!-- ======================================================================= --> 886<h3> 887 <a name="ds_sequential">Sequential Containers (std::vector, std::list, etc)</a> 888</h3> 889 890<div> 891There are a variety of sequential containers available for you, based on your 892needs. Pick the first in this section that will do what you want. 893 894<!-- _______________________________________________________________________ --> 895<h4> 896 <a name="dss_arrayref">llvm/ADT/ArrayRef.h</a> 897</h4> 898 899<div> 900<p>The llvm::ArrayRef class is the preferred class to use in an interface that 901 accepts a sequential list of elements in memory and just reads from them. By 902 taking an ArrayRef, the API can be passed a fixed size array, an std::vector, 903 an llvm::SmallVector and anything else that is contiguous in memory. 904</p> 905</div> 906 907 908 909<!-- _______________________________________________________________________ --> 910<h4> 911 <a name="dss_fixedarrays">Fixed Size Arrays</a> 912</h4> 913 914<div> 915<p>Fixed size arrays are very simple and very fast. They are good if you know 916exactly how many elements you have, or you have a (low) upper bound on how many 917you have.</p> 918</div> 919 920<!-- _______________________________________________________________________ --> 921<h4> 922 <a name="dss_heaparrays">Heap Allocated Arrays</a> 923</h4> 924 925<div> 926<p>Heap allocated arrays (new[] + delete[]) are also simple. They are good if 927the number of elements is variable, if you know how many elements you will need 928before the array is allocated, and if the array is usually large (if not, 929consider a <a href="#dss_smallvector">SmallVector</a>). The cost of a heap 930allocated array is the cost of the new/delete (aka malloc/free). Also note that 931if you are allocating an array of a type with a constructor, the constructor and 932destructors will be run for every element in the array (re-sizable vectors only 933construct those elements actually used).</p> 934</div> 935 936<!-- _______________________________________________________________________ --> 937<h4> 938 <a name="dss_tinyptrvector">"llvm/ADT/TinyPtrVector.h"</a> 939</h4> 940 941 942<div> 943<p><tt>TinyPtrVector<Type></tt> is a highly specialized collection class 944that is optimized to avoid allocation in the case when a vector has zero or one 945elements. It has two major restrictions: 1) it can only hold values of pointer 946type, and 2) it cannot hold a null pointer.</p> 947 948<p>Since this container is highly specialized, it is rarely used.</p> 949 950</div> 951 952<!-- _______________________________________________________________________ --> 953<h4> 954 <a name="dss_smallvector">"llvm/ADT/SmallVector.h"</a> 955</h4> 956 957<div> 958<p><tt>SmallVector<Type, N></tt> is a simple class that looks and smells 959just like <tt>vector<Type></tt>: 960it supports efficient iteration, lays out elements in memory order (so you can 961do pointer arithmetic between elements), supports efficient push_back/pop_back 962operations, supports efficient random access to its elements, etc.</p> 963 964<p>The advantage of SmallVector is that it allocates space for 965some number of elements (N) <b>in the object itself</b>. Because of this, if 966the SmallVector is dynamically smaller than N, no malloc is performed. This can 967be a big win in cases where the malloc/free call is far more expensive than the 968code that fiddles around with the elements.</p> 969 970<p>This is good for vectors that are "usually small" (e.g. the number of 971predecessors/successors of a block is usually less than 8). On the other hand, 972this makes the size of the SmallVector itself large, so you don't want to 973allocate lots of them (doing so will waste a lot of space). As such, 974SmallVectors are most useful when on the stack.</p> 975 976<p>SmallVector also provides a nice portable and efficient replacement for 977<tt>alloca</tt>.</p> 978 979</div> 980 981<!-- _______________________________________________________________________ --> 982<h4> 983 <a name="dss_vector"><vector></a> 984</h4> 985 986<div> 987<p> 988std::vector is well loved and respected. It is useful when SmallVector isn't: 989when the size of the vector is often large (thus the small optimization will 990rarely be a benefit) or if you will be allocating many instances of the vector 991itself (which would waste space for elements that aren't in the container). 992vector is also useful when interfacing with code that expects vectors :). 993</p> 994 995<p>One worthwhile note about std::vector: avoid code like this:</p> 996 997<div class="doc_code"> 998<pre> 999for ( ... ) { 1000 std::vector<foo> V; 1001 use V; 1002} 1003</pre> 1004</div> 1005 1006<p>Instead, write this as:</p> 1007 1008<div class="doc_code"> 1009<pre> 1010std::vector<foo> V; 1011for ( ... ) { 1012 use V; 1013 V.clear(); 1014} 1015</pre> 1016</div> 1017 1018<p>Doing so will save (at least) one heap allocation and free per iteration of 1019the loop.</p> 1020 1021</div> 1022 1023<!-- _______________________________________________________________________ --> 1024<h4> 1025 <a name="dss_deque"><deque></a> 1026</h4> 1027 1028<div> 1029<p>std::deque is, in some senses, a generalized version of std::vector. Like 1030std::vector, it provides constant time random access and other similar 1031properties, but it also provides efficient access to the front of the list. It 1032does not guarantee continuity of elements within memory.</p> 1033 1034<p>In exchange for this extra flexibility, std::deque has significantly higher 1035constant factor costs than std::vector. If possible, use std::vector or 1036something cheaper.</p> 1037</div> 1038 1039<!-- _______________________________________________________________________ --> 1040<h4> 1041 <a name="dss_list"><list></a> 1042</h4> 1043 1044<div> 1045<p>std::list is an extremely inefficient class that is rarely useful. 1046It performs a heap allocation for every element inserted into it, thus having an 1047extremely high constant factor, particularly for small data types. std::list 1048also only supports bidirectional iteration, not random access iteration.</p> 1049 1050<p>In exchange for this high cost, std::list supports efficient access to both 1051ends of the list (like std::deque, but unlike std::vector or SmallVector). In 1052addition, the iterator invalidation characteristics of std::list are stronger 1053than that of a vector class: inserting or removing an element into the list does 1054not invalidate iterator or pointers to other elements in the list.</p> 1055</div> 1056 1057<!-- _______________________________________________________________________ --> 1058<h4> 1059 <a name="dss_ilist">llvm/ADT/ilist.h</a> 1060</h4> 1061 1062<div> 1063<p><tt>ilist<T></tt> implements an 'intrusive' doubly-linked list. It is 1064intrusive, because it requires the element to store and provide access to the 1065prev/next pointers for the list.</p> 1066 1067<p><tt>ilist</tt> has the same drawbacks as <tt>std::list</tt>, and additionally 1068requires an <tt>ilist_traits</tt> implementation for the element type, but it 1069provides some novel characteristics. In particular, it can efficiently store 1070polymorphic objects, the traits class is informed when an element is inserted or 1071removed from the list, and <tt>ilist</tt>s are guaranteed to support a 1072constant-time splice operation.</p> 1073 1074<p>These properties are exactly what we want for things like 1075<tt>Instruction</tt>s and basic blocks, which is why these are implemented with 1076<tt>ilist</tt>s.</p> 1077 1078Related classes of interest are explained in the following subsections: 1079 <ul> 1080 <li><a href="#dss_ilist_traits">ilist_traits</a></li> 1081 <li><a href="#dss_iplist">iplist</a></li> 1082 <li><a href="#dss_ilist_node">llvm/ADT/ilist_node.h</a></li> 1083 <li><a href="#dss_ilist_sentinel">Sentinels</a></li> 1084 </ul> 1085</div> 1086 1087<!-- _______________________________________________________________________ --> 1088<h4> 1089 <a name="dss_packedvector">llvm/ADT/PackedVector.h</a> 1090</h4> 1091 1092<div> 1093<p> 1094Useful for storing a vector of values using only a few number of bits for each 1095value. Apart from the standard operations of a vector-like container, it can 1096also perform an 'or' set operation. 1097</p> 1098 1099<p>For example:</p> 1100 1101<div class="doc_code"> 1102<pre> 1103enum State { 1104 None = 0x0, 1105 FirstCondition = 0x1, 1106 SecondCondition = 0x2, 1107 Both = 0x3 1108}; 1109 1110State get() { 1111 PackedVector<State, 2> Vec1; 1112 Vec1.push_back(FirstCondition); 1113 1114 PackedVector<State, 2> Vec2; 1115 Vec2.push_back(SecondCondition); 1116 1117 Vec1 |= Vec2; 1118 return Vec1[0]; // returns 'Both'. 1119} 1120</pre> 1121</div> 1122 1123</div> 1124 1125<!-- _______________________________________________________________________ --> 1126<h4> 1127 <a name="dss_ilist_traits">ilist_traits</a> 1128</h4> 1129 1130<div> 1131<p><tt>ilist_traits<T></tt> is <tt>ilist<T></tt>'s customization 1132mechanism. <tt>iplist<T></tt> (and consequently <tt>ilist<T></tt>) 1133publicly derive from this traits class.</p> 1134</div> 1135 1136<!-- _______________________________________________________________________ --> 1137<h4> 1138 <a name="dss_iplist">iplist</a> 1139</h4> 1140 1141<div> 1142<p><tt>iplist<T></tt> is <tt>ilist<T></tt>'s base and as such 1143supports a slightly narrower interface. Notably, inserters from 1144<tt>T&</tt> are absent.</p> 1145 1146<p><tt>ilist_traits<T></tt> is a public base of this class and can be 1147used for a wide variety of customizations.</p> 1148</div> 1149 1150<!-- _______________________________________________________________________ --> 1151<h4> 1152 <a name="dss_ilist_node">llvm/ADT/ilist_node.h</a> 1153</h4> 1154 1155<div> 1156<p><tt>ilist_node<T></tt> implements a the forward and backward links 1157that are expected by the <tt>ilist<T></tt> (and analogous containers) 1158in the default manner.</p> 1159 1160<p><tt>ilist_node<T></tt>s are meant to be embedded in the node type 1161<tt>T</tt>, usually <tt>T</tt> publicly derives from 1162<tt>ilist_node<T></tt>.</p> 1163</div> 1164 1165<!-- _______________________________________________________________________ --> 1166<h4> 1167 <a name="dss_ilist_sentinel">Sentinels</a> 1168</h4> 1169 1170<div> 1171<p><tt>ilist</tt>s have another specialty that must be considered. To be a good 1172citizen in the C++ ecosystem, it needs to support the standard container 1173operations, such as <tt>begin</tt> and <tt>end</tt> iterators, etc. Also, the 1174<tt>operator--</tt> must work correctly on the <tt>end</tt> iterator in the 1175case of non-empty <tt>ilist</tt>s.</p> 1176 1177<p>The only sensible solution to this problem is to allocate a so-called 1178<i>sentinel</i> along with the intrusive list, which serves as the <tt>end</tt> 1179iterator, providing the back-link to the last element. However conforming to the 1180C++ convention it is illegal to <tt>operator++</tt> beyond the sentinel and it 1181also must not be dereferenced.</p> 1182 1183<p>These constraints allow for some implementation freedom to the <tt>ilist</tt> 1184how to allocate and store the sentinel. The corresponding policy is dictated 1185by <tt>ilist_traits<T></tt>. By default a <tt>T</tt> gets heap-allocated 1186whenever the need for a sentinel arises.</p> 1187 1188<p>While the default policy is sufficient in most cases, it may break down when 1189<tt>T</tt> does not provide a default constructor. Also, in the case of many 1190instances of <tt>ilist</tt>s, the memory overhead of the associated sentinels 1191is wasted. To alleviate the situation with numerous and voluminous 1192<tt>T</tt>-sentinels, sometimes a trick is employed, leading to <i>ghostly 1193sentinels</i>.</p> 1194 1195<p>Ghostly sentinels are obtained by specially-crafted <tt>ilist_traits<T></tt> 1196which superpose the sentinel with the <tt>ilist</tt> instance in memory. Pointer 1197arithmetic is used to obtain the sentinel, which is relative to the 1198<tt>ilist</tt>'s <tt>this</tt> pointer. The <tt>ilist</tt> is augmented by an 1199extra pointer, which serves as the back-link of the sentinel. This is the only 1200field in the ghostly sentinel which can be legally accessed.</p> 1201</div> 1202 1203<!-- _______________________________________________________________________ --> 1204<h4> 1205 <a name="dss_other">Other Sequential Container options</a> 1206</h4> 1207 1208<div> 1209<p>Other STL containers are available, such as std::string.</p> 1210 1211<p>There are also various STL adapter classes such as std::queue, 1212std::priority_queue, std::stack, etc. These provide simplified access to an 1213underlying container but don't affect the cost of the container itself.</p> 1214 1215</div> 1216</div> 1217 1218<!-- ======================================================================= --> 1219<h3> 1220 <a name="ds_string">String-like containers</a> 1221</h3> 1222 1223<div> 1224 1225<p> 1226There are a variety of ways to pass around and use strings in C and C++, and 1227LLVM adds a few new options to choose from. Pick the first option on this list 1228that will do what you need, they are ordered according to their relative cost. 1229</p> 1230<p> 1231Note that is is generally preferred to <em>not</em> pass strings around as 1232"<tt>const char*</tt>"'s. These have a number of problems, including the fact 1233that they cannot represent embedded nul ("\0") characters, and do not have a 1234length available efficiently. The general replacement for '<tt>const 1235char*</tt>' is StringRef. 1236</p> 1237 1238<p>For more information on choosing string containers for APIs, please see 1239<a href="#string_apis">Passing strings</a>.</p> 1240 1241 1242<!-- _______________________________________________________________________ --> 1243<h4> 1244 <a name="dss_stringref">llvm/ADT/StringRef.h</a> 1245</h4> 1246 1247<div> 1248<p> 1249The StringRef class is a simple value class that contains a pointer to a 1250character and a length, and is quite related to the <a 1251href="#dss_arrayref">ArrayRef</a> class (but specialized for arrays of 1252characters). Because StringRef carries a length with it, it safely handles 1253strings with embedded nul characters in it, getting the length does not require 1254a strlen call, and it even has very convenient APIs for slicing and dicing the 1255character range that it represents. 1256</p> 1257 1258<p> 1259StringRef is ideal for passing simple strings around that are known to be live, 1260either because they are C string literals, std::string, a C array, or a 1261SmallVector. Each of these cases has an efficient implicit conversion to 1262StringRef, which doesn't result in a dynamic strlen being executed. 1263</p> 1264 1265<p>StringRef has a few major limitations which make more powerful string 1266containers useful:</p> 1267 1268<ol> 1269<li>You cannot directly convert a StringRef to a 'const char*' because there is 1270no way to add a trailing nul (unlike the .c_str() method on various stronger 1271classes).</li> 1272 1273 1274<li>StringRef doesn't own or keep alive the underlying string bytes. 1275As such it can easily lead to dangling pointers, and is not suitable for 1276embedding in datastructures in most cases (instead, use an std::string or 1277something like that).</li> 1278 1279<li>For the same reason, StringRef cannot be used as the return value of a 1280method if the method "computes" the result string. Instead, use 1281std::string.</li> 1282 1283<li>StringRef's do not allow you to mutate the pointed-to string bytes and it 1284doesn't allow you to insert or remove bytes from the range. For editing 1285operations like this, it interoperates with the <a 1286href="#dss_twine">Twine</a> class.</li> 1287</ol> 1288 1289<p>Because of its strengths and limitations, it is very common for a function to 1290take a StringRef and for a method on an object to return a StringRef that 1291points into some string that it owns.</p> 1292 1293</div> 1294 1295<!-- _______________________________________________________________________ --> 1296<h4> 1297 <a name="dss_twine">llvm/ADT/Twine.h</a> 1298</h4> 1299 1300<div> 1301 <p> 1302 The Twine class is used as an intermediary datatype for APIs that want to take 1303 a string that can be constructed inline with a series of concatenations. 1304 Twine works by forming recursive instances of the Twine datatype (a simple 1305 value object) on the stack as temporary objects, linking them together into a 1306 tree which is then linearized when the Twine is consumed. Twine is only safe 1307 to use as the argument to a function, and should always be a const reference, 1308 e.g.: 1309 </p> 1310 1311 <pre> 1312 void foo(const Twine &T); 1313 ... 1314 StringRef X = ... 1315 unsigned i = ... 1316 foo(X + "." + Twine(i)); 1317 </pre> 1318 1319 <p>This example forms a string like "blarg.42" by concatenating the values 1320 together, and does not form intermediate strings containing "blarg" or 1321 "blarg.". 1322 </p> 1323 1324 <p>Because Twine is constructed with temporary objects on the stack, and 1325 because these instances are destroyed at the end of the current statement, 1326 it is an inherently dangerous API. For example, this simple variant contains 1327 undefined behavior and will probably crash:</p> 1328 1329 <pre> 1330 void foo(const Twine &T); 1331 ... 1332 StringRef X = ... 1333 unsigned i = ... 1334 const Twine &Tmp = X + "." + Twine(i); 1335 foo(Tmp); 1336 </pre> 1337 1338 <p>... because the temporaries are destroyed before the call. That said, 1339 Twine's are much more efficient than intermediate std::string temporaries, and 1340 they work really well with StringRef. Just be aware of their limitations.</p> 1341 1342</div> 1343 1344 1345<!-- _______________________________________________________________________ --> 1346<h4> 1347 <a name="dss_smallstring">llvm/ADT/SmallString.h</a> 1348</h4> 1349 1350<div> 1351 1352<p>SmallString is a subclass of <a href="#dss_smallvector">SmallVector</a> that 1353adds some convenience APIs like += that takes StringRef's. SmallString avoids 1354allocating memory in the case when the preallocated space is enough to hold its 1355data, and it calls back to general heap allocation when required. Since it owns 1356its data, it is very safe to use and supports full mutation of the string.</p> 1357 1358<p>Like SmallVector's, the big downside to SmallString is their sizeof. While 1359they are optimized for small strings, they themselves are not particularly 1360small. This means that they work great for temporary scratch buffers on the 1361stack, but should not generally be put into the heap: it is very rare to 1362see a SmallString as the member of a frequently-allocated heap data structure 1363or returned by-value. 1364</p> 1365 1366</div> 1367 1368<!-- _______________________________________________________________________ --> 1369<h4> 1370 <a name="dss_stdstring">std::string</a> 1371</h4> 1372 1373<div> 1374 1375 <p>The standard C++ std::string class is a very general class that (like 1376 SmallString) owns its underlying data. sizeof(std::string) is very reasonable 1377 so it can be embedded into heap data structures and returned by-value. 1378 On the other hand, std::string is highly inefficient for inline editing (e.g. 1379 concatenating a bunch of stuff together) and because it is provided by the 1380 standard library, its performance characteristics depend a lot of the host 1381 standard library (e.g. libc++ and MSVC provide a highly optimized string 1382 class, GCC contains a really slow implementation). 1383 </p> 1384 1385 <p>The major disadvantage of std::string is that almost every operation that 1386 makes them larger can allocate memory, which is slow. As such, it is better 1387 to use SmallVector or Twine as a scratch buffer, but then use std::string to 1388 persist the result.</p> 1389 1390 1391</div> 1392 1393<!-- end of strings --> 1394</div> 1395 1396 1397<!-- ======================================================================= --> 1398<h3> 1399 <a name="ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a> 1400</h3> 1401 1402<div> 1403 1404<p>Set-like containers are useful when you need to canonicalize multiple values 1405into a single representation. There are several different choices for how to do 1406this, providing various trade-offs.</p> 1407 1408<!-- _______________________________________________________________________ --> 1409<h4> 1410 <a name="dss_sortedvectorset">A sorted 'vector'</a> 1411</h4> 1412 1413<div> 1414 1415<p>If you intend to insert a lot of elements, then do a lot of queries, a 1416great approach is to use a vector (or other sequential container) with 1417std::sort+std::unique to remove duplicates. This approach works really well if 1418your usage pattern has these two distinct phases (insert then query), and can be 1419coupled with a good choice of <a href="#ds_sequential">sequential container</a>. 1420</p> 1421 1422<p> 1423This combination provides the several nice properties: the result data is 1424contiguous in memory (good for cache locality), has few allocations, is easy to 1425address (iterators in the final vector are just indices or pointers), and can be 1426efficiently queried with a standard binary or radix search.</p> 1427 1428</div> 1429 1430<!-- _______________________________________________________________________ --> 1431<h4> 1432 <a name="dss_smallset">"llvm/ADT/SmallSet.h"</a> 1433</h4> 1434 1435<div> 1436 1437<p>If you have a set-like data structure that is usually small and whose elements 1438are reasonably small, a <tt>SmallSet<Type, N></tt> is a good choice. This set 1439has space for N elements in place (thus, if the set is dynamically smaller than 1440N, no malloc traffic is required) and accesses them with a simple linear search. 1441When the set grows beyond 'N' elements, it allocates a more expensive representation that 1442guarantees efficient access (for most types, it falls back to std::set, but for 1443pointers it uses something far better, <a 1444href="#dss_smallptrset">SmallPtrSet</a>).</p> 1445 1446<p>The magic of this class is that it handles small sets extremely efficiently, 1447but gracefully handles extremely large sets without loss of efficiency. The 1448drawback is that the interface is quite small: it supports insertion, queries 1449and erasing, but does not support iteration.</p> 1450 1451</div> 1452 1453<!-- _______________________________________________________________________ --> 1454<h4> 1455 <a name="dss_smallptrset">"llvm/ADT/SmallPtrSet.h"</a> 1456</h4> 1457 1458<div> 1459 1460<p>SmallPtrSet has all the advantages of <tt>SmallSet</tt> (and a <tt>SmallSet</tt> of pointers is 1461transparently implemented with a <tt>SmallPtrSet</tt>), but also supports iterators. If 1462more than 'N' insertions are performed, a single quadratically 1463probed hash table is allocated and grows as needed, providing extremely 1464efficient access (constant time insertion/deleting/queries with low constant 1465factors) and is very stingy with malloc traffic.</p> 1466 1467<p>Note that, unlike <tt>std::set</tt>, the iterators of <tt>SmallPtrSet</tt> are invalidated 1468whenever an insertion occurs. Also, the values visited by the iterators are not 1469visited in sorted order.</p> 1470 1471</div> 1472 1473<!-- _______________________________________________________________________ --> 1474<h4> 1475 <a name="dss_denseset">"llvm/ADT/DenseSet.h"</a> 1476</h4> 1477 1478<div> 1479 1480<p> 1481DenseSet is a simple quadratically probed hash table. It excels at supporting 1482small values: it uses a single allocation to hold all of the pairs that 1483are currently inserted in the set. DenseSet is a great way to unique small 1484values that are not simple pointers (use <a 1485href="#dss_smallptrset">SmallPtrSet</a> for pointers). Note that DenseSet has 1486the same requirements for the value type that <a 1487href="#dss_densemap">DenseMap</a> has. 1488</p> 1489 1490</div> 1491 1492<!-- _______________________________________________________________________ --> 1493<h4> 1494 <a name="dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a> 1495</h4> 1496 1497<div> 1498 1499<p> 1500FoldingSet is an aggregate class that is really good at uniquing 1501expensive-to-create or polymorphic objects. It is a combination of a chained 1502hash table with intrusive links (uniqued objects are required to inherit from 1503FoldingSetNode) that uses <a href="#dss_smallvector">SmallVector</a> as part of 1504its ID process.</p> 1505 1506<p>Consider a case where you want to implement a "getOrCreateFoo" method for 1507a complex object (for example, a node in the code generator). The client has a 1508description of *what* it wants to generate (it knows the opcode and all the 1509operands), but we don't want to 'new' a node, then try inserting it into a set 1510only to find out it already exists, at which point we would have to delete it 1511and return the node that already exists. 1512</p> 1513 1514<p>To support this style of client, FoldingSet perform a query with a 1515FoldingSetNodeID (which wraps SmallVector) that can be used to describe the 1516element that we want to query for. The query either returns the element 1517matching the ID or it returns an opaque ID that indicates where insertion should 1518take place. Construction of the ID usually does not require heap traffic.</p> 1519 1520<p>Because FoldingSet uses intrusive links, it can support polymorphic objects 1521in the set (for example, you can have SDNode instances mixed with LoadSDNodes). 1522Because the elements are individually allocated, pointers to the elements are 1523stable: inserting or removing elements does not invalidate any pointers to other 1524elements. 1525</p> 1526 1527</div> 1528 1529<!-- _______________________________________________________________________ --> 1530<h4> 1531 <a name="dss_set"><set></a> 1532</h4> 1533 1534<div> 1535 1536<p><tt>std::set</tt> is a reasonable all-around set class, which is decent at 1537many things but great at nothing. std::set allocates memory for each element 1538inserted (thus it is very malloc intensive) and typically stores three pointers 1539per element in the set (thus adding a large amount of per-element space 1540overhead). It offers guaranteed log(n) performance, which is not particularly 1541fast from a complexity standpoint (particularly if the elements of the set are 1542expensive to compare, like strings), and has extremely high constant factors for 1543lookup, insertion and removal.</p> 1544 1545<p>The advantages of std::set are that its iterators are stable (deleting or 1546inserting an element from the set does not affect iterators or pointers to other 1547elements) and that iteration over the set is guaranteed to be in sorted order. 1548If the elements in the set are large, then the relative overhead of the pointers 1549and malloc traffic is not a big deal, but if the elements of the set are small, 1550std::set is almost never a good choice.</p> 1551 1552</div> 1553 1554<!-- _______________________________________________________________________ --> 1555<h4> 1556 <a name="dss_setvector">"llvm/ADT/SetVector.h"</a> 1557</h4> 1558 1559<div> 1560<p>LLVM's SetVector<Type> is an adapter class that combines your choice of 1561a set-like container along with a <a href="#ds_sequential">Sequential 1562Container</a>. The important property 1563that this provides is efficient insertion with uniquing (duplicate elements are 1564ignored) with iteration support. It implements this by inserting elements into 1565both a set-like container and the sequential container, using the set-like 1566container for uniquing and the sequential container for iteration. 1567</p> 1568 1569<p>The difference between SetVector and other sets is that the order of 1570iteration is guaranteed to match the order of insertion into the SetVector. 1571This property is really important for things like sets of pointers. Because 1572pointer values are non-deterministic (e.g. vary across runs of the program on 1573different machines), iterating over the pointers in the set will 1574not be in a well-defined order.</p> 1575 1576<p> 1577The drawback of SetVector is that it requires twice as much space as a normal 1578set and has the sum of constant factors from the set-like container and the 1579sequential container that it uses. Use it *only* if you need to iterate over 1580the elements in a deterministic order. SetVector is also expensive to delete 1581elements out of (linear time), unless you use it's "pop_back" method, which is 1582faster. 1583</p> 1584 1585<p><tt>SetVector</tt> is an adapter class that defaults to 1586 using <tt>std::vector</tt> and a size 16 <tt>SmallSet</tt> for the underlying 1587 containers, so it is quite expensive. However, 1588 <tt>"llvm/ADT/SetVector.h"</tt> also provides a <tt>SmallSetVector</tt> 1589 class, which defaults to using a <tt>SmallVector</tt> and <tt>SmallSet</tt> 1590 of a specified size. If you use this, and if your sets are dynamically 1591 smaller than <tt>N</tt>, you will save a lot of heap traffic.</p> 1592 1593</div> 1594 1595<!-- _______________________________________________________________________ --> 1596<h4> 1597 <a name="dss_uniquevector">"llvm/ADT/UniqueVector.h"</a> 1598</h4> 1599 1600<div> 1601 1602<p> 1603UniqueVector is similar to <a href="#dss_setvector">SetVector</a>, but it 1604retains a unique ID for each element inserted into the set. It internally 1605contains a map and a vector, and it assigns a unique ID for each value inserted 1606into the set.</p> 1607 1608<p>UniqueVector is very expensive: its cost is the sum of the cost of 1609maintaining both the map and vector, it has high complexity, high constant 1610factors, and produces a lot of malloc traffic. It should be avoided.</p> 1611 1612</div> 1613 1614 1615<!-- _______________________________________________________________________ --> 1616<h4> 1617 <a name="dss_otherset">Other Set-Like Container Options</a> 1618</h4> 1619 1620<div> 1621 1622<p> 1623The STL provides several other options, such as std::multiset and the various 1624"hash_set" like containers (whether from C++ TR1 or from the SGI library). We 1625never use hash_set and unordered_set because they are generally very expensive 1626(each insertion requires a malloc) and very non-portable. 1627</p> 1628 1629<p>std::multiset is useful if you're not interested in elimination of 1630duplicates, but has all the drawbacks of std::set. A sorted vector (where you 1631don't delete duplicate entries) or some other approach is almost always 1632better.</p> 1633 1634</div> 1635 1636</div> 1637 1638<!-- ======================================================================= --> 1639<h3> 1640 <a name="ds_map">Map-Like Containers (std::map, DenseMap, etc)</a> 1641</h3> 1642 1643<div> 1644Map-like containers are useful when you want to associate data to a key. As 1645usual, there are a lot of different ways to do this. :) 1646 1647<!-- _______________________________________________________________________ --> 1648<h4> 1649 <a name="dss_sortedvectormap">A sorted 'vector'</a> 1650</h4> 1651 1652<div> 1653 1654<p> 1655If your usage pattern follows a strict insert-then-query approach, you can 1656trivially use the same approach as <a href="#dss_sortedvectorset">sorted vectors 1657for set-like containers</a>. The only difference is that your query function 1658(which uses std::lower_bound to get efficient log(n) lookup) should only compare 1659the key, not both the key and value. This yields the same advantages as sorted 1660vectors for sets. 1661</p> 1662</div> 1663 1664<!-- _______________________________________________________________________ --> 1665<h4> 1666 <a name="dss_stringmap">"llvm/ADT/StringMap.h"</a> 1667</h4> 1668 1669<div> 1670 1671<p> 1672Strings are commonly used as keys in maps, and they are difficult to support 1673efficiently: they are variable length, inefficient to hash and compare when 1674long, expensive to copy, etc. StringMap is a specialized container designed to 1675cope with these issues. It supports mapping an arbitrary range of bytes to an 1676arbitrary other object.</p> 1677 1678<p>The StringMap implementation uses a quadratically-probed hash table, where 1679the buckets store a pointer to the heap allocated entries (and some other 1680stuff). The entries in the map must be heap allocated because the strings are 1681variable length. The string data (key) and the element object (value) are 1682stored in the same allocation with the string data immediately after the element 1683object. This container guarantees the "<tt>(char*)(&Value+1)</tt>" points 1684to the key string for a value.</p> 1685 1686<p>The StringMap is very fast for several reasons: quadratic probing is very 1687cache efficient for lookups, the hash value of strings in buckets is not 1688recomputed when looking up an element, StringMap rarely has to touch the 1689memory for unrelated objects when looking up a value (even when hash collisions 1690happen), hash table growth does not recompute the hash values for strings 1691already in the table, and each pair in the map is store in a single allocation 1692(the string data is stored in the same allocation as the Value of a pair).</p> 1693 1694<p>StringMap also provides query methods that take byte ranges, so it only ever 1695copies a string if a value is inserted into the table.</p> 1696</div> 1697 1698<!-- _______________________________________________________________________ --> 1699<h4> 1700 <a name="dss_indexedmap">"llvm/ADT/IndexedMap.h"</a> 1701</h4> 1702 1703<div> 1704<p> 1705IndexedMap is a specialized container for mapping small dense integers (or 1706values that can be mapped to small dense integers) to some other type. It is 1707internally implemented as a vector with a mapping function that maps the keys to 1708the dense integer range. 1709</p> 1710 1711<p> 1712This is useful for cases like virtual registers in the LLVM code generator: they 1713have a dense mapping that is offset by a compile-time constant (the first 1714virtual register ID).</p> 1715 1716</div> 1717 1718<!-- _______________________________________________________________________ --> 1719<h4> 1720 <a name="dss_densemap">"llvm/ADT/DenseMap.h"</a> 1721</h4> 1722 1723<div> 1724 1725<p> 1726DenseMap is a simple quadratically probed hash table. It excels at supporting 1727small keys and values: it uses a single allocation to hold all of the pairs that 1728are currently inserted in the map. DenseMap is a great way to map pointers to 1729pointers, or map other small types to each other. 1730</p> 1731 1732<p> 1733There are several aspects of DenseMap that you should be aware of, however. The 1734iterators in a densemap are invalidated whenever an insertion occurs, unlike 1735map. Also, because DenseMap allocates space for a large number of key/value 1736pairs (it starts with 64 by default), it will waste a lot of space if your keys 1737or values are large. Finally, you must implement a partial specialization of 1738DenseMapInfo for the key that you want, if it isn't already supported. This 1739is required to tell DenseMap about two special marker values (which can never be 1740inserted into the map) that it needs internally.</p> 1741 1742</div> 1743 1744<!-- _______________________________________________________________________ --> 1745<h4> 1746 <a name="dss_valuemap">"llvm/ADT/ValueMap.h"</a> 1747</h4> 1748 1749<div> 1750 1751<p> 1752ValueMap is a wrapper around a <a href="#dss_densemap">DenseMap</a> mapping 1753Value*s (or subclasses) to another type. When a Value is deleted or RAUW'ed, 1754ValueMap will update itself so the new version of the key is mapped to the same 1755value, just as if the key were a WeakVH. You can configure exactly how this 1756happens, and what else happens on these two events, by passing 1757a <code>Config</code> parameter to the ValueMap template.</p> 1758 1759</div> 1760 1761<!-- _______________________________________________________________________ --> 1762<h4> 1763 <a name="dss_intervalmap">"llvm/ADT/IntervalMap.h"</a> 1764</h4> 1765 1766<div> 1767 1768<p> IntervalMap is a compact map for small keys and values. It maps key 1769intervals instead of single keys, and it will automatically coalesce adjacent 1770intervals. When then map only contains a few intervals, they are stored in the 1771map object itself to avoid allocations.</p> 1772 1773<p> The IntervalMap iterators are quite big, so they should not be passed around 1774as STL iterators. The heavyweight iterators allow a smaller data structure.</p> 1775 1776</div> 1777 1778<!-- _______________________________________________________________________ --> 1779<h4> 1780 <a name="dss_map"><map></a> 1781</h4> 1782 1783<div> 1784 1785<p> 1786std::map has similar characteristics to <a href="#dss_set">std::set</a>: it uses 1787a single allocation per pair inserted into the map, it offers log(n) lookup with 1788an extremely large constant factor, imposes a space penalty of 3 pointers per 1789pair in the map, etc.</p> 1790 1791<p>std::map is most useful when your keys or values are very large, if you need 1792to iterate over the collection in sorted order, or if you need stable iterators 1793into the map (i.e. they don't get invalidated if an insertion or deletion of 1794another element takes place).</p> 1795 1796</div> 1797 1798<!-- _______________________________________________________________________ --> 1799<h4> 1800 <a name="dss_inteqclasses">"llvm/ADT/IntEqClasses.h"</a> 1801</h4> 1802 1803<div> 1804 1805<p>IntEqClasses provides a compact representation of equivalence classes of 1806small integers. Initially, each integer in the range 0..n-1 has its own 1807equivalence class. Classes can be joined by passing two class representatives to 1808the join(a, b) method. Two integers are in the same class when findLeader() 1809returns the same representative.</p> 1810 1811<p>Once all equivalence classes are formed, the map can be compressed so each 1812integer 0..n-1 maps to an equivalence class number in the range 0..m-1, where m 1813is the total number of equivalence classes. The map must be uncompressed before 1814it can be edited again.</p> 1815 1816</div> 1817 1818<!-- _______________________________________________________________________ --> 1819<h4> 1820 <a name="dss_othermap">Other Map-Like Container Options</a> 1821</h4> 1822 1823<div> 1824 1825<p> 1826The STL provides several other options, such as std::multimap and the various 1827"hash_map" like containers (whether from C++ TR1 or from the SGI library). We 1828never use hash_set and unordered_set because they are generally very expensive 1829(each insertion requires a malloc) and very non-portable.</p> 1830 1831<p>std::multimap is useful if you want to map a key to multiple values, but has 1832all the drawbacks of std::map. A sorted vector or some other approach is almost 1833always better.</p> 1834 1835</div> 1836 1837</div> 1838 1839<!-- ======================================================================= --> 1840<h3> 1841 <a name="ds_bit">Bit storage containers (BitVector, SparseBitVector)</a> 1842</h3> 1843 1844<div> 1845<p>Unlike the other containers, there are only two bit storage containers, and 1846choosing when to use each is relatively straightforward.</p> 1847 1848<p>One additional option is 1849<tt>std::vector<bool></tt>: we discourage its use for two reasons 1) the 1850implementation in many common compilers (e.g. commonly available versions of 1851GCC) is extremely inefficient and 2) the C++ standards committee is likely to 1852deprecate this container and/or change it significantly somehow. In any case, 1853please don't use it.</p> 1854 1855<!-- _______________________________________________________________________ --> 1856<h4> 1857 <a name="dss_bitvector">BitVector</a> 1858</h4> 1859 1860<div> 1861<p> The BitVector container provides a dynamic size set of bits for manipulation. 1862It supports individual bit setting/testing, as well as set operations. The set 1863operations take time O(size of bitvector), but operations are performed one word 1864at a time, instead of one bit at a time. This makes the BitVector very fast for 1865set operations compared to other containers. Use the BitVector when you expect 1866the number of set bits to be high (IE a dense set). 1867</p> 1868</div> 1869 1870<!-- _______________________________________________________________________ --> 1871<h4> 1872 <a name="dss_smallbitvector">SmallBitVector</a> 1873</h4> 1874 1875<div> 1876<p> The SmallBitVector container provides the same interface as BitVector, but 1877it is optimized for the case where only a small number of bits, less than 187825 or so, are needed. It also transparently supports larger bit counts, but 1879slightly less efficiently than a plain BitVector, so SmallBitVector should 1880only be used when larger counts are rare. 1881</p> 1882 1883<p> 1884At this time, SmallBitVector does not support set operations (and, or, xor), 1885and its operator[] does not provide an assignable lvalue. 1886</p> 1887</div> 1888 1889<!-- _______________________________________________________________________ --> 1890<h4> 1891 <a name="dss_sparsebitvector">SparseBitVector</a> 1892</h4> 1893 1894<div> 1895<p> The SparseBitVector container is much like BitVector, with one major 1896difference: Only the bits that are set, are stored. This makes the 1897SparseBitVector much more space efficient than BitVector when the set is sparse, 1898as well as making set operations O(number of set bits) instead of O(size of 1899universe). The downside to the SparseBitVector is that setting and testing of random bits is O(N), and on large SparseBitVectors, this can be slower than BitVector. In our implementation, setting or testing bits in sorted order 1900(either forwards or reverse) is O(1) worst case. Testing and setting bits within 128 bits (depends on size) of the current bit is also O(1). As a general statement, testing/setting bits in a SparseBitVector is O(distance away from last set bit). 1901</p> 1902</div> 1903 1904</div> 1905 1906</div> 1907 1908<!-- *********************************************************************** --> 1909<h2> 1910 <a name="common">Helpful Hints for Common Operations</a> 1911</h2> 1912<!-- *********************************************************************** --> 1913 1914<div> 1915 1916<p>This section describes how to perform some very simple transformations of 1917LLVM code. This is meant to give examples of common idioms used, showing the 1918practical side of LLVM transformations. <p> Because this is a "how-to" section, 1919you should also read about the main classes that you will be working with. The 1920<a href="#coreclasses">Core LLVM Class Hierarchy Reference</a> contains details 1921and descriptions of the main classes that you should know about.</p> 1922 1923<!-- NOTE: this section should be heavy on example code --> 1924<!-- ======================================================================= --> 1925<h3> 1926 <a name="inspection">Basic Inspection and Traversal Routines</a> 1927</h3> 1928 1929<div> 1930 1931<p>The LLVM compiler infrastructure have many different data structures that may 1932be traversed. Following the example of the C++ standard template library, the 1933techniques used to traverse these various data structures are all basically the 1934same. For a enumerable sequence of values, the <tt>XXXbegin()</tt> function (or 1935method) returns an iterator to the start of the sequence, the <tt>XXXend()</tt> 1936function returns an iterator pointing to one past the last valid element of the 1937sequence, and there is some <tt>XXXiterator</tt> data type that is common 1938between the two operations.</p> 1939 1940<p>Because the pattern for iteration is common across many different aspects of 1941the program representation, the standard template library algorithms may be used 1942on them, and it is easier to remember how to iterate. First we show a few common 1943examples of the data structures that need to be traversed. Other data 1944structures are traversed in very similar ways.</p> 1945 1946<!-- _______________________________________________________________________ --> 1947<h4> 1948 <a name="iterate_function">Iterating over the </a><a 1949 href="#BasicBlock"><tt>BasicBlock</tt></a>s in a <a 1950 href="#Function"><tt>Function</tt></a> 1951</h4> 1952 1953<div> 1954 1955<p>It's quite common to have a <tt>Function</tt> instance that you'd like to 1956transform in some way; in particular, you'd like to manipulate its 1957<tt>BasicBlock</tt>s. To facilitate this, you'll need to iterate over all of 1958the <tt>BasicBlock</tt>s that constitute the <tt>Function</tt>. The following is 1959an example that prints the name of a <tt>BasicBlock</tt> and the number of 1960<tt>Instruction</tt>s it contains:</p> 1961 1962<div class="doc_code"> 1963<pre> 1964// <i>func is a pointer to a Function instance</i> 1965for (Function::iterator i = func->begin(), e = func->end(); i != e; ++i) 1966 // <i>Print out the name of the basic block if it has one, and then the</i> 1967 // <i>number of instructions that it contains</i> 1968 errs() << "Basic block (name=" << i->getName() << ") has " 1969 << i->size() << " instructions.\n"; 1970</pre> 1971</div> 1972 1973<p>Note that i can be used as if it were a pointer for the purposes of 1974invoking member functions of the <tt>Instruction</tt> class. This is 1975because the indirection operator is overloaded for the iterator 1976classes. In the above code, the expression <tt>i->size()</tt> is 1977exactly equivalent to <tt>(*i).size()</tt> just like you'd expect.</p> 1978 1979</div> 1980 1981<!-- _______________________________________________________________________ --> 1982<h4> 1983 <a name="iterate_basicblock">Iterating over the </a><a 1984 href="#Instruction"><tt>Instruction</tt></a>s in a <a 1985 href="#BasicBlock"><tt>BasicBlock</tt></a> 1986</h4> 1987 1988<div> 1989 1990<p>Just like when dealing with <tt>BasicBlock</tt>s in <tt>Function</tt>s, it's 1991easy to iterate over the individual instructions that make up 1992<tt>BasicBlock</tt>s. Here's a code snippet that prints out each instruction in 1993a <tt>BasicBlock</tt>:</p> 1994 1995<div class="doc_code"> 1996<pre> 1997// <i>blk is a pointer to a BasicBlock instance</i> 1998for (BasicBlock::iterator i = blk->begin(), e = blk->end(); i != e; ++i) 1999 // <i>The next statement works since operator<<(ostream&,...)</i> 2000 // <i>is overloaded for Instruction&</i> 2001 errs() << *i << "\n"; 2002</pre> 2003</div> 2004 2005<p>However, this isn't really the best way to print out the contents of a 2006<tt>BasicBlock</tt>! Since the ostream operators are overloaded for virtually 2007anything you'll care about, you could have just invoked the print routine on the 2008basic block itself: <tt>errs() << *blk << "\n";</tt>.</p> 2009 2010</div> 2011 2012<!-- _______________________________________________________________________ --> 2013<h4> 2014 <a name="iterate_institer">Iterating over the </a><a 2015 href="#Instruction"><tt>Instruction</tt></a>s in a <a 2016 href="#Function"><tt>Function</tt></a> 2017</h4> 2018 2019<div> 2020 2021<p>If you're finding that you commonly iterate over a <tt>Function</tt>'s 2022<tt>BasicBlock</tt>s and then that <tt>BasicBlock</tt>'s <tt>Instruction</tt>s, 2023<tt>InstIterator</tt> should be used instead. You'll need to include <a 2024href="/doxygen/InstIterator_8h-source.html"><tt>llvm/Support/InstIterator.h</tt></a>, 2025and then instantiate <tt>InstIterator</tt>s explicitly in your code. Here's a 2026small example that shows how to dump all instructions in a function to the standard error stream:<p> 2027 2028<div class="doc_code"> 2029<pre> 2030#include "<a href="/doxygen/InstIterator_8h-source.html">llvm/Support/InstIterator.h</a>" 2031 2032// <i>F is a pointer to a Function instance</i> 2033for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 2034 errs() << *I << "\n"; 2035</pre> 2036</div> 2037 2038<p>Easy, isn't it? You can also use <tt>InstIterator</tt>s to fill a 2039work list with its initial contents. For example, if you wanted to 2040initialize a work list to contain all instructions in a <tt>Function</tt> 2041F, all you would need to do is something like:</p> 2042 2043<div class="doc_code"> 2044<pre> 2045std::set<Instruction*> worklist; 2046// or better yet, SmallPtrSet<Instruction*, 64> worklist; 2047 2048for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 2049 worklist.insert(&*I); 2050</pre> 2051</div> 2052 2053<p>The STL set <tt>worklist</tt> would now contain all instructions in the 2054<tt>Function</tt> pointed to by F.</p> 2055 2056</div> 2057 2058<!-- _______________________________________________________________________ --> 2059<h4> 2060 <a name="iterate_convert">Turning an iterator into a class pointer (and 2061 vice-versa)</a> 2062</h4> 2063 2064<div> 2065 2066<p>Sometimes, it'll be useful to grab a reference (or pointer) to a class 2067instance when all you've got at hand is an iterator. Well, extracting 2068a reference or a pointer from an iterator is very straight-forward. 2069Assuming that <tt>i</tt> is a <tt>BasicBlock::iterator</tt> and <tt>j</tt> 2070is a <tt>BasicBlock::const_iterator</tt>:</p> 2071 2072<div class="doc_code"> 2073<pre> 2074Instruction& inst = *i; // <i>Grab reference to instruction reference</i> 2075Instruction* pinst = &*i; // <i>Grab pointer to instruction reference</i> 2076const Instruction& inst = *j; 2077</pre> 2078</div> 2079 2080<p>However, the iterators you'll be working with in the LLVM framework are 2081special: they will automatically convert to a ptr-to-instance type whenever they 2082need to. Instead of dereferencing the iterator and then taking the address of 2083the result, you can simply assign the iterator to the proper pointer type and 2084you get the dereference and address-of operation as a result of the assignment 2085(behind the scenes, this is a result of overloading casting mechanisms). Thus 2086the last line of the last example,</p> 2087 2088<div class="doc_code"> 2089<pre> 2090Instruction *pinst = &*i; 2091</pre> 2092</div> 2093 2094<p>is semantically equivalent to</p> 2095 2096<div class="doc_code"> 2097<pre> 2098Instruction *pinst = i; 2099</pre> 2100</div> 2101 2102<p>It's also possible to turn a class pointer into the corresponding iterator, 2103and this is a constant time operation (very efficient). The following code 2104snippet illustrates use of the conversion constructors provided by LLVM 2105iterators. By using these, you can explicitly grab the iterator of something 2106without actually obtaining it via iteration over some structure:</p> 2107 2108<div class="doc_code"> 2109<pre> 2110void printNextInstruction(Instruction* inst) { 2111 BasicBlock::iterator it(inst); 2112 ++it; // <i>After this line, it refers to the instruction after *inst</i> 2113 if (it != inst->getParent()->end()) errs() << *it << "\n"; 2114} 2115</pre> 2116</div> 2117 2118<p>Unfortunately, these implicit conversions come at a cost; they prevent 2119these iterators from conforming to standard iterator conventions, and thus 2120from being usable with standard algorithms and containers. For example, they 2121prevent the following code, where <tt>B</tt> is a <tt>BasicBlock</tt>, 2122from compiling:</p> 2123 2124<div class="doc_code"> 2125<pre> 2126 llvm::SmallVector<llvm::Instruction *, 16>(B->begin(), B->end()); 2127</pre> 2128</div> 2129 2130<p>Because of this, these implicit conversions may be removed some day, 2131and <tt>operator*</tt> changed to return a pointer instead of a reference.</p> 2132 2133</div> 2134 2135<!--_______________________________________________________________________--> 2136<h4> 2137 <a name="iterate_complex">Finding call sites: a slightly more complex 2138 example</a> 2139</h4> 2140 2141<div> 2142 2143<p>Say that you're writing a FunctionPass and would like to count all the 2144locations in the entire module (that is, across every <tt>Function</tt>) where a 2145certain function (i.e., some <tt>Function</tt>*) is already in scope. As you'll 2146learn later, you may want to use an <tt>InstVisitor</tt> to accomplish this in a 2147much more straight-forward manner, but this example will allow us to explore how 2148you'd do it if you didn't have <tt>InstVisitor</tt> around. In pseudo-code, this 2149is what we want to do:</p> 2150 2151<div class="doc_code"> 2152<pre> 2153initialize callCounter to zero 2154for each Function f in the Module 2155 for each BasicBlock b in f 2156 for each Instruction i in b 2157 if (i is a CallInst and calls the given function) 2158 increment callCounter 2159</pre> 2160</div> 2161 2162<p>And the actual code is (remember, because we're writing a 2163<tt>FunctionPass</tt>, our <tt>FunctionPass</tt>-derived class simply has to 2164override the <tt>runOnFunction</tt> method):</p> 2165 2166<div class="doc_code"> 2167<pre> 2168Function* targetFunc = ...; 2169 2170class OurFunctionPass : public FunctionPass { 2171 public: 2172 OurFunctionPass(): callCounter(0) { } 2173 2174 virtual runOnFunction(Function& F) { 2175 for (Function::iterator b = F.begin(), be = F.end(); b != be; ++b) { 2176 for (BasicBlock::iterator i = b->begin(), ie = b->end(); i != ie; ++i) { 2177 if (<a href="#CallInst">CallInst</a>* callInst = <a href="#isa">dyn_cast</a><<a 2178 href="#CallInst">CallInst</a>>(&*i)) { 2179 // <i>We know we've encountered a call instruction, so we</i> 2180 // <i>need to determine if it's a call to the</i> 2181 // <i>function pointed to by m_func or not.</i> 2182 if (callInst->getCalledFunction() == targetFunc) 2183 ++callCounter; 2184 } 2185 } 2186 } 2187 } 2188 2189 private: 2190 unsigned callCounter; 2191}; 2192</pre> 2193</div> 2194 2195</div> 2196 2197<!--_______________________________________________________________________--> 2198<h4> 2199 <a name="calls_and_invokes">Treating calls and invokes the same way</a> 2200</h4> 2201 2202<div> 2203 2204<p>You may have noticed that the previous example was a bit oversimplified in 2205that it did not deal with call sites generated by 'invoke' instructions. In 2206this, and in other situations, you may find that you want to treat 2207<tt>CallInst</tt>s and <tt>InvokeInst</tt>s the same way, even though their 2208most-specific common base class is <tt>Instruction</tt>, which includes lots of 2209less closely-related things. For these cases, LLVM provides a handy wrapper 2210class called <a 2211href="http://llvm.org/doxygen/classllvm_1_1CallSite.html"><tt>CallSite</tt></a>. 2212It is essentially a wrapper around an <tt>Instruction</tt> pointer, with some 2213methods that provide functionality common to <tt>CallInst</tt>s and 2214<tt>InvokeInst</tt>s.</p> 2215 2216<p>This class has "value semantics": it should be passed by value, not by 2217reference and it should not be dynamically allocated or deallocated using 2218<tt>operator new</tt> or <tt>operator delete</tt>. It is efficiently copyable, 2219assignable and constructable, with costs equivalents to that of a bare pointer. 2220If you look at its definition, it has only a single pointer member.</p> 2221 2222</div> 2223 2224<!--_______________________________________________________________________--> 2225<h4> 2226 <a name="iterate_chains">Iterating over def-use & use-def chains</a> 2227</h4> 2228 2229<div> 2230 2231<p>Frequently, we might have an instance of the <a 2232href="/doxygen/classllvm_1_1Value.html">Value Class</a> and we want to 2233determine which <tt>User</tt>s use the <tt>Value</tt>. The list of all 2234<tt>User</tt>s of a particular <tt>Value</tt> is called a <i>def-use</i> chain. 2235For example, let's say we have a <tt>Function*</tt> named <tt>F</tt> to a 2236particular function <tt>foo</tt>. Finding all of the instructions that 2237<i>use</i> <tt>foo</tt> is as simple as iterating over the <i>def-use</i> chain 2238of <tt>F</tt>:</p> 2239 2240<div class="doc_code"> 2241<pre> 2242Function *F = ...; 2243 2244for (Value::use_iterator i = F->use_begin(), e = F->use_end(); i != e; ++i) 2245 if (Instruction *Inst = dyn_cast<Instruction>(*i)) { 2246 errs() << "F is used in instruction:\n"; 2247 errs() << *Inst << "\n"; 2248 } 2249</pre> 2250</div> 2251 2252<p>Note that dereferencing a <tt>Value::use_iterator</tt> is not a very cheap 2253operation. Instead of performing <tt>*i</tt> above several times, consider 2254doing it only once in the loop body and reusing its result.</p> 2255 2256<p>Alternatively, it's common to have an instance of the <a 2257href="/doxygen/classllvm_1_1User.html">User Class</a> and need to know what 2258<tt>Value</tt>s are used by it. The list of all <tt>Value</tt>s used by a 2259<tt>User</tt> is known as a <i>use-def</i> chain. Instances of class 2260<tt>Instruction</tt> are common <tt>User</tt>s, so we might want to iterate over 2261all of the values that a particular instruction uses (that is, the operands of 2262the particular <tt>Instruction</tt>):</p> 2263 2264<div class="doc_code"> 2265<pre> 2266Instruction *pi = ...; 2267 2268for (User::op_iterator i = pi->op_begin(), e = pi->op_end(); i != e; ++i) { 2269 Value *v = *i; 2270 // <i>...</i> 2271} 2272</pre> 2273</div> 2274 2275<p>Declaring objects as <tt>const</tt> is an important tool of enforcing 2276mutation free algorithms (such as analyses, etc.). For this purpose above 2277iterators come in constant flavors as <tt>Value::const_use_iterator</tt> 2278and <tt>Value::const_op_iterator</tt>. They automatically arise when 2279calling <tt>use/op_begin()</tt> on <tt>const Value*</tt>s or 2280<tt>const User*</tt>s respectively. Upon dereferencing, they return 2281<tt>const Use*</tt>s. Otherwise the above patterns remain unchanged.</p> 2282 2283</div> 2284 2285<!--_______________________________________________________________________--> 2286<h4> 2287 <a name="iterate_preds">Iterating over predecessors & 2288successors of blocks</a> 2289</h4> 2290 2291<div> 2292 2293<p>Iterating over the predecessors and successors of a block is quite easy 2294with the routines defined in <tt>"llvm/Support/CFG.h"</tt>. Just use code like 2295this to iterate over all predecessors of BB:</p> 2296 2297<div class="doc_code"> 2298<pre> 2299#include "llvm/Support/CFG.h" 2300BasicBlock *BB = ...; 2301 2302for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 2303 BasicBlock *Pred = *PI; 2304 // <i>...</i> 2305} 2306</pre> 2307</div> 2308 2309<p>Similarly, to iterate over successors use 2310succ_iterator/succ_begin/succ_end.</p> 2311 2312</div> 2313 2314</div> 2315 2316<!-- ======================================================================= --> 2317<h3> 2318 <a name="simplechanges">Making simple changes</a> 2319</h3> 2320 2321<div> 2322 2323<p>There are some primitive transformation operations present in the LLVM 2324infrastructure that are worth knowing about. When performing 2325transformations, it's fairly common to manipulate the contents of basic 2326blocks. This section describes some of the common methods for doing so 2327and gives example code.</p> 2328 2329<!--_______________________________________________________________________--> 2330<h4> 2331 <a name="schanges_creating">Creating and inserting new 2332 <tt>Instruction</tt>s</a> 2333</h4> 2334 2335<div> 2336 2337<p><i>Instantiating Instructions</i></p> 2338 2339<p>Creation of <tt>Instruction</tt>s is straight-forward: simply call the 2340constructor for the kind of instruction to instantiate and provide the necessary 2341parameters. For example, an <tt>AllocaInst</tt> only <i>requires</i> a 2342(const-ptr-to) <tt>Type</tt>. Thus:</p> 2343 2344<div class="doc_code"> 2345<pre> 2346AllocaInst* ai = new AllocaInst(Type::Int32Ty); 2347</pre> 2348</div> 2349 2350<p>will create an <tt>AllocaInst</tt> instance that represents the allocation of 2351one integer in the current stack frame, at run time. Each <tt>Instruction</tt> 2352subclass is likely to have varying default parameters which change the semantics 2353of the instruction, so refer to the <a 2354href="/doxygen/classllvm_1_1Instruction.html">doxygen documentation for the subclass of 2355Instruction</a> that you're interested in instantiating.</p> 2356 2357<p><i>Naming values</i></p> 2358 2359<p>It is very useful to name the values of instructions when you're able to, as 2360this facilitates the debugging of your transformations. If you end up looking 2361at generated LLVM machine code, you definitely want to have logical names 2362associated with the results of instructions! By supplying a value for the 2363<tt>Name</tt> (default) parameter of the <tt>Instruction</tt> constructor, you 2364associate a logical name with the result of the instruction's execution at 2365run time. For example, say that I'm writing a transformation that dynamically 2366allocates space for an integer on the stack, and that integer is going to be 2367used as some kind of index by some other code. To accomplish this, I place an 2368<tt>AllocaInst</tt> at the first point in the first <tt>BasicBlock</tt> of some 2369<tt>Function</tt>, and I'm intending to use it within the same 2370<tt>Function</tt>. I might do:</p> 2371 2372<div class="doc_code"> 2373<pre> 2374AllocaInst* pa = new AllocaInst(Type::Int32Ty, 0, "indexLoc"); 2375</pre> 2376</div> 2377 2378<p>where <tt>indexLoc</tt> is now the logical name of the instruction's 2379execution value, which is a pointer to an integer on the run time stack.</p> 2380 2381<p><i>Inserting instructions</i></p> 2382 2383<p>There are essentially two ways to insert an <tt>Instruction</tt> 2384into an existing sequence of instructions that form a <tt>BasicBlock</tt>:</p> 2385 2386<ul> 2387 <li>Insertion into an explicit instruction list 2388 2389 <p>Given a <tt>BasicBlock* pb</tt>, an <tt>Instruction* pi</tt> within that 2390 <tt>BasicBlock</tt>, and a newly-created instruction we wish to insert 2391 before <tt>*pi</tt>, we do the following: </p> 2392 2393<div class="doc_code"> 2394<pre> 2395BasicBlock *pb = ...; 2396Instruction *pi = ...; 2397Instruction *newInst = new Instruction(...); 2398 2399pb->getInstList().insert(pi, newInst); // <i>Inserts newInst before pi in pb</i> 2400</pre> 2401</div> 2402 2403 <p>Appending to the end of a <tt>BasicBlock</tt> is so common that 2404 the <tt>Instruction</tt> class and <tt>Instruction</tt>-derived 2405 classes provide constructors which take a pointer to a 2406 <tt>BasicBlock</tt> to be appended to. For example code that 2407 looked like: </p> 2408 2409<div class="doc_code"> 2410<pre> 2411BasicBlock *pb = ...; 2412Instruction *newInst = new Instruction(...); 2413 2414pb->getInstList().push_back(newInst); // <i>Appends newInst to pb</i> 2415</pre> 2416</div> 2417 2418 <p>becomes: </p> 2419 2420<div class="doc_code"> 2421<pre> 2422BasicBlock *pb = ...; 2423Instruction *newInst = new Instruction(..., pb); 2424</pre> 2425</div> 2426 2427 <p>which is much cleaner, especially if you are creating 2428 long instruction streams.</p></li> 2429 2430 <li>Insertion into an implicit instruction list 2431 2432 <p><tt>Instruction</tt> instances that are already in <tt>BasicBlock</tt>s 2433 are implicitly associated with an existing instruction list: the instruction 2434 list of the enclosing basic block. Thus, we could have accomplished the same 2435 thing as the above code without being given a <tt>BasicBlock</tt> by doing: 2436 </p> 2437 2438<div class="doc_code"> 2439<pre> 2440Instruction *pi = ...; 2441Instruction *newInst = new Instruction(...); 2442 2443pi->getParent()->getInstList().insert(pi, newInst); 2444</pre> 2445</div> 2446 2447 <p>In fact, this sequence of steps occurs so frequently that the 2448 <tt>Instruction</tt> class and <tt>Instruction</tt>-derived classes provide 2449 constructors which take (as a default parameter) a pointer to an 2450 <tt>Instruction</tt> which the newly-created <tt>Instruction</tt> should 2451 precede. That is, <tt>Instruction</tt> constructors are capable of 2452 inserting the newly-created instance into the <tt>BasicBlock</tt> of a 2453 provided instruction, immediately before that instruction. Using an 2454 <tt>Instruction</tt> constructor with a <tt>insertBefore</tt> (default) 2455 parameter, the above code becomes:</p> 2456 2457<div class="doc_code"> 2458<pre> 2459Instruction* pi = ...; 2460Instruction* newInst = new Instruction(..., pi); 2461</pre> 2462</div> 2463 2464 <p>which is much cleaner, especially if you're creating a lot of 2465 instructions and adding them to <tt>BasicBlock</tt>s.</p></li> 2466</ul> 2467 2468</div> 2469 2470<!--_______________________________________________________________________--> 2471<h4> 2472 <a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a> 2473</h4> 2474 2475<div> 2476 2477<p>Deleting an instruction from an existing sequence of instructions that form a 2478<a href="#BasicBlock"><tt>BasicBlock</tt></a> is very straight-forward: just 2479call the instruction's eraseFromParent() method. For example:</p> 2480 2481<div class="doc_code"> 2482<pre> 2483<a href="#Instruction">Instruction</a> *I = .. ; 2484I->eraseFromParent(); 2485</pre> 2486</div> 2487 2488<p>This unlinks the instruction from its containing basic block and deletes 2489it. If you'd just like to unlink the instruction from its containing basic 2490block but not delete it, you can use the <tt>removeFromParent()</tt> method.</p> 2491 2492</div> 2493 2494<!--_______________________________________________________________________--> 2495<h4> 2496 <a name="schanges_replacing">Replacing an <tt>Instruction</tt> with another 2497 <tt>Value</tt></a> 2498</h4> 2499 2500<div> 2501 2502<p><i>Replacing individual instructions</i></p> 2503 2504<p>Including "<a href="/doxygen/BasicBlockUtils_8h-source.html">llvm/Transforms/Utils/BasicBlockUtils.h</a>" 2505permits use of two very useful replace functions: <tt>ReplaceInstWithValue</tt> 2506and <tt>ReplaceInstWithInst</tt>.</p> 2507 2508<h5><a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a></h5> 2509 2510<ul> 2511 <li><tt>ReplaceInstWithValue</tt> 2512 2513 <p>This function replaces all uses of a given instruction with a value, 2514 and then removes the original instruction. The following example 2515 illustrates the replacement of the result of a particular 2516 <tt>AllocaInst</tt> that allocates memory for a single integer with a null 2517 pointer to an integer.</p> 2518 2519<div class="doc_code"> 2520<pre> 2521AllocaInst* instToReplace = ...; 2522BasicBlock::iterator ii(instToReplace); 2523 2524ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii, 2525 Constant::getNullValue(PointerType::getUnqual(Type::Int32Ty))); 2526</pre></div></li> 2527 2528 <li><tt>ReplaceInstWithInst</tt> 2529 2530 <p>This function replaces a particular instruction with another 2531 instruction, inserting the new instruction into the basic block at the 2532 location where the old instruction was, and replacing any uses of the old 2533 instruction with the new instruction. The following example illustrates 2534 the replacement of one <tt>AllocaInst</tt> with another.</p> 2535 2536<div class="doc_code"> 2537<pre> 2538AllocaInst* instToReplace = ...; 2539BasicBlock::iterator ii(instToReplace); 2540 2541ReplaceInstWithInst(instToReplace->getParent()->getInstList(), ii, 2542 new AllocaInst(Type::Int32Ty, 0, "ptrToReplacedInt")); 2543</pre></div></li> 2544</ul> 2545 2546<p><i>Replacing multiple uses of <tt>User</tt>s and <tt>Value</tt>s</i></p> 2547 2548<p>You can use <tt>Value::replaceAllUsesWith</tt> and 2549<tt>User::replaceUsesOfWith</tt> to change more than one use at a time. See the 2550doxygen documentation for the <a href="/doxygen/classllvm_1_1Value.html">Value Class</a> 2551and <a href="/doxygen/classllvm_1_1User.html">User Class</a>, respectively, for more 2552information.</p> 2553 2554<!-- Value::replaceAllUsesWith User::replaceUsesOfWith Point out: 2555include/llvm/Transforms/Utils/ especially BasicBlockUtils.h with: 2556ReplaceInstWithValue, ReplaceInstWithInst --> 2557 2558</div> 2559 2560<!--_______________________________________________________________________--> 2561<h4> 2562 <a name="schanges_deletingGV">Deleting <tt>GlobalVariable</tt>s</a> 2563</h4> 2564 2565<div> 2566 2567<p>Deleting a global variable from a module is just as easy as deleting an 2568Instruction. First, you must have a pointer to the global variable that you wish 2569 to delete. You use this pointer to erase it from its parent, the module. 2570 For example:</p> 2571 2572<div class="doc_code"> 2573<pre> 2574<a href="#GlobalVariable">GlobalVariable</a> *GV = .. ; 2575 2576GV->eraseFromParent(); 2577</pre> 2578</div> 2579 2580</div> 2581 2582</div> 2583 2584<!-- ======================================================================= --> 2585<h3> 2586 <a name="create_types">How to Create Types</a> 2587</h3> 2588 2589<div> 2590 2591<p>In generating IR, you may need some complex types. If you know these types 2592statically, you can use <tt>TypeBuilder<...>::get()</tt>, defined 2593in <tt>llvm/Support/TypeBuilder.h</tt>, to retrieve them. <tt>TypeBuilder</tt> 2594has two forms depending on whether you're building types for cross-compilation 2595or native library use. <tt>TypeBuilder<T, true></tt> requires 2596that <tt>T</tt> be independent of the host environment, meaning that it's built 2597out of types from 2598the <a href="/doxygen/namespacellvm_1_1types.html"><tt>llvm::types</tt></a> 2599namespace and pointers, functions, arrays, etc. built of 2600those. <tt>TypeBuilder<T, false></tt> additionally allows native C types 2601whose size may depend on the host compiler. For example,</p> 2602 2603<div class="doc_code"> 2604<pre> 2605FunctionType *ft = TypeBuilder<types::i<8>(types::i<32>*), true>::get(); 2606</pre> 2607</div> 2608 2609<p>is easier to read and write than the equivalent</p> 2610 2611<div class="doc_code"> 2612<pre> 2613std::vector<const Type*> params; 2614params.push_back(PointerType::getUnqual(Type::Int32Ty)); 2615FunctionType *ft = FunctionType::get(Type::Int8Ty, params, false); 2616</pre> 2617</div> 2618 2619<p>See the <a href="/doxygen/TypeBuilder_8h-source.html#l00001">class 2620comment</a> for more details.</p> 2621 2622</div> 2623 2624</div> 2625 2626<!-- *********************************************************************** --> 2627<h2> 2628 <a name="threading">Threads and LLVM</a> 2629</h2> 2630<!-- *********************************************************************** --> 2631 2632<div> 2633<p> 2634This section describes the interaction of the LLVM APIs with multithreading, 2635both on the part of client applications, and in the JIT, in the hosted 2636application. 2637</p> 2638 2639<p> 2640Note that LLVM's support for multithreading is still relatively young. Up 2641through version 2.5, the execution of threaded hosted applications was 2642supported, but not threaded client access to the APIs. While this use case is 2643now supported, clients <em>must</em> adhere to the guidelines specified below to 2644ensure proper operation in multithreaded mode. 2645</p> 2646 2647<p> 2648Note that, on Unix-like platforms, LLVM requires the presence of GCC's atomic 2649intrinsics in order to support threaded operation. If you need a 2650multhreading-capable LLVM on a platform without a suitably modern system 2651compiler, consider compiling LLVM and LLVM-GCC in single-threaded mode, and 2652using the resultant compiler to build a copy of LLVM with multithreading 2653support. 2654</p> 2655 2656<!-- ======================================================================= --> 2657<h3> 2658 <a name="startmultithreaded">Entering and Exiting Multithreaded Mode</a> 2659</h3> 2660 2661<div> 2662 2663<p> 2664In order to properly protect its internal data structures while avoiding 2665excessive locking overhead in the single-threaded case, the LLVM must intialize 2666certain data structures necessary to provide guards around its internals. To do 2667so, the client program must invoke <tt>llvm_start_multithreaded()</tt> before 2668making any concurrent LLVM API calls. To subsequently tear down these 2669structures, use the <tt>llvm_stop_multithreaded()</tt> call. You can also use 2670the <tt>llvm_is_multithreaded()</tt> call to check the status of multithreaded 2671mode. 2672</p> 2673 2674<p> 2675Note that both of these calls must be made <em>in isolation</em>. That is to 2676say that no other LLVM API calls may be executing at any time during the 2677execution of <tt>llvm_start_multithreaded()</tt> or <tt>llvm_stop_multithreaded 2678</tt>. It's is the client's responsibility to enforce this isolation. 2679</p> 2680 2681<p> 2682The return value of <tt>llvm_start_multithreaded()</tt> indicates the success or 2683failure of the initialization. Failure typically indicates that your copy of 2684LLVM was built without multithreading support, typically because GCC atomic 2685intrinsics were not found in your system compiler. In this case, the LLVM API 2686will not be safe for concurrent calls. However, it <em>will</em> be safe for 2687hosting threaded applications in the JIT, though <a href="#jitthreading">care 2688must be taken</a> to ensure that side exits and the like do not accidentally 2689result in concurrent LLVM API calls. 2690</p> 2691</div> 2692 2693<!-- ======================================================================= --> 2694<h3> 2695 <a name="shutdown">Ending Execution with <tt>llvm_shutdown()</tt></a> 2696</h3> 2697 2698<div> 2699<p> 2700When you are done using the LLVM APIs, you should call <tt>llvm_shutdown()</tt> 2701to deallocate memory used for internal structures. This will also invoke 2702<tt>llvm_stop_multithreaded()</tt> if LLVM is operating in multithreaded mode. 2703As such, <tt>llvm_shutdown()</tt> requires the same isolation guarantees as 2704<tt>llvm_stop_multithreaded()</tt>. 2705</p> 2706 2707<p> 2708Note that, if you use scope-based shutdown, you can use the 2709<tt>llvm_shutdown_obj</tt> class, which calls <tt>llvm_shutdown()</tt> in its 2710destructor. 2711</div> 2712 2713<!-- ======================================================================= --> 2714<h3> 2715 <a name="managedstatic">Lazy Initialization with <tt>ManagedStatic</tt></a> 2716</h3> 2717 2718<div> 2719<p> 2720<tt>ManagedStatic</tt> is a utility class in LLVM used to implement static 2721initialization of static resources, such as the global type tables. Before the 2722invocation of <tt>llvm_shutdown()</tt>, it implements a simple lazy 2723initialization scheme. Once <tt>llvm_start_multithreaded()</tt> returns, 2724however, it uses double-checked locking to implement thread-safe lazy 2725initialization. 2726</p> 2727 2728<p> 2729Note that, because no other threads are allowed to issue LLVM API calls before 2730<tt>llvm_start_multithreaded()</tt> returns, it is possible to have 2731<tt>ManagedStatic</tt>s of <tt>llvm::sys::Mutex</tt>s. 2732</p> 2733 2734<p> 2735The <tt>llvm_acquire_global_lock()</tt> and <tt>llvm_release_global_lock</tt> 2736APIs provide access to the global lock used to implement the double-checked 2737locking for lazy initialization. These should only be used internally to LLVM, 2738and only if you know what you're doing! 2739</p> 2740</div> 2741 2742<!-- ======================================================================= --> 2743<h3> 2744 <a name="llvmcontext">Achieving Isolation with <tt>LLVMContext</tt></a> 2745</h3> 2746 2747<div> 2748<p> 2749<tt>LLVMContext</tt> is an opaque class in the LLVM API which clients can use 2750to operate multiple, isolated instances of LLVM concurrently within the same 2751address space. For instance, in a hypothetical compile-server, the compilation 2752of an individual translation unit is conceptually independent from all the 2753others, and it would be desirable to be able to compile incoming translation 2754units concurrently on independent server threads. Fortunately, 2755<tt>LLVMContext</tt> exists to enable just this kind of scenario! 2756</p> 2757 2758<p> 2759Conceptually, <tt>LLVMContext</tt> provides isolation. Every LLVM entity 2760(<tt>Module</tt>s, <tt>Value</tt>s, <tt>Type</tt>s, <tt>Constant</tt>s, etc.) 2761in LLVM's in-memory IR belongs to an <tt>LLVMContext</tt>. Entities in 2762different contexts <em>cannot</em> interact with each other: <tt>Module</tt>s in 2763different contexts cannot be linked together, <tt>Function</tt>s cannot be added 2764to <tt>Module</tt>s in different contexts, etc. What this means is that is is 2765safe to compile on multiple threads simultaneously, as long as no two threads 2766operate on entities within the same context. 2767</p> 2768 2769<p> 2770In practice, very few places in the API require the explicit specification of a 2771<tt>LLVMContext</tt>, other than the <tt>Type</tt> creation/lookup APIs. 2772Because every <tt>Type</tt> carries a reference to its owning context, most 2773other entities can determine what context they belong to by looking at their 2774own <tt>Type</tt>. If you are adding new entities to LLVM IR, please try to 2775maintain this interface design. 2776</p> 2777 2778<p> 2779For clients that do <em>not</em> require the benefits of isolation, LLVM 2780provides a convenience API <tt>getGlobalContext()</tt>. This returns a global, 2781lazily initialized <tt>LLVMContext</tt> that may be used in situations where 2782isolation is not a concern. 2783</p> 2784</div> 2785 2786<!-- ======================================================================= --> 2787<h3> 2788 <a name="jitthreading">Threads and the JIT</a> 2789</h3> 2790 2791<div> 2792<p> 2793LLVM's "eager" JIT compiler is safe to use in threaded programs. Multiple 2794threads can call <tt>ExecutionEngine::getPointerToFunction()</tt> or 2795<tt>ExecutionEngine::runFunction()</tt> concurrently, and multiple threads can 2796run code output by the JIT concurrently. The user must still ensure that only 2797one thread accesses IR in a given <tt>LLVMContext</tt> while another thread 2798might be modifying it. One way to do that is to always hold the JIT lock while 2799accessing IR outside the JIT (the JIT <em>modifies</em> the IR by adding 2800<tt>CallbackVH</tt>s). Another way is to only 2801call <tt>getPointerToFunction()</tt> from the <tt>LLVMContext</tt>'s thread. 2802</p> 2803 2804<p>When the JIT is configured to compile lazily (using 2805<tt>ExecutionEngine::DisableLazyCompilation(false)</tt>), there is currently a 2806<a href="http://llvm.org/bugs/show_bug.cgi?id=5184">race condition</a> in 2807updating call sites after a function is lazily-jitted. It's still possible to 2808use the lazy JIT in a threaded program if you ensure that only one thread at a 2809time can call any particular lazy stub and that the JIT lock guards any IR 2810access, but we suggest using only the eager JIT in threaded programs. 2811</p> 2812</div> 2813 2814</div> 2815 2816<!-- *********************************************************************** --> 2817<h2> 2818 <a name="advanced">Advanced Topics</a> 2819</h2> 2820<!-- *********************************************************************** --> 2821 2822<div> 2823<p> 2824This section describes some of the advanced or obscure API's that most clients 2825do not need to be aware of. These API's tend manage the inner workings of the 2826LLVM system, and only need to be accessed in unusual circumstances. 2827</p> 2828 2829 2830<!-- ======================================================================= --> 2831<h3> 2832 <a name="SymbolTable">The <tt>ValueSymbolTable</tt> class</a> 2833</h3> 2834 2835<div> 2836<p>The <tt><a href="http://llvm.org/doxygen/classllvm_1_1ValueSymbolTable.html"> 2837ValueSymbolTable</a></tt> class provides a symbol table that the <a 2838href="#Function"><tt>Function</tt></a> and <a href="#Module"> 2839<tt>Module</tt></a> classes use for naming value definitions. The symbol table 2840can provide a name for any <a href="#Value"><tt>Value</tt></a>. 2841</p> 2842 2843<p>Note that the <tt>SymbolTable</tt> class should not be directly accessed 2844by most clients. It should only be used when iteration over the symbol table 2845names themselves are required, which is very special purpose. Note that not 2846all LLVM 2847<tt><a href="#Value">Value</a></tt>s have names, and those without names (i.e. they have 2848an empty name) do not exist in the symbol table. 2849</p> 2850 2851<p>Symbol tables support iteration over the values in the symbol 2852table with <tt>begin/end/iterator</tt> and supports querying to see if a 2853specific name is in the symbol table (with <tt>lookup</tt>). The 2854<tt>ValueSymbolTable</tt> class exposes no public mutator methods, instead, 2855simply call <tt>setName</tt> on a value, which will autoinsert it into the 2856appropriate symbol table.</p> 2857 2858</div> 2859 2860 2861 2862<!-- ======================================================================= --> 2863<h3> 2864 <a name="UserLayout">The <tt>User</tt> and owned <tt>Use</tt> classes' memory layout</a> 2865</h3> 2866 2867<div> 2868<p>The <tt><a href="http://llvm.org/doxygen/classllvm_1_1User.html"> 2869User</a></tt> class provides a basis for expressing the ownership of <tt>User</tt> 2870towards other <tt><a href="http://llvm.org/doxygen/classllvm_1_1Value.html"> 2871Value</a></tt>s. The <tt><a href="http://llvm.org/doxygen/classllvm_1_1Use.html"> 2872Use</a></tt> helper class is employed to do the bookkeeping and to facilitate <i>O(1)</i> 2873addition and removal.</p> 2874 2875<!-- ______________________________________________________________________ --> 2876<h4> 2877 <a name="Use2User"> 2878 Interaction and relationship between <tt>User</tt> and <tt>Use</tt> objects 2879 </a> 2880</h4> 2881 2882<div> 2883<p> 2884A subclass of <tt>User</tt> can choose between incorporating its <tt>Use</tt> objects 2885or refer to them out-of-line by means of a pointer. A mixed variant 2886(some <tt>Use</tt>s inline others hung off) is impractical and breaks the invariant 2887that the <tt>Use</tt> objects belonging to the same <tt>User</tt> form a contiguous array. 2888</p> 2889 2890<p> 2891We have 2 different layouts in the <tt>User</tt> (sub)classes: 2892<ul> 2893<li><p>Layout a) 2894The <tt>Use</tt> object(s) are inside (resp. at fixed offset) of the <tt>User</tt> 2895object and there are a fixed number of them.</p> 2896 2897<li><p>Layout b) 2898The <tt>Use</tt> object(s) are referenced by a pointer to an 2899array from the <tt>User</tt> object and there may be a variable 2900number of them.</p> 2901</ul> 2902<p> 2903As of v2.4 each layout still possesses a direct pointer to the 2904start of the array of <tt>Use</tt>s. Though not mandatory for layout a), 2905we stick to this redundancy for the sake of simplicity. 2906The <tt>User</tt> object also stores the number of <tt>Use</tt> objects it 2907has. (Theoretically this information can also be calculated 2908given the scheme presented below.)</p> 2909<p> 2910Special forms of allocation operators (<tt>operator new</tt>) 2911enforce the following memory layouts:</p> 2912 2913<ul> 2914<li><p>Layout a) is modelled by prepending the <tt>User</tt> object by the <tt>Use[]</tt> array.</p> 2915 2916<pre> 2917...---.---.---.---.-------... 2918 | P | P | P | P | User 2919'''---'---'---'---'-------''' 2920</pre> 2921 2922<li><p>Layout b) is modelled by pointing at the <tt>Use[]</tt> array.</p> 2923<pre> 2924.-------... 2925| User 2926'-------''' 2927 | 2928 v 2929 .---.---.---.---... 2930 | P | P | P | P | 2931 '---'---'---'---''' 2932</pre> 2933</ul> 2934<i>(In the above figures '<tt>P</tt>' stands for the <tt>Use**</tt> that 2935 is stored in each <tt>Use</tt> object in the member <tt>Use::Prev</tt>)</i> 2936 2937</div> 2938 2939<!-- ______________________________________________________________________ --> 2940<h4> 2941 <a name="Waymarking">The waymarking algorithm</a> 2942</h4> 2943 2944<div> 2945<p> 2946Since the <tt>Use</tt> objects are deprived of the direct (back)pointer to 2947their <tt>User</tt> objects, there must be a fast and exact method to 2948recover it. This is accomplished by the following scheme:</p> 2949 2950A bit-encoding in the 2 LSBits (least significant bits) of the <tt>Use::Prev</tt> allows to find the 2951start of the <tt>User</tt> object: 2952<ul> 2953<li><tt>00</tt> —> binary digit 0</li> 2954<li><tt>01</tt> —> binary digit 1</li> 2955<li><tt>10</tt> —> stop and calculate (<tt>s</tt>)</li> 2956<li><tt>11</tt> —> full stop (<tt>S</tt>)</li> 2957</ul> 2958<p> 2959Given a <tt>Use*</tt>, all we have to do is to walk till we get 2960a stop and we either have a <tt>User</tt> immediately behind or 2961we have to walk to the next stop picking up digits 2962and calculating the offset:</p> 2963<pre> 2964.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---------------- 2965| 1 | s | 1 | 0 | 1 | 0 | s | 1 | 1 | 0 | s | 1 | 1 | s | 1 | S | User (or User*) 2966'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---------------- 2967 |+15 |+10 |+6 |+3 |+1 2968 | | | | |__> 2969 | | | |__________> 2970 | | |______________________> 2971 | |______________________________________> 2972 |__________________________________________________________> 2973</pre> 2974<p> 2975Only the significant number of bits need to be stored between the 2976stops, so that the <i>worst case is 20 memory accesses</i> when there are 29771000 <tt>Use</tt> objects associated with a <tt>User</tt>.</p> 2978 2979</div> 2980 2981<!-- ______________________________________________________________________ --> 2982<h4> 2983 <a name="ReferenceImpl">Reference implementation</a> 2984</h4> 2985 2986<div> 2987<p> 2988The following literate Haskell fragment demonstrates the concept:</p> 2989 2990<div class="doc_code"> 2991<pre> 2992> import Test.QuickCheck 2993> 2994> digits :: Int -> [Char] -> [Char] 2995> digits 0 acc = '0' : acc 2996> digits 1 acc = '1' : acc 2997> digits n acc = digits (n `div` 2) $ digits (n `mod` 2) acc 2998> 2999> dist :: Int -> [Char] -> [Char] 3000> dist 0 [] = ['S'] 3001> dist 0 acc = acc 3002> dist 1 acc = let r = dist 0 acc in 's' : digits (length r) r 3003> dist n acc = dist (n - 1) $ dist 1 acc 3004> 3005> takeLast n ss = reverse $ take n $ reverse ss 3006> 3007> test = takeLast 40 $ dist 20 [] 3008> 3009</pre> 3010</div> 3011<p> 3012Printing <test> gives: <tt>"1s100000s11010s10100s1111s1010s110s11s1S"</tt></p> 3013<p> 3014The reverse algorithm computes the length of the string just by examining 3015a certain prefix:</p> 3016 3017<div class="doc_code"> 3018<pre> 3019> pref :: [Char] -> Int 3020> pref "S" = 1 3021> pref ('s':'1':rest) = decode 2 1 rest 3022> pref (_:rest) = 1 + pref rest 3023> 3024> decode walk acc ('0':rest) = decode (walk + 1) (acc * 2) rest 3025> decode walk acc ('1':rest) = decode (walk + 1) (acc * 2 + 1) rest 3026> decode walk acc _ = walk + acc 3027> 3028</pre> 3029</div> 3030<p> 3031Now, as expected, printing <pref test> gives <tt>40</tt>.</p> 3032<p> 3033We can <i>quickCheck</i> this with following property:</p> 3034 3035<div class="doc_code"> 3036<pre> 3037> testcase = dist 2000 [] 3038> testcaseLength = length testcase 3039> 3040> identityProp n = n > 0 && n <= testcaseLength ==> length arr == pref arr 3041> where arr = takeLast n testcase 3042> 3043</pre> 3044</div> 3045<p> 3046As expected <quickCheck identityProp> gives:</p> 3047 3048<pre> 3049*Main> quickCheck identityProp 3050OK, passed 100 tests. 3051</pre> 3052<p> 3053Let's be a bit more exhaustive:</p> 3054 3055<div class="doc_code"> 3056<pre> 3057> 3058> deepCheck p = check (defaultConfig { configMaxTest = 500 }) p 3059> 3060</pre> 3061</div> 3062<p> 3063And here is the result of <deepCheck identityProp>:</p> 3064 3065<pre> 3066*Main> deepCheck identityProp 3067OK, passed 500 tests. 3068</pre> 3069 3070</div> 3071 3072<!-- ______________________________________________________________________ --> 3073<h4> 3074 <a name="Tagging">Tagging considerations</a> 3075</h4> 3076 3077<div> 3078 3079<p> 3080To maintain the invariant that the 2 LSBits of each <tt>Use**</tt> in <tt>Use</tt> 3081never change after being set up, setters of <tt>Use::Prev</tt> must re-tag the 3082new <tt>Use**</tt> on every modification. Accordingly getters must strip the 3083tag bits.</p> 3084<p> 3085For layout b) instead of the <tt>User</tt> we find a pointer (<tt>User*</tt> with LSBit set). 3086Following this pointer brings us to the <tt>User</tt>. A portable trick ensures 3087that the first bytes of <tt>User</tt> (if interpreted as a pointer) never has 3088the LSBit set. (Portability is relying on the fact that all known compilers place the 3089<tt>vptr</tt> in the first word of the instances.)</p> 3090 3091</div> 3092 3093</div> 3094 3095</div> 3096 3097<!-- *********************************************************************** --> 3098<h2> 3099 <a name="coreclasses">The Core LLVM Class Hierarchy Reference </a> 3100</h2> 3101<!-- *********************************************************************** --> 3102 3103<div> 3104<p><tt>#include "<a href="/doxygen/Type_8h-source.html">llvm/Type.h</a>"</tt> 3105<br>doxygen info: <a href="/doxygen/classllvm_1_1Type.html">Type Class</a></p> 3106 3107<p>The Core LLVM classes are the primary means of representing the program 3108being inspected or transformed. The core LLVM classes are defined in 3109header files in the <tt>include/llvm/</tt> directory, and implemented in 3110the <tt>lib/VMCore</tt> directory.</p> 3111 3112<!-- ======================================================================= --> 3113<h3> 3114 <a name="Type">The <tt>Type</tt> class and Derived Types</a> 3115</h3> 3116 3117<div> 3118 3119 <p><tt>Type</tt> is a superclass of all type classes. Every <tt>Value</tt> has 3120 a <tt>Type</tt>. <tt>Type</tt> cannot be instantiated directly but only 3121 through its subclasses. Certain primitive types (<tt>VoidType</tt>, 3122 <tt>LabelType</tt>, <tt>FloatType</tt> and <tt>DoubleType</tt>) have hidden 3123 subclasses. They are hidden because they offer no useful functionality beyond 3124 what the <tt>Type</tt> class offers except to distinguish themselves from 3125 other subclasses of <tt>Type</tt>.</p> 3126 <p>All other types are subclasses of <tt>DerivedType</tt>. Types can be 3127 named, but this is not a requirement. There exists exactly 3128 one instance of a given shape at any one time. This allows type equality to 3129 be performed with address equality of the Type Instance. That is, given two 3130 <tt>Type*</tt> values, the types are identical if the pointers are identical. 3131 </p> 3132 3133<!-- _______________________________________________________________________ --> 3134<h4> 3135 <a name="m_Type">Important Public Methods</a> 3136</h4> 3137 3138<div> 3139 3140<ul> 3141 <li><tt>bool isIntegerTy() const</tt>: Returns true for any integer type.</li> 3142 3143 <li><tt>bool isFloatingPointTy()</tt>: Return true if this is one of the five 3144 floating point types.</li> 3145 3146 <li><tt>bool isSized()</tt>: Return true if the type has known size. Things 3147 that don't have a size are abstract types, labels and void.</li> 3148 3149</ul> 3150</div> 3151 3152<!-- _______________________________________________________________________ --> 3153<h4> 3154 <a name="derivedtypes">Important Derived Types</a> 3155</h4> 3156<div> 3157<dl> 3158 <dt><tt>IntegerType</tt></dt> 3159 <dd>Subclass of DerivedType that represents integer types of any bit width. 3160 Any bit width between <tt>IntegerType::MIN_INT_BITS</tt> (1) and 3161 <tt>IntegerType::MAX_INT_BITS</tt> (~8 million) can be represented. 3162 <ul> 3163 <li><tt>static const IntegerType* get(unsigned NumBits)</tt>: get an integer 3164 type of a specific bit width.</li> 3165 <li><tt>unsigned getBitWidth() const</tt>: Get the bit width of an integer 3166 type.</li> 3167 </ul> 3168 </dd> 3169 <dt><tt>SequentialType</tt></dt> 3170 <dd>This is subclassed by ArrayType, PointerType and VectorType. 3171 <ul> 3172 <li><tt>const Type * getElementType() const</tt>: Returns the type of each 3173 of the elements in the sequential type. </li> 3174 </ul> 3175 </dd> 3176 <dt><tt>ArrayType</tt></dt> 3177 <dd>This is a subclass of SequentialType and defines the interface for array 3178 types. 3179 <ul> 3180 <li><tt>unsigned getNumElements() const</tt>: Returns the number of 3181 elements in the array. </li> 3182 </ul> 3183 </dd> 3184 <dt><tt>PointerType</tt></dt> 3185 <dd>Subclass of SequentialType for pointer types.</dd> 3186 <dt><tt>VectorType</tt></dt> 3187 <dd>Subclass of SequentialType for vector types. A 3188 vector type is similar to an ArrayType but is distinguished because it is 3189 a first class type whereas ArrayType is not. Vector types are used for 3190 vector operations and are usually small vectors of of an integer or floating 3191 point type.</dd> 3192 <dt><tt>StructType</tt></dt> 3193 <dd>Subclass of DerivedTypes for struct types.</dd> 3194 <dt><tt><a name="FunctionType">FunctionType</a></tt></dt> 3195 <dd>Subclass of DerivedTypes for function types. 3196 <ul> 3197 <li><tt>bool isVarArg() const</tt>: Returns true if it's a vararg 3198 function</li> 3199 <li><tt> const Type * getReturnType() const</tt>: Returns the 3200 return type of the function.</li> 3201 <li><tt>const Type * getParamType (unsigned i)</tt>: Returns 3202 the type of the ith parameter.</li> 3203 <li><tt> const unsigned getNumParams() const</tt>: Returns the 3204 number of formal parameters.</li> 3205 </ul> 3206 </dd> 3207</dl> 3208</div> 3209 3210</div> 3211 3212<!-- ======================================================================= --> 3213<h3> 3214 <a name="Module">The <tt>Module</tt> class</a> 3215</h3> 3216 3217<div> 3218 3219<p><tt>#include "<a 3220href="/doxygen/Module_8h-source.html">llvm/Module.h</a>"</tt><br> doxygen info: 3221<a href="/doxygen/classllvm_1_1Module.html">Module Class</a></p> 3222 3223<p>The <tt>Module</tt> class represents the top level structure present in LLVM 3224programs. An LLVM module is effectively either a translation unit of the 3225original program or a combination of several translation units merged by the 3226linker. The <tt>Module</tt> class keeps track of a list of <a 3227href="#Function"><tt>Function</tt></a>s, a list of <a 3228href="#GlobalVariable"><tt>GlobalVariable</tt></a>s, and a <a 3229href="#SymbolTable"><tt>SymbolTable</tt></a>. Additionally, it contains a few 3230helpful member functions that try to make common operations easy.</p> 3231 3232<!-- _______________________________________________________________________ --> 3233<h4> 3234 <a name="m_Module">Important Public Members of the <tt>Module</tt> class</a> 3235</h4> 3236 3237<div> 3238 3239<ul> 3240 <li><tt>Module::Module(std::string name = "")</tt></li> 3241</ul> 3242 3243<p>Constructing a <a href="#Module">Module</a> is easy. You can optionally 3244provide a name for it (probably based on the name of the translation unit).</p> 3245 3246<ul> 3247 <li><tt>Module::iterator</tt> - Typedef for function list iterator<br> 3248 <tt>Module::const_iterator</tt> - Typedef for const_iterator.<br> 3249 3250 <tt>begin()</tt>, <tt>end()</tt> 3251 <tt>size()</tt>, <tt>empty()</tt> 3252 3253 <p>These are forwarding methods that make it easy to access the contents of 3254 a <tt>Module</tt> object's <a href="#Function"><tt>Function</tt></a> 3255 list.</p></li> 3256 3257 <li><tt>Module::FunctionListType &getFunctionList()</tt> 3258 3259 <p> Returns the list of <a href="#Function"><tt>Function</tt></a>s. This is 3260 necessary to use when you need to update the list or perform a complex 3261 action that doesn't have a forwarding method.</p> 3262 3263 <p><!-- Global Variable --></p></li> 3264</ul> 3265 3266<hr> 3267 3268<ul> 3269 <li><tt>Module::global_iterator</tt> - Typedef for global variable list iterator<br> 3270 3271 <tt>Module::const_global_iterator</tt> - Typedef for const_iterator.<br> 3272 3273 <tt>global_begin()</tt>, <tt>global_end()</tt> 3274 <tt>global_size()</tt>, <tt>global_empty()</tt> 3275 3276 <p> These are forwarding methods that make it easy to access the contents of 3277 a <tt>Module</tt> object's <a 3278 href="#GlobalVariable"><tt>GlobalVariable</tt></a> list.</p></li> 3279 3280 <li><tt>Module::GlobalListType &getGlobalList()</tt> 3281 3282 <p>Returns the list of <a 3283 href="#GlobalVariable"><tt>GlobalVariable</tt></a>s. This is necessary to 3284 use when you need to update the list or perform a complex action that 3285 doesn't have a forwarding method.</p> 3286 3287 <p><!-- Symbol table stuff --> </p></li> 3288</ul> 3289 3290<hr> 3291 3292<ul> 3293 <li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt> 3294 3295 <p>Return a reference to the <a href="#SymbolTable"><tt>SymbolTable</tt></a> 3296 for this <tt>Module</tt>.</p> 3297 3298 <p><!-- Convenience methods --></p></li> 3299</ul> 3300 3301<hr> 3302 3303<ul> 3304 <li><tt><a href="#Function">Function</a> *getFunction(const std::string 3305 &Name, const <a href="#FunctionType">FunctionType</a> *Ty)</tt> 3306 3307 <p>Look up the specified function in the <tt>Module</tt> <a 3308 href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, return 3309 <tt>null</tt>.</p></li> 3310 3311 <li><tt><a href="#Function">Function</a> *getOrInsertFunction(const 3312 std::string &Name, const <a href="#FunctionType">FunctionType</a> *T)</tt> 3313 3314 <p>Look up the specified function in the <tt>Module</tt> <a 3315 href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, add an 3316 external declaration for the function and return it.</p></li> 3317 3318 <li><tt>std::string getTypeName(const <a href="#Type">Type</a> *Ty)</tt> 3319 3320 <p>If there is at least one entry in the <a 3321 href="#SymbolTable"><tt>SymbolTable</tt></a> for the specified <a 3322 href="#Type"><tt>Type</tt></a>, return it. Otherwise return the empty 3323 string.</p></li> 3324 3325 <li><tt>bool addTypeName(const std::string &Name, const <a 3326 href="#Type">Type</a> *Ty)</tt> 3327 3328 <p>Insert an entry in the <a href="#SymbolTable"><tt>SymbolTable</tt></a> 3329 mapping <tt>Name</tt> to <tt>Ty</tt>. If there is already an entry for this 3330 name, true is returned and the <a 3331 href="#SymbolTable"><tt>SymbolTable</tt></a> is not modified.</p></li> 3332</ul> 3333 3334</div> 3335 3336</div> 3337 3338<!-- ======================================================================= --> 3339<h3> 3340 <a name="Value">The <tt>Value</tt> class</a> 3341</h3> 3342 3343<div> 3344 3345<p><tt>#include "<a href="/doxygen/Value_8h-source.html">llvm/Value.h</a>"</tt> 3346<br> 3347doxygen info: <a href="/doxygen/classllvm_1_1Value.html">Value Class</a></p> 3348 3349<p>The <tt>Value</tt> class is the most important class in the LLVM Source 3350base. It represents a typed value that may be used (among other things) as an 3351operand to an instruction. There are many different types of <tt>Value</tt>s, 3352such as <a href="#Constant"><tt>Constant</tt></a>s,<a 3353href="#Argument"><tt>Argument</tt></a>s. Even <a 3354href="#Instruction"><tt>Instruction</tt></a>s and <a 3355href="#Function"><tt>Function</tt></a>s are <tt>Value</tt>s.</p> 3356 3357<p>A particular <tt>Value</tt> may be used many times in the LLVM representation 3358for a program. For example, an incoming argument to a function (represented 3359with an instance of the <a href="#Argument">Argument</a> class) is "used" by 3360every instruction in the function that references the argument. To keep track 3361of this relationship, the <tt>Value</tt> class keeps a list of all of the <a 3362href="#User"><tt>User</tt></a>s that is using it (the <a 3363href="#User"><tt>User</tt></a> class is a base class for all nodes in the LLVM 3364graph that can refer to <tt>Value</tt>s). This use list is how LLVM represents 3365def-use information in the program, and is accessible through the <tt>use_</tt>* 3366methods, shown below.</p> 3367 3368<p>Because LLVM is a typed representation, every LLVM <tt>Value</tt> is typed, 3369and this <a href="#Type">Type</a> is available through the <tt>getType()</tt> 3370method. In addition, all LLVM values can be named. The "name" of the 3371<tt>Value</tt> is a symbolic string printed in the LLVM code:</p> 3372 3373<div class="doc_code"> 3374<pre> 3375%<b>foo</b> = add i32 1, 2 3376</pre> 3377</div> 3378 3379<p><a name="nameWarning">The name of this instruction is "foo".</a> <b>NOTE</b> 3380that the name of any value may be missing (an empty string), so names should 3381<b>ONLY</b> be used for debugging (making the source code easier to read, 3382debugging printouts), they should not be used to keep track of values or map 3383between them. For this purpose, use a <tt>std::map</tt> of pointers to the 3384<tt>Value</tt> itself instead.</p> 3385 3386<p>One important aspect of LLVM is that there is no distinction between an SSA 3387variable and the operation that produces it. Because of this, any reference to 3388the value produced by an instruction (or the value available as an incoming 3389argument, for example) is represented as a direct pointer to the instance of 3390the class that 3391represents this value. Although this may take some getting used to, it 3392simplifies the representation and makes it easier to manipulate.</p> 3393 3394<!-- _______________________________________________________________________ --> 3395<h4> 3396 <a name="m_Value">Important Public Members of the <tt>Value</tt> class</a> 3397</h4> 3398 3399<div> 3400 3401<ul> 3402 <li><tt>Value::use_iterator</tt> - Typedef for iterator over the 3403use-list<br> 3404 <tt>Value::const_use_iterator</tt> - Typedef for const_iterator over 3405the use-list<br> 3406 <tt>unsigned use_size()</tt> - Returns the number of users of the 3407value.<br> 3408 <tt>bool use_empty()</tt> - Returns true if there are no users.<br> 3409 <tt>use_iterator use_begin()</tt> - Get an iterator to the start of 3410the use-list.<br> 3411 <tt>use_iterator use_end()</tt> - Get an iterator to the end of the 3412use-list.<br> 3413 <tt><a href="#User">User</a> *use_back()</tt> - Returns the last 3414element in the list. 3415 <p> These methods are the interface to access the def-use 3416information in LLVM. As with all other iterators in LLVM, the naming 3417conventions follow the conventions defined by the <a href="#stl">STL</a>.</p> 3418 </li> 3419 <li><tt><a href="#Type">Type</a> *getType() const</tt> 3420 <p>This method returns the Type of the Value.</p> 3421 </li> 3422 <li><tt>bool hasName() const</tt><br> 3423 <tt>std::string getName() const</tt><br> 3424 <tt>void setName(const std::string &Name)</tt> 3425 <p> This family of methods is used to access and assign a name to a <tt>Value</tt>, 3426be aware of the <a href="#nameWarning">precaution above</a>.</p> 3427 </li> 3428 <li><tt>void replaceAllUsesWith(Value *V)</tt> 3429 3430 <p>This method traverses the use list of a <tt>Value</tt> changing all <a 3431 href="#User"><tt>User</tt>s</a> of the current value to refer to 3432 "<tt>V</tt>" instead. For example, if you detect that an instruction always 3433 produces a constant value (for example through constant folding), you can 3434 replace all uses of the instruction with the constant like this:</p> 3435 3436<div class="doc_code"> 3437<pre> 3438Inst->replaceAllUsesWith(ConstVal); 3439</pre> 3440</div> 3441 3442</ul> 3443 3444</div> 3445 3446</div> 3447 3448<!-- ======================================================================= --> 3449<h3> 3450 <a name="User">The <tt>User</tt> class</a> 3451</h3> 3452 3453<div> 3454 3455<p> 3456<tt>#include "<a href="/doxygen/User_8h-source.html">llvm/User.h</a>"</tt><br> 3457doxygen info: <a href="/doxygen/classllvm_1_1User.html">User Class</a><br> 3458Superclass: <a href="#Value"><tt>Value</tt></a></p> 3459 3460<p>The <tt>User</tt> class is the common base class of all LLVM nodes that may 3461refer to <a href="#Value"><tt>Value</tt></a>s. It exposes a list of "Operands" 3462that are all of the <a href="#Value"><tt>Value</tt></a>s that the User is 3463referring to. The <tt>User</tt> class itself is a subclass of 3464<tt>Value</tt>.</p> 3465 3466<p>The operands of a <tt>User</tt> point directly to the LLVM <a 3467href="#Value"><tt>Value</tt></a> that it refers to. Because LLVM uses Static 3468Single Assignment (SSA) form, there can only be one definition referred to, 3469allowing this direct connection. This connection provides the use-def 3470information in LLVM.</p> 3471 3472<!-- _______________________________________________________________________ --> 3473<h4> 3474 <a name="m_User">Important Public Members of the <tt>User</tt> class</a> 3475</h4> 3476 3477<div> 3478 3479<p>The <tt>User</tt> class exposes the operand list in two ways: through 3480an index access interface and through an iterator based interface.</p> 3481 3482<ul> 3483 <li><tt>Value *getOperand(unsigned i)</tt><br> 3484 <tt>unsigned getNumOperands()</tt> 3485 <p> These two methods expose the operands of the <tt>User</tt> in a 3486convenient form for direct access.</p></li> 3487 3488 <li><tt>User::op_iterator</tt> - Typedef for iterator over the operand 3489list<br> 3490 <tt>op_iterator op_begin()</tt> - Get an iterator to the start of 3491the operand list.<br> 3492 <tt>op_iterator op_end()</tt> - Get an iterator to the end of the 3493operand list. 3494 <p> Together, these methods make up the iterator based interface to 3495the operands of a <tt>User</tt>.</p></li> 3496</ul> 3497 3498</div> 3499 3500</div> 3501 3502<!-- ======================================================================= --> 3503<h3> 3504 <a name="Instruction">The <tt>Instruction</tt> class</a> 3505</h3> 3506 3507<div> 3508 3509<p><tt>#include "</tt><tt><a 3510href="/doxygen/Instruction_8h-source.html">llvm/Instruction.h</a>"</tt><br> 3511doxygen info: <a href="/doxygen/classllvm_1_1Instruction.html">Instruction Class</a><br> 3512Superclasses: <a href="#User"><tt>User</tt></a>, <a 3513href="#Value"><tt>Value</tt></a></p> 3514 3515<p>The <tt>Instruction</tt> class is the common base class for all LLVM 3516instructions. It provides only a few methods, but is a very commonly used 3517class. The primary data tracked by the <tt>Instruction</tt> class itself is the 3518opcode (instruction type) and the parent <a 3519href="#BasicBlock"><tt>BasicBlock</tt></a> the <tt>Instruction</tt> is embedded 3520into. To represent a specific type of instruction, one of many subclasses of 3521<tt>Instruction</tt> are used.</p> 3522 3523<p> Because the <tt>Instruction</tt> class subclasses the <a 3524href="#User"><tt>User</tt></a> class, its operands can be accessed in the same 3525way as for other <a href="#User"><tt>User</tt></a>s (with the 3526<tt>getOperand()</tt>/<tt>getNumOperands()</tt> and 3527<tt>op_begin()</tt>/<tt>op_end()</tt> methods).</p> <p> An important file for 3528the <tt>Instruction</tt> class is the <tt>llvm/Instruction.def</tt> file. This 3529file contains some meta-data about the various different types of instructions 3530in LLVM. It describes the enum values that are used as opcodes (for example 3531<tt>Instruction::Add</tt> and <tt>Instruction::ICmp</tt>), as well as the 3532concrete sub-classes of <tt>Instruction</tt> that implement the instruction (for 3533example <tt><a href="#BinaryOperator">BinaryOperator</a></tt> and <tt><a 3534href="#CmpInst">CmpInst</a></tt>). Unfortunately, the use of macros in 3535this file confuses doxygen, so these enum values don't show up correctly in the 3536<a href="/doxygen/classllvm_1_1Instruction.html">doxygen output</a>.</p> 3537 3538<!-- _______________________________________________________________________ --> 3539<h4> 3540 <a name="s_Instruction"> 3541 Important Subclasses of the <tt>Instruction</tt> class 3542 </a> 3543</h4> 3544<div> 3545 <ul> 3546 <li><tt><a name="BinaryOperator">BinaryOperator</a></tt> 3547 <p>This subclasses represents all two operand instructions whose operands 3548 must be the same type, except for the comparison instructions.</p></li> 3549 <li><tt><a name="CastInst">CastInst</a></tt> 3550 <p>This subclass is the parent of the 12 casting instructions. It provides 3551 common operations on cast instructions.</p> 3552 <li><tt><a name="CmpInst">CmpInst</a></tt> 3553 <p>This subclass respresents the two comparison instructions, 3554 <a href="LangRef.html#i_icmp">ICmpInst</a> (integer opreands), and 3555 <a href="LangRef.html#i_fcmp">FCmpInst</a> (floating point operands).</p> 3556 <li><tt><a name="TerminatorInst">TerminatorInst</a></tt> 3557 <p>This subclass is the parent of all terminator instructions (those which 3558 can terminate a block).</p> 3559 </ul> 3560 </div> 3561 3562<!-- _______________________________________________________________________ --> 3563<h4> 3564 <a name="m_Instruction"> 3565 Important Public Members of the <tt>Instruction</tt> class 3566 </a> 3567</h4> 3568 3569<div> 3570 3571<ul> 3572 <li><tt><a href="#BasicBlock">BasicBlock</a> *getParent()</tt> 3573 <p>Returns the <a href="#BasicBlock"><tt>BasicBlock</tt></a> that 3574this <tt>Instruction</tt> is embedded into.</p></li> 3575 <li><tt>bool mayWriteToMemory()</tt> 3576 <p>Returns true if the instruction writes to memory, i.e. it is a 3577 <tt>call</tt>,<tt>free</tt>,<tt>invoke</tt>, or <tt>store</tt>.</p></li> 3578 <li><tt>unsigned getOpcode()</tt> 3579 <p>Returns the opcode for the <tt>Instruction</tt>.</p></li> 3580 <li><tt><a href="#Instruction">Instruction</a> *clone() const</tt> 3581 <p>Returns another instance of the specified instruction, identical 3582in all ways to the original except that the instruction has no parent 3583(ie it's not embedded into a <a href="#BasicBlock"><tt>BasicBlock</tt></a>), 3584and it has no name</p></li> 3585</ul> 3586 3587</div> 3588 3589</div> 3590 3591<!-- ======================================================================= --> 3592<h3> 3593 <a name="Constant">The <tt>Constant</tt> class and subclasses</a> 3594</h3> 3595 3596<div> 3597 3598<p>Constant represents a base class for different types of constants. It 3599is subclassed by ConstantInt, ConstantArray, etc. for representing 3600the various types of Constants. <a href="#GlobalValue">GlobalValue</a> is also 3601a subclass, which represents the address of a global variable or function. 3602</p> 3603 3604<!-- _______________________________________________________________________ --> 3605<h4>Important Subclasses of Constant</h4> 3606<div> 3607<ul> 3608 <li>ConstantInt : This subclass of Constant represents an integer constant of 3609 any width. 3610 <ul> 3611 <li><tt>const APInt& getValue() const</tt>: Returns the underlying 3612 value of this constant, an APInt value.</li> 3613 <li><tt>int64_t getSExtValue() const</tt>: Converts the underlying APInt 3614 value to an int64_t via sign extension. If the value (not the bit width) 3615 of the APInt is too large to fit in an int64_t, an assertion will result. 3616 For this reason, use of this method is discouraged.</li> 3617 <li><tt>uint64_t getZExtValue() const</tt>: Converts the underlying APInt 3618 value to a uint64_t via zero extension. IF the value (not the bit width) 3619 of the APInt is too large to fit in a uint64_t, an assertion will result. 3620 For this reason, use of this method is discouraged.</li> 3621 <li><tt>static ConstantInt* get(const APInt& Val)</tt>: Returns the 3622 ConstantInt object that represents the value provided by <tt>Val</tt>. 3623 The type is implied as the IntegerType that corresponds to the bit width 3624 of <tt>Val</tt>.</li> 3625 <li><tt>static ConstantInt* get(const Type *Ty, uint64_t Val)</tt>: 3626 Returns the ConstantInt object that represents the value provided by 3627 <tt>Val</tt> for integer type <tt>Ty</tt>.</li> 3628 </ul> 3629 </li> 3630 <li>ConstantFP : This class represents a floating point constant. 3631 <ul> 3632 <li><tt>double getValue() const</tt>: Returns the underlying value of 3633 this constant. </li> 3634 </ul> 3635 </li> 3636 <li>ConstantArray : This represents a constant array. 3637 <ul> 3638 <li><tt>const std::vector<Use> &getValues() const</tt>: Returns 3639 a vector of component constants that makeup this array. </li> 3640 </ul> 3641 </li> 3642 <li>ConstantStruct : This represents a constant struct. 3643 <ul> 3644 <li><tt>const std::vector<Use> &getValues() const</tt>: Returns 3645 a vector of component constants that makeup this array. </li> 3646 </ul> 3647 </li> 3648 <li>GlobalValue : This represents either a global variable or a function. In 3649 either case, the value is a constant fixed address (after linking). 3650 </li> 3651</ul> 3652</div> 3653 3654</div> 3655 3656<!-- ======================================================================= --> 3657<h3> 3658 <a name="GlobalValue">The <tt>GlobalValue</tt> class</a> 3659</h3> 3660 3661<div> 3662 3663<p><tt>#include "<a 3664href="/doxygen/GlobalValue_8h-source.html">llvm/GlobalValue.h</a>"</tt><br> 3665doxygen info: <a href="/doxygen/classllvm_1_1GlobalValue.html">GlobalValue 3666Class</a><br> 3667Superclasses: <a href="#Constant"><tt>Constant</tt></a>, 3668<a href="#User"><tt>User</tt></a>, <a href="#Value"><tt>Value</tt></a></p> 3669 3670<p>Global values (<a href="#GlobalVariable"><tt>GlobalVariable</tt></a>s or <a 3671href="#Function"><tt>Function</tt></a>s) are the only LLVM values that are 3672visible in the bodies of all <a href="#Function"><tt>Function</tt></a>s. 3673Because they are visible at global scope, they are also subject to linking with 3674other globals defined in different translation units. To control the linking 3675process, <tt>GlobalValue</tt>s know their linkage rules. Specifically, 3676<tt>GlobalValue</tt>s know whether they have internal or external linkage, as 3677defined by the <tt>LinkageTypes</tt> enumeration.</p> 3678 3679<p>If a <tt>GlobalValue</tt> has internal linkage (equivalent to being 3680<tt>static</tt> in C), it is not visible to code outside the current translation 3681unit, and does not participate in linking. If it has external linkage, it is 3682visible to external code, and does participate in linking. In addition to 3683linkage information, <tt>GlobalValue</tt>s keep track of which <a 3684href="#Module"><tt>Module</tt></a> they are currently part of.</p> 3685 3686<p>Because <tt>GlobalValue</tt>s are memory objects, they are always referred to 3687by their <b>address</b>. As such, the <a href="#Type"><tt>Type</tt></a> of a 3688global is always a pointer to its contents. It is important to remember this 3689when using the <tt>GetElementPtrInst</tt> instruction because this pointer must 3690be dereferenced first. For example, if you have a <tt>GlobalVariable</tt> (a 3691subclass of <tt>GlobalValue)</tt> that is an array of 24 ints, type <tt>[24 x 3692i32]</tt>, then the <tt>GlobalVariable</tt> is a pointer to that array. Although 3693the address of the first element of this array and the value of the 3694<tt>GlobalVariable</tt> are the same, they have different types. The 3695<tt>GlobalVariable</tt>'s type is <tt>[24 x i32]</tt>. The first element's type 3696is <tt>i32.</tt> Because of this, accessing a global value requires you to 3697dereference the pointer with <tt>GetElementPtrInst</tt> first, then its elements 3698can be accessed. This is explained in the <a href="LangRef.html#globalvars">LLVM 3699Language Reference Manual</a>.</p> 3700 3701<!-- _______________________________________________________________________ --> 3702<h4> 3703 <a name="m_GlobalValue"> 3704 Important Public Members of the <tt>GlobalValue</tt> class 3705 </a> 3706</h4> 3707 3708<div> 3709 3710<ul> 3711 <li><tt>bool hasInternalLinkage() const</tt><br> 3712 <tt>bool hasExternalLinkage() const</tt><br> 3713 <tt>void setInternalLinkage(bool HasInternalLinkage)</tt> 3714 <p> These methods manipulate the linkage characteristics of the <tt>GlobalValue</tt>.</p> 3715 <p> </p> 3716 </li> 3717 <li><tt><a href="#Module">Module</a> *getParent()</tt> 3718 <p> This returns the <a href="#Module"><tt>Module</tt></a> that the 3719GlobalValue is currently embedded into.</p></li> 3720</ul> 3721 3722</div> 3723 3724</div> 3725 3726<!-- ======================================================================= --> 3727<h3> 3728 <a name="Function">The <tt>Function</tt> class</a> 3729</h3> 3730 3731<div> 3732 3733<p><tt>#include "<a 3734href="/doxygen/Function_8h-source.html">llvm/Function.h</a>"</tt><br> doxygen 3735info: <a href="/doxygen/classllvm_1_1Function.html">Function Class</a><br> 3736Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>, 3737<a href="#Constant"><tt>Constant</tt></a>, 3738<a href="#User"><tt>User</tt></a>, 3739<a href="#Value"><tt>Value</tt></a></p> 3740 3741<p>The <tt>Function</tt> class represents a single procedure in LLVM. It is 3742actually one of the more complex classes in the LLVM hierarchy because it must 3743keep track of a large amount of data. The <tt>Function</tt> class keeps track 3744of a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, a list of formal 3745<a href="#Argument"><tt>Argument</tt></a>s, and a 3746<a href="#SymbolTable"><tt>SymbolTable</tt></a>.</p> 3747 3748<p>The list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s is the most 3749commonly used part of <tt>Function</tt> objects. The list imposes an implicit 3750ordering of the blocks in the function, which indicate how the code will be 3751laid out by the backend. Additionally, the first <a 3752href="#BasicBlock"><tt>BasicBlock</tt></a> is the implicit entry node for the 3753<tt>Function</tt>. It is not legal in LLVM to explicitly branch to this initial 3754block. There are no implicit exit nodes, and in fact there may be multiple exit 3755nodes from a single <tt>Function</tt>. If the <a 3756href="#BasicBlock"><tt>BasicBlock</tt></a> list is empty, this indicates that 3757the <tt>Function</tt> is actually a function declaration: the actual body of the 3758function hasn't been linked in yet.</p> 3759 3760<p>In addition to a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, the 3761<tt>Function</tt> class also keeps track of the list of formal <a 3762href="#Argument"><tt>Argument</tt></a>s that the function receives. This 3763container manages the lifetime of the <a href="#Argument"><tt>Argument</tt></a> 3764nodes, just like the <a href="#BasicBlock"><tt>BasicBlock</tt></a> list does for 3765the <a href="#BasicBlock"><tt>BasicBlock</tt></a>s.</p> 3766 3767<p>The <a href="#SymbolTable"><tt>SymbolTable</tt></a> is a very rarely used 3768LLVM feature that is only used when you have to look up a value by name. Aside 3769from that, the <a href="#SymbolTable"><tt>SymbolTable</tt></a> is used 3770internally to make sure that there are not conflicts between the names of <a 3771href="#Instruction"><tt>Instruction</tt></a>s, <a 3772href="#BasicBlock"><tt>BasicBlock</tt></a>s, or <a 3773href="#Argument"><tt>Argument</tt></a>s in the function body.</p> 3774 3775<p>Note that <tt>Function</tt> is a <a href="#GlobalValue">GlobalValue</a> 3776and therefore also a <a href="#Constant">Constant</a>. The value of the function 3777is its address (after linking) which is guaranteed to be constant.</p> 3778 3779<!-- _______________________________________________________________________ --> 3780<h4> 3781 <a name="m_Function"> 3782 Important Public Members of the <tt>Function</tt> class 3783 </a> 3784</h4> 3785 3786<div> 3787 3788<ul> 3789 <li><tt>Function(const </tt><tt><a href="#FunctionType">FunctionType</a> 3790 *Ty, LinkageTypes Linkage, const std::string &N = "", Module* Parent = 0)</tt> 3791 3792 <p>Constructor used when you need to create new <tt>Function</tt>s to add 3793 the the program. The constructor must specify the type of the function to 3794 create and what type of linkage the function should have. The <a 3795 href="#FunctionType"><tt>FunctionType</tt></a> argument 3796 specifies the formal arguments and return value for the function. The same 3797 <a href="#FunctionType"><tt>FunctionType</tt></a> value can be used to 3798 create multiple functions. The <tt>Parent</tt> argument specifies the Module 3799 in which the function is defined. If this argument is provided, the function 3800 will automatically be inserted into that module's list of 3801 functions.</p></li> 3802 3803 <li><tt>bool isDeclaration()</tt> 3804 3805 <p>Return whether or not the <tt>Function</tt> has a body defined. If the 3806 function is "external", it does not have a body, and thus must be resolved 3807 by linking with a function defined in a different translation unit.</p></li> 3808 3809 <li><tt>Function::iterator</tt> - Typedef for basic block list iterator<br> 3810 <tt>Function::const_iterator</tt> - Typedef for const_iterator.<br> 3811 3812 <tt>begin()</tt>, <tt>end()</tt> 3813 <tt>size()</tt>, <tt>empty()</tt> 3814 3815 <p>These are forwarding methods that make it easy to access the contents of 3816 a <tt>Function</tt> object's <a href="#BasicBlock"><tt>BasicBlock</tt></a> 3817 list.</p></li> 3818 3819 <li><tt>Function::BasicBlockListType &getBasicBlockList()</tt> 3820 3821 <p>Returns the list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s. This 3822 is necessary to use when you need to update the list or perform a complex 3823 action that doesn't have a forwarding method.</p></li> 3824 3825 <li><tt>Function::arg_iterator</tt> - Typedef for the argument list 3826iterator<br> 3827 <tt>Function::const_arg_iterator</tt> - Typedef for const_iterator.<br> 3828 3829 <tt>arg_begin()</tt>, <tt>arg_end()</tt> 3830 <tt>arg_size()</tt>, <tt>arg_empty()</tt> 3831 3832 <p>These are forwarding methods that make it easy to access the contents of 3833 a <tt>Function</tt> object's <a href="#Argument"><tt>Argument</tt></a> 3834 list.</p></li> 3835 3836 <li><tt>Function::ArgumentListType &getArgumentList()</tt> 3837 3838 <p>Returns the list of <a href="#Argument"><tt>Argument</tt></a>s. This is 3839 necessary to use when you need to update the list or perform a complex 3840 action that doesn't have a forwarding method.</p></li> 3841 3842 <li><tt><a href="#BasicBlock">BasicBlock</a> &getEntryBlock()</tt> 3843 3844 <p>Returns the entry <a href="#BasicBlock"><tt>BasicBlock</tt></a> for the 3845 function. Because the entry block for the function is always the first 3846 block, this returns the first block of the <tt>Function</tt>.</p></li> 3847 3848 <li><tt><a href="#Type">Type</a> *getReturnType()</tt><br> 3849 <tt><a href="#FunctionType">FunctionType</a> *getFunctionType()</tt> 3850 3851 <p>This traverses the <a href="#Type"><tt>Type</tt></a> of the 3852 <tt>Function</tt> and returns the return type of the function, or the <a 3853 href="#FunctionType"><tt>FunctionType</tt></a> of the actual 3854 function.</p></li> 3855 3856 <li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt> 3857 3858 <p> Return a pointer to the <a href="#SymbolTable"><tt>SymbolTable</tt></a> 3859 for this <tt>Function</tt>.</p></li> 3860</ul> 3861 3862</div> 3863 3864</div> 3865 3866<!-- ======================================================================= --> 3867<h3> 3868 <a name="GlobalVariable">The <tt>GlobalVariable</tt> class</a> 3869</h3> 3870 3871<div> 3872 3873<p><tt>#include "<a 3874href="/doxygen/GlobalVariable_8h-source.html">llvm/GlobalVariable.h</a>"</tt> 3875<br> 3876doxygen info: <a href="/doxygen/classllvm_1_1GlobalVariable.html">GlobalVariable 3877 Class</a><br> 3878Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>, 3879<a href="#Constant"><tt>Constant</tt></a>, 3880<a href="#User"><tt>User</tt></a>, 3881<a href="#Value"><tt>Value</tt></a></p> 3882 3883<p>Global variables are represented with the (surprise surprise) 3884<tt>GlobalVariable</tt> class. Like functions, <tt>GlobalVariable</tt>s are also 3885subclasses of <a href="#GlobalValue"><tt>GlobalValue</tt></a>, and as such are 3886always referenced by their address (global values must live in memory, so their 3887"name" refers to their constant address). See 3888<a href="#GlobalValue"><tt>GlobalValue</tt></a> for more on this. Global 3889variables may have an initial value (which must be a 3890<a href="#Constant"><tt>Constant</tt></a>), and if they have an initializer, 3891they may be marked as "constant" themselves (indicating that their contents 3892never change at runtime).</p> 3893 3894<!-- _______________________________________________________________________ --> 3895<h4> 3896 <a name="m_GlobalVariable"> 3897 Important Public Members of the <tt>GlobalVariable</tt> class 3898 </a> 3899</h4> 3900 3901<div> 3902 3903<ul> 3904 <li><tt>GlobalVariable(const </tt><tt><a href="#Type">Type</a> *Ty, bool 3905 isConstant, LinkageTypes& Linkage, <a href="#Constant">Constant</a> 3906 *Initializer = 0, const std::string &Name = "", Module* Parent = 0)</tt> 3907 3908 <p>Create a new global variable of the specified type. If 3909 <tt>isConstant</tt> is true then the global variable will be marked as 3910 unchanging for the program. The Linkage parameter specifies the type of 3911 linkage (internal, external, weak, linkonce, appending) for the variable. 3912 If the linkage is InternalLinkage, WeakAnyLinkage, WeakODRLinkage, 3913 LinkOnceAnyLinkage or LinkOnceODRLinkage, then the resultant 3914 global variable will have internal linkage. AppendingLinkage concatenates 3915 together all instances (in different translation units) of the variable 3916 into a single variable but is only applicable to arrays. See 3917 the <a href="LangRef.html#modulestructure">LLVM Language Reference</a> for 3918 further details on linkage types. Optionally an initializer, a name, and the 3919 module to put the variable into may be specified for the global variable as 3920 well.</p></li> 3921 3922 <li><tt>bool isConstant() const</tt> 3923 3924 <p>Returns true if this is a global variable that is known not to 3925 be modified at runtime.</p></li> 3926 3927 <li><tt>bool hasInitializer()</tt> 3928 3929 <p>Returns true if this <tt>GlobalVariable</tt> has an intializer.</p></li> 3930 3931 <li><tt><a href="#Constant">Constant</a> *getInitializer()</tt> 3932 3933 <p>Returns the initial value for a <tt>GlobalVariable</tt>. It is not legal 3934 to call this method if there is no initializer.</p></li> 3935</ul> 3936 3937</div> 3938 3939</div> 3940 3941<!-- ======================================================================= --> 3942<h3> 3943 <a name="BasicBlock">The <tt>BasicBlock</tt> class</a> 3944</h3> 3945 3946<div> 3947 3948<p><tt>#include "<a 3949href="/doxygen/BasicBlock_8h-source.html">llvm/BasicBlock.h</a>"</tt><br> 3950doxygen info: <a href="/doxygen/classllvm_1_1BasicBlock.html">BasicBlock 3951Class</a><br> 3952Superclass: <a href="#Value"><tt>Value</tt></a></p> 3953 3954<p>This class represents a single entry single exit section of the code, 3955commonly known as a basic block by the compiler community. The 3956<tt>BasicBlock</tt> class maintains a list of <a 3957href="#Instruction"><tt>Instruction</tt></a>s, which form the body of the block. 3958Matching the language definition, the last element of this list of instructions 3959is always a terminator instruction (a subclass of the <a 3960href="#TerminatorInst"><tt>TerminatorInst</tt></a> class).</p> 3961 3962<p>In addition to tracking the list of instructions that make up the block, the 3963<tt>BasicBlock</tt> class also keeps track of the <a 3964href="#Function"><tt>Function</tt></a> that it is embedded into.</p> 3965 3966<p>Note that <tt>BasicBlock</tt>s themselves are <a 3967href="#Value"><tt>Value</tt></a>s, because they are referenced by instructions 3968like branches and can go in the switch tables. <tt>BasicBlock</tt>s have type 3969<tt>label</tt>.</p> 3970 3971<!-- _______________________________________________________________________ --> 3972<h4> 3973 <a name="m_BasicBlock"> 3974 Important Public Members of the <tt>BasicBlock</tt> class 3975 </a> 3976</h4> 3977 3978<div> 3979<ul> 3980 3981<li><tt>BasicBlock(const std::string &Name = "", </tt><tt><a 3982 href="#Function">Function</a> *Parent = 0)</tt> 3983 3984<p>The <tt>BasicBlock</tt> constructor is used to create new basic blocks for 3985insertion into a function. The constructor optionally takes a name for the new 3986block, and a <a href="#Function"><tt>Function</tt></a> to insert it into. If 3987the <tt>Parent</tt> parameter is specified, the new <tt>BasicBlock</tt> is 3988automatically inserted at the end of the specified <a 3989href="#Function"><tt>Function</tt></a>, if not specified, the BasicBlock must be 3990manually inserted into the <a href="#Function"><tt>Function</tt></a>.</p></li> 3991 3992<li><tt>BasicBlock::iterator</tt> - Typedef for instruction list iterator<br> 3993<tt>BasicBlock::const_iterator</tt> - Typedef for const_iterator.<br> 3994<tt>begin()</tt>, <tt>end()</tt>, <tt>front()</tt>, <tt>back()</tt>, 3995<tt>size()</tt>, <tt>empty()</tt> 3996STL-style functions for accessing the instruction list. 3997 3998<p>These methods and typedefs are forwarding functions that have the same 3999semantics as the standard library methods of the same names. These methods 4000expose the underlying instruction list of a basic block in a way that is easy to 4001manipulate. To get the full complement of container operations (including 4002operations to update the list), you must use the <tt>getInstList()</tt> 4003method.</p></li> 4004 4005<li><tt>BasicBlock::InstListType &getInstList()</tt> 4006 4007<p>This method is used to get access to the underlying container that actually 4008holds the Instructions. This method must be used when there isn't a forwarding 4009function in the <tt>BasicBlock</tt> class for the operation that you would like 4010to perform. Because there are no forwarding functions for "updating" 4011operations, you need to use this if you want to update the contents of a 4012<tt>BasicBlock</tt>.</p></li> 4013 4014<li><tt><a href="#Function">Function</a> *getParent()</tt> 4015 4016<p> Returns a pointer to <a href="#Function"><tt>Function</tt></a> the block is 4017embedded into, or a null pointer if it is homeless.</p></li> 4018 4019<li><tt><a href="#TerminatorInst">TerminatorInst</a> *getTerminator()</tt> 4020 4021<p> Returns a pointer to the terminator instruction that appears at the end of 4022the <tt>BasicBlock</tt>. If there is no terminator instruction, or if the last 4023instruction in the block is not a terminator, then a null pointer is 4024returned.</p></li> 4025 4026</ul> 4027 4028</div> 4029 4030</div> 4031 4032<!-- ======================================================================= --> 4033<h3> 4034 <a name="Argument">The <tt>Argument</tt> class</a> 4035</h3> 4036 4037<div> 4038 4039<p>This subclass of Value defines the interface for incoming formal 4040arguments to a function. A Function maintains a list of its formal 4041arguments. An argument has a pointer to the parent Function.</p> 4042 4043</div> 4044 4045</div> 4046 4047<!-- *********************************************************************** --> 4048<hr> 4049<address> 4050 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 4051 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 4052 <a href="http://validator.w3.org/check/referer"><img 4053 src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01 Strict"></a> 4054 4055 <a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a> and 4056 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br> 4057 <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br> 4058 Last modified: $Date: 2011-10-11 02:33:56 -0400 (Tue, 11 Oct 2011) $ 4059</address> 4060 4061</body> 4062</html> 4063