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>Writing an LLVM Pass</title> 7 <link rel="stylesheet" href="llvm.css" type="text/css"> 8</head> 9<body> 10 11<h1> 12 Writing an LLVM Pass 13</h1> 14 15<ol> 16 <li><a href="#introduction">Introduction - What is a pass?</a></li> 17 <li><a href="#quickstart">Quick Start - Writing hello world</a> 18 <ul> 19 <li><a href="#makefile">Setting up the build environment</a></li> 20 <li><a href="#basiccode">Basic code required</a></li> 21 <li><a href="#running">Running a pass with <tt>opt</tt></a></li> 22 </ul></li> 23 <li><a href="#passtype">Pass classes and requirements</a> 24 <ul> 25 <li><a href="#ImmutablePass">The <tt>ImmutablePass</tt> class</a></li> 26 <li><a href="#ModulePass">The <tt>ModulePass</tt> class</a> 27 <ul> 28 <li><a href="#runOnModule">The <tt>runOnModule</tt> method</a></li> 29 </ul></li> 30 <li><a href="#CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a> 31 <ul> 32 <li><a href="#doInitialization_scc">The <tt>doInitialization(CallGraph 33 &)</tt> method</a></li> 34 <li><a href="#runOnSCC">The <tt>runOnSCC</tt> method</a></li> 35 <li><a href="#doFinalization_scc">The <tt>doFinalization(CallGraph 36 &)</tt> method</a></li> 37 </ul></li> 38 <li><a href="#FunctionPass">The <tt>FunctionPass</tt> class</a> 39 <ul> 40 <li><a href="#doInitialization_mod">The <tt>doInitialization(Module 41 &)</tt> method</a></li> 42 <li><a href="#runOnFunction">The <tt>runOnFunction</tt> method</a></li> 43 <li><a href="#doFinalization_mod">The <tt>doFinalization(Module 44 &)</tt> method</a></li> 45 </ul></li> 46 <li><a href="#LoopPass">The <tt>LoopPass</tt> class</a> 47 <ul> 48 <li><a href="#doInitialization_loop">The <tt>doInitialization(Loop *, 49 LPPassManager &)</tt> method</a></li> 50 <li><a href="#runOnLoop">The <tt>runOnLoop</tt> method</a></li> 51 <li><a href="#doFinalization_loop">The <tt>doFinalization() 52 </tt> method</a></li> 53 </ul></li> 54 <li><a href="#RegionPass">The <tt>RegionPass</tt> class</a> 55 <ul> 56 <li><a href="#doInitialization_region">The <tt>doInitialization(Region *, 57 RGPassManager &)</tt> method</a></li> 58 <li><a href="#runOnRegion">The <tt>runOnRegion</tt> method</a></li> 59 <li><a href="#doFinalization_region">The <tt>doFinalization() 60 </tt> method</a></li> 61 </ul></li> 62 <li><a href="#BasicBlockPass">The <tt>BasicBlockPass</tt> class</a> 63 <ul> 64 <li><a href="#doInitialization_fn">The <tt>doInitialization(Function 65 &)</tt> method</a></li> 66 <li><a href="#runOnBasicBlock">The <tt>runOnBasicBlock</tt> 67 method</a></li> 68 <li><a href="#doFinalization_fn">The <tt>doFinalization(Function 69 &)</tt> method</a></li> 70 </ul></li> 71 <li><a href="#MachineFunctionPass">The <tt>MachineFunctionPass</tt> 72 class</a> 73 <ul> 74 <li><a href="#runOnMachineFunction">The 75 <tt>runOnMachineFunction(MachineFunction &)</tt> method</a></li> 76 </ul></li> 77 </ul> 78 <li><a href="#registration">Pass Registration</a> 79 <ul> 80 <li><a href="#print">The <tt>print</tt> method</a></li> 81 </ul></li> 82 <li><a href="#interaction">Specifying interactions between passes</a> 83 <ul> 84 <li><a href="#getAnalysisUsage">The <tt>getAnalysisUsage</tt> 85 method</a></li> 86 <li><a href="#AU::addRequired">The <tt>AnalysisUsage::addRequired<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods</a></li> 87 <li><a href="#AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a></li> 88 <li><a href="#AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a></li> 89 <li><a href="#getAnalysis">The <tt>getAnalysis<></tt> and 90<tt>getAnalysisIfAvailable<></tt> methods</a></li> 91 </ul></li> 92 <li><a href="#analysisgroup">Implementing Analysis Groups</a> 93 <ul> 94 <li><a href="#agconcepts">Analysis Group Concepts</a></li> 95 <li><a href="#registerag">Using <tt>RegisterAnalysisGroup</tt></a></li> 96 </ul></li> 97 <li><a href="#passStatistics">Pass Statistics</a> 98 <li><a href="#passmanager">What PassManager does</a> 99 <ul> 100 <li><a href="#releaseMemory">The <tt>releaseMemory</tt> method</a></li> 101 </ul></li> 102 <li><a href="#registering">Registering dynamically loaded passes</a> 103 <ul> 104 <li><a href="#registering_existing">Using existing registries</a></li> 105 <li><a href="#registering_new">Creating new registries</a></li> 106 </ul></li> 107 <li><a href="#debughints">Using GDB with dynamically loaded passes</a> 108 <ul> 109 <li><a href="#breakpoint">Setting a breakpoint in your pass</a></li> 110 <li><a href="#debugmisc">Miscellaneous Problems</a></li> 111 </ul></li> 112 <li><a href="#future">Future extensions planned</a> 113 <ul> 114 <li><a href="#SMP">Multithreaded LLVM</a></li> 115 </ul></li> 116</ol> 117 118<div class="doc_author"> 119 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and 120 <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p> 121</div> 122 123<!-- *********************************************************************** --> 124<h2> 125 <a name="introduction">Introduction - What is a pass?</a> 126</h2> 127<!-- *********************************************************************** --> 128 129<div> 130 131<p>The LLVM Pass Framework is an important part of the LLVM system, because LLVM 132passes are where most of the interesting parts of the compiler exist. Passes 133perform the transformations and optimizations that make up the compiler, they 134build the analysis results that are used by these transformations, and they are, 135above all, a structuring technique for compiler code.</p> 136 137<p>All LLVM passes are subclasses of the <tt><a 138href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt> 139class, which implement functionality by overriding virtual methods inherited 140from <tt>Pass</tt>. Depending on how your pass works, you should inherit from 141the <tt><a href="#ModulePass">ModulePass</a></tt>, <tt><a 142href="#CallGraphSCCPass">CallGraphSCCPass</a></tt>, <tt><a 143href="#FunctionPass">FunctionPass</a></tt>, or <tt><a 144href="#LoopPass">LoopPass</a></tt>, or <tt><a 145href="#RegionPass">RegionPass</a></tt>, or <tt><a 146href="#BasicBlockPass">BasicBlockPass</a></tt> classes, which gives the system 147more information about what your pass does, and how it can be combined with 148other passes. One of the main features of the LLVM Pass Framework is that it 149schedules passes to run in an efficient way based on the constraints that your 150pass meets (which are indicated by which class they derive from).</p> 151 152<p>We start by showing you how to construct a pass, everything from setting up 153the code, to compiling, loading, and executing it. After the basics are down, 154more advanced features are discussed.</p> 155 156</div> 157 158<!-- *********************************************************************** --> 159<h2> 160 <a name="quickstart">Quick Start - Writing hello world</a> 161</h2> 162<!-- *********************************************************************** --> 163 164<div> 165 166<p>Here we describe how to write the "hello world" of passes. The "Hello" pass 167is designed to simply print out the name of non-external functions that exist in 168the program being compiled. It does not modify the program at all, it just 169inspects it. The source code and files for this pass are available in the LLVM 170source tree in the <tt>lib/Transforms/Hello</tt> directory.</p> 171 172<!-- ======================================================================= --> 173<h3> 174 <a name="makefile">Setting up the build environment</a> 175</h3> 176 177<div> 178 179 <p>First, configure and build LLVM. This needs to be done directly inside the 180 LLVM source tree rather than in a separate objects directory. 181 Next, you need to create a new directory somewhere in the LLVM source 182 base. For this example, we'll assume that you made 183 <tt>lib/Transforms/Hello</tt>. Finally, you must set up a build script 184 (Makefile) that will compile the source code for the new pass. To do this, 185 copy the following into <tt>Makefile</tt>:</p> 186 <hr> 187 188<div class="doc_code"><pre> 189# Makefile for hello pass 190 191# Path to top level of LLVM hierarchy 192LEVEL = ../../.. 193 194# Name of the library to build 195LIBRARYNAME = Hello 196 197# Make the shared library become a loadable module so the tools can 198# dlopen/dlsym on the resulting library. 199LOADABLE_MODULE = 1 200 201# Include the makefile implementation stuff 202include $(LEVEL)/Makefile.common 203</pre></div> 204 205<p>This makefile specifies that all of the <tt>.cpp</tt> files in the current 206directory are to be compiled and linked together into a shared object 207<tt>$(LEVEL)/Debug+Asserts/lib/Hello.so</tt> that can be dynamically loaded by 208the <tt>opt</tt> or <tt>bugpoint</tt> tools via their <tt>-load</tt> options. 209If your operating system uses a suffix other than .so (such as windows or 210Mac OS/X), the appropriate extension will be used.</p> 211 212<p>If you are used CMake to build LLVM, see 213<a href="CMake.html#passdev">Developing an LLVM pass with CMake</a>.</p> 214 215<p>Now that we have the build scripts set up, we just need to write the code for 216the pass itself.</p> 217 218</div> 219 220<!-- ======================================================================= --> 221<h3> 222 <a name="basiccode">Basic code required</a> 223</h3> 224 225<div> 226 227<p>Now that we have a way to compile our new pass, we just have to write it. 228Start out with:</p> 229 230<div class="doc_code"><pre> 231<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>" 232<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>" 233<b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>" 234</pre></div> 235 236<p>Which are needed because we are writing a <tt><a 237href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>, 238we are operating on <tt><a 239href="http://llvm.org/doxygen/classllvm_1_1Function.html">Function</a></tt>'s, 240and we will be doing some printing.</p> 241 242<p>Next we have:</p> 243<div class="doc_code"><pre> 244<b>using namespace llvm;</b> 245</pre></div> 246<p>... which is required because the functions from the include files 247live in the llvm namespace. 248</p> 249 250<p>Next we have:</p> 251 252<div class="doc_code"><pre> 253<b>namespace</b> { 254</pre></div> 255 256<p>... which starts out an anonymous namespace. Anonymous namespaces are to C++ 257what the "<tt>static</tt>" keyword is to C (at global scope). It makes the 258things declared inside of the anonymous namespace only visible to the current 259file. If you're not familiar with them, consult a decent C++ book for more 260information.</p> 261 262<p>Next, we declare our pass itself:</p> 263 264<div class="doc_code"><pre> 265 <b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> { 266</pre></div><p> 267 268<p>This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a 269href="http://llvm.org/doxygen/classllvm_1_1FunctionPass.html">FunctionPass</a></tt>. 270The different builtin pass subclasses are described in detail <a 271href="#passtype">later</a>, but for now, know that <a 272href="#FunctionPass"><tt>FunctionPass</tt></a>'s operate a function at a 273time.</p> 274 275<div class="doc_code"><pre> 276 static char ID; 277 Hello() : FunctionPass(ID) {} 278</pre></div><p> 279 280<p> This declares pass identifier used by LLVM to identify pass. This allows LLVM to 281avoid using expensive C++ runtime information.</p> 282 283<div class="doc_code"><pre> 284 <b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) { 285 errs() << "<i>Hello: </i>" << F.getName() << "\n"; 286 <b>return false</b>; 287 } 288 }; <i>// end of struct Hello</i> 289</pre></div> 290 291<p>We declare a "<a href="#runOnFunction"><tt>runOnFunction</tt></a>" method, 292which overloads an abstract virtual method inherited from <a 293href="#FunctionPass"><tt>FunctionPass</tt></a>. This is where we are supposed 294to do our thing, so we just print out our message with the name of each 295function.</p> 296 297<div class="doc_code"><pre> 298 char Hello::ID = 0; 299</pre></div> 300 301<p> We initialize pass ID here. LLVM uses ID's address to identify pass so 302initialization value is not important.</p> 303 304<div class="doc_code"><pre> 305 static RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>", 306 false /* Only looks at CFG */, 307 false /* Analysis Pass */); 308} <i>// end of anonymous namespace</i> 309</pre></div> 310 311<p>Lastly, we <a href="#registration">register our class</a> <tt>Hello</tt>, 312giving it a command line 313argument "<tt>hello</tt>", and a name "<tt>Hello World Pass</tt>". 314Last two arguments describe its behavior. 315If a pass walks CFG without modifying it then third argument is set to true. 316If a pass is an analysis pass, for example dominator tree pass, then true 317is supplied as fourth argument. </p> 318 319<p>As a whole, the <tt>.cpp</tt> file looks like:</p> 320 321<div class="doc_code"><pre> 322<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>" 323<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>" 324<b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>" 325 326<b>using namespace llvm;</b> 327 328<b>namespace</b> { 329 <b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> { 330 331 static char ID; 332 Hello() : FunctionPass(ID) {} 333 334 <b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) { 335 errs() << "<i>Hello: </i>" << F.getName() << "\n"; 336 <b>return false</b>; 337 } 338 }; 339 340 char Hello::ID = 0; 341 static RegisterPass<Hello> X("hello", "Hello World Pass", false, false); 342} 343 344</pre></div> 345 346<p>Now that it's all together, compile the file with a simple "<tt>gmake</tt>" 347command in the local directory and you should get a new file 348"<tt>Debug+Asserts/lib/Hello.so</tt>" under the top level directory of the LLVM 349source tree (not in the local directory). Note that everything in this file is 350contained in an anonymous namespace: this reflects the fact that passes are self 351contained units that do not need external interfaces (although they can have 352them) to be useful.</p> 353 354</div> 355 356<!-- ======================================================================= --> 357<h3> 358 <a name="running">Running a pass with <tt>opt</tt></a> 359</h3> 360 361<div> 362 363<p>Now that you have a brand new shiny shared object file, we can use the 364<tt>opt</tt> command to run an LLVM program through your pass. Because you 365registered your pass with <tt>RegisterPass</tt>, you will be able to 366use the <tt>opt</tt> tool to access it, once loaded.</p> 367 368<p>To test it, follow the example at the end of the <a 369href="GettingStarted.html">Getting Started Guide</a> to compile "Hello World" to 370LLVM. We can now run the bitcode file (<tt>hello.bc</tt>) for the program 371through our transformation like this (or course, any bitcode file will 372work):</p> 373 374<div class="doc_code"><pre> 375$ opt -load ../../../Debug+Asserts/lib/Hello.so -hello < hello.bc > /dev/null 376Hello: __main 377Hello: puts 378Hello: main 379</pre></div> 380 381<p>The '<tt>-load</tt>' option specifies that '<tt>opt</tt>' should load your 382pass as a shared object, which makes '<tt>-hello</tt>' a valid command line 383argument (which is one reason you need to <a href="#registration">register your 384pass</a>). Because the hello pass does not modify the program in any 385interesting way, we just throw away the result of <tt>opt</tt> (sending it to 386<tt>/dev/null</tt>).</p> 387 388<p>To see what happened to the other string you registered, try running 389<tt>opt</tt> with the <tt>-help</tt> option:</p> 390 391<div class="doc_code"><pre> 392$ opt -load ../../../Debug+Asserts/lib/Hello.so -help 393OVERVIEW: llvm .bc -> .bc modular optimizer 394 395USAGE: opt [options] <input bitcode> 396 397OPTIONS: 398 Optimizations available: 399... 400 -funcresolve - Resolve Functions 401 -gcse - Global Common Subexpression Elimination 402 -globaldce - Dead Global Elimination 403 <b>-hello - Hello World Pass</b> 404 -indvars - Canonicalize Induction Variables 405 -inline - Function Integration/Inlining 406 -instcombine - Combine redundant instructions 407... 408</pre></div> 409 410<p>The pass name get added as the information string for your pass, giving some 411documentation to users of <tt>opt</tt>. Now that you have a working pass, you 412would go ahead and make it do the cool transformations you want. Once you get 413it all working and tested, it may become useful to find out how fast your pass 414is. The <a href="#passManager"><tt>PassManager</tt></a> provides a nice command 415line option (<tt>--time-passes</tt>) that allows you to get information about 416the execution time of your pass along with the other passes you queue up. For 417example:</p> 418 419<div class="doc_code"><pre> 420$ opt -load ../../../Debug+Asserts/lib/Hello.so -hello -time-passes < hello.bc > /dev/null 421Hello: __main 422Hello: puts 423Hello: main 424=============================================================================== 425 ... Pass execution timing report ... 426=============================================================================== 427 Total Execution Time: 0.02 seconds (0.0479059 wall clock) 428 429 ---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name --- 430 0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bitcode Writer 431 0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction 432 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier 433 <b> 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass</b> 434 0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL 435</pre></div> 436 437<p>As you can see, our implementation above is pretty fast :). The additional 438passes listed are automatically inserted by the '<tt>opt</tt>' tool to verify 439that the LLVM emitted by your pass is still valid and well formed LLVM, which 440hasn't been broken somehow.</p> 441 442<p>Now that you have seen the basics of the mechanics behind passes, we can talk 443about some more details of how they work and how to use them.</p> 444 445</div> 446 447</div> 448 449<!-- *********************************************************************** --> 450<h2> 451 <a name="passtype">Pass classes and requirements</a> 452</h2> 453<!-- *********************************************************************** --> 454 455<div> 456 457<p>One of the first things that you should do when designing a new pass is to 458decide what class you should subclass for your pass. The <a 459href="#basiccode">Hello World</a> example uses the <tt><a 460href="#FunctionPass">FunctionPass</a></tt> class for its implementation, but we 461did not discuss why or when this should occur. Here we talk about the classes 462available, from the most general to the most specific.</p> 463 464<p>When choosing a superclass for your Pass, you should choose the <b>most 465specific</b> class possible, while still being able to meet the requirements 466listed. This gives the LLVM Pass Infrastructure information necessary to 467optimize how passes are run, so that the resultant compiler isn't unnecessarily 468slow.</p> 469 470<!-- ======================================================================= --> 471<h3> 472 <a name="ImmutablePass">The <tt>ImmutablePass</tt> class</a> 473</h3> 474 475<div> 476 477<p>The most plain and boring type of pass is the "<tt><a 478href="http://llvm.org/doxygen/classllvm_1_1ImmutablePass.html">ImmutablePass</a></tt>" 479class. This pass type is used for passes that do not have to be run, do not 480change state, and never need to be updated. This is not a normal type of 481transformation or analysis, but can provide information about the current 482compiler configuration.</p> 483 484<p>Although this pass class is very infrequently used, it is important for 485providing information about the current target machine being compiled for, and 486other static information that can affect the various transformations.</p> 487 488<p><tt>ImmutablePass</tt>es never invalidate other transformations, are never 489invalidated, and are never "run".</p> 490 491</div> 492 493<!-- ======================================================================= --> 494<h3> 495 <a name="ModulePass">The <tt>ModulePass</tt> class</a> 496</h3> 497 498<div> 499 500<p>The "<tt><a 501href="http://llvm.org/doxygen/classllvm_1_1ModulePass.html">ModulePass</a></tt>" 502class is the most general of all superclasses that you can use. Deriving from 503<tt>ModulePass</tt> indicates that your pass uses the entire program as a unit, 504referring to function bodies in no predictable order, or adding and removing 505functions. Because nothing is known about the behavior of <tt>ModulePass</tt> 506subclasses, no optimization can be done for their execution.</p> 507 508<p>A module pass can use function level passes (e.g. dominators) using 509the getAnalysis interface 510<tt>getAnalysis<DominatorTree>(llvm::Function *)</tt> to provide the 511function to retrieve analysis result for, if the function pass does not require 512any module or immutable passes. Note that this can only be done for functions for which the 513analysis ran, e.g. in the case of dominators you should only ask for the 514DominatorTree for function definitions, not declarations.</p> 515 516<p>To write a correct <tt>ModulePass</tt> subclass, derive from 517<tt>ModulePass</tt> and overload the <tt>runOnModule</tt> method with the 518following signature:</p> 519 520<!-- _______________________________________________________________________ --> 521<h4> 522 <a name="runOnModule">The <tt>runOnModule</tt> method</a> 523</h4> 524 525<div> 526 527<div class="doc_code"><pre> 528 <b>virtual bool</b> runOnModule(Module &M) = 0; 529</pre></div> 530 531<p>The <tt>runOnModule</tt> method performs the interesting work of the pass. 532It should return true if the module was modified by the transformation and 533false otherwise.</p> 534 535</div> 536 537</div> 538 539<!-- ======================================================================= --> 540<h3> 541 <a name="CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a> 542</h3> 543 544<div> 545 546<p>The "<tt><a 547href="http://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html">CallGraphSCCPass</a></tt>" 548is used by passes that need to traverse the program bottom-up on the call graph 549(callees before callers). Deriving from CallGraphSCCPass provides some 550mechanics for building and traversing the CallGraph, but also allows the system 551to optimize execution of CallGraphSCCPass's. If your pass meets the 552requirements outlined below, and doesn't meet the requirements of a <tt><a 553href="#FunctionPass">FunctionPass</a></tt> or <tt><a 554href="#BasicBlockPass">BasicBlockPass</a></tt>, you should derive from 555<tt>CallGraphSCCPass</tt>.</p> 556 557<p><b>TODO</b>: explain briefly what SCC, Tarjan's algo, and B-U mean.</p> 558 559<p>To be explicit, <tt>CallGraphSCCPass</tt> subclasses are:</p> 560 561<ol> 562 563<li>... <em>not allowed</em> to inspect or modify any <tt>Function</tt>s other 564than those in the current SCC and the direct callers and direct callees of the 565SCC.</li> 566 567<li>... <em>required</em> to preserve the current CallGraph object, updating it 568to reflect any changes made to the program.</li> 569 570<li>... <em>not allowed</em> to add or remove SCC's from the current Module, 571though they may change the contents of an SCC.</li> 572 573<li>... <em>allowed</em> to add or remove global variables from the current 574Module.</li> 575 576<li>... <em>allowed</em> to maintain state across invocations of 577 <a href="#runOnSCC"><tt>runOnSCC</tt></a> (including global data).</li> 578</ol> 579 580<p>Implementing a <tt>CallGraphSCCPass</tt> is slightly tricky in some cases 581because it has to handle SCCs with more than one node in it. All of the virtual 582methods described below should return true if they modified the program, or 583false if they didn't.</p> 584 585<!-- _______________________________________________________________________ --> 586<h4> 587 <a name="doInitialization_scc"> 588 The <tt>doInitialization(CallGraph &)</tt> method 589 </a> 590</h4> 591 592<div> 593 594<div class="doc_code"><pre> 595 <b>virtual bool</b> doInitialization(CallGraph &CG); 596</pre></div> 597 598<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that 599<tt>CallGraphSCCPass</tt>'s are not allowed to do. They can add and remove 600functions, get pointers to functions, etc. The <tt>doInitialization</tt> method 601is designed to do simple initialization type of stuff that does not depend on 602the SCCs being processed. The <tt>doInitialization</tt> method call is not 603scheduled to overlap with any other pass executions (thus it should be very 604fast).</p> 605 606</div> 607 608<!-- _______________________________________________________________________ --> 609<h4> 610 <a name="runOnSCC">The <tt>runOnSCC</tt> method</a> 611</h4> 612 613<div> 614 615<div class="doc_code"><pre> 616 <b>virtual bool</b> runOnSCC(CallGraphSCC &SCC) = 0; 617</pre></div> 618 619<p>The <tt>runOnSCC</tt> method performs the interesting work of the pass, and 620should return true if the module was modified by the transformation, false 621otherwise.</p> 622 623</div> 624 625<!-- _______________________________________________________________________ --> 626<h4> 627 <a name="doFinalization_scc"> 628 The <tt>doFinalization(CallGraph &)</tt> method 629 </a> 630</h4> 631 632<div> 633 634<div class="doc_code"><pre> 635 <b>virtual bool</b> doFinalization(CallGraph &CG); 636</pre></div> 637 638<p>The <tt>doFinalization</tt> method is an infrequently used method that is 639called when the pass framework has finished calling <a 640href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the 641program being compiled.</p> 642 643</div> 644 645</div> 646 647<!-- ======================================================================= --> 648<h3> 649 <a name="FunctionPass">The <tt>FunctionPass</tt> class</a> 650</h3> 651 652<div> 653 654<p>In contrast to <tt>ModulePass</tt> subclasses, <tt><a 655href="http://llvm.org/doxygen/classllvm_1_1Pass.html">FunctionPass</a></tt> 656subclasses do have a predictable, local behavior that can be expected by the 657system. All <tt>FunctionPass</tt> execute on each function in the program 658independent of all of the other functions in the program. 659<tt>FunctionPass</tt>'s do not require that they are executed in a particular 660order, and <tt>FunctionPass</tt>'s do not modify external functions.</p> 661 662<p>To be explicit, <tt>FunctionPass</tt> subclasses are not allowed to:</p> 663 664<ol> 665<li>Modify a Function other than the one currently being processed.</li> 666<li>Add or remove Function's from the current Module.</li> 667<li>Add or remove global variables from the current Module.</li> 668<li>Maintain state across invocations of 669 <a href="#runOnFunction"><tt>runOnFunction</tt></a> (including global data)</li> 670</ol> 671 672<p>Implementing a <tt>FunctionPass</tt> is usually straightforward (See the <a 673href="#basiccode">Hello World</a> pass for example). <tt>FunctionPass</tt>'s 674may overload three virtual methods to do their work. All of these methods 675should return true if they modified the program, or false if they didn't.</p> 676 677<!-- _______________________________________________________________________ --> 678<h4> 679 <a name="doInitialization_mod"> 680 The <tt>doInitialization(Module &)</tt> method 681 </a> 682</h4> 683 684<div> 685 686<div class="doc_code"><pre> 687 <b>virtual bool</b> doInitialization(Module &M); 688</pre></div> 689 690<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that 691<tt>FunctionPass</tt>'s are not allowed to do. They can add and remove 692functions, get pointers to functions, etc. The <tt>doInitialization</tt> method 693is designed to do simple initialization type of stuff that does not depend on 694the functions being processed. The <tt>doInitialization</tt> method call is not 695scheduled to overlap with any other pass executions (thus it should be very 696fast).</p> 697 698<p>A good example of how this method should be used is the <a 699href="http://llvm.org/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a> 700pass. This pass converts <tt>malloc</tt> and <tt>free</tt> instructions into 701platform dependent <tt>malloc()</tt> and <tt>free()</tt> function calls. It 702uses the <tt>doInitialization</tt> method to get a reference to the malloc and 703free functions that it needs, adding prototypes to the module if necessary.</p> 704 705</div> 706 707<!-- _______________________________________________________________________ --> 708<h4> 709 <a name="runOnFunction">The <tt>runOnFunction</tt> method</a> 710</h4> 711 712<div> 713 714<div class="doc_code"><pre> 715 <b>virtual bool</b> runOnFunction(Function &F) = 0; 716</pre></div><p> 717 718<p>The <tt>runOnFunction</tt> method must be implemented by your subclass to do 719the transformation or analysis work of your pass. As usual, a true value should 720be returned if the function is modified.</p> 721 722</div> 723 724<!-- _______________________________________________________________________ --> 725<h4> 726 <a name="doFinalization_mod"> 727 The <tt>doFinalization(Module &)</tt> method 728 </a> 729</h4> 730 731<div> 732 733<div class="doc_code"><pre> 734 <b>virtual bool</b> doFinalization(Module &M); 735</pre></div> 736 737<p>The <tt>doFinalization</tt> method is an infrequently used method that is 738called when the pass framework has finished calling <a 739href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the 740program being compiled.</p> 741 742</div> 743 744</div> 745 746<!-- ======================================================================= --> 747<h3> 748 <a name="LoopPass">The <tt>LoopPass</tt> class </a> 749</h3> 750 751<div> 752 753<p> All <tt>LoopPass</tt> execute on each loop in the function independent of 754all of the other loops in the function. <tt>LoopPass</tt> processes loops in 755loop nest order such that outer most loop is processed last. </p> 756 757<p> <tt>LoopPass</tt> subclasses are allowed to update loop nest using 758<tt>LPPassManager</tt> interface. Implementing a loop pass is usually 759straightforward. <tt>LoopPass</tt>'s may overload three virtual methods to 760do their work. All these methods should return true if they modified the 761program, or false if they didn't. </p> 762 763<!-- _______________________________________________________________________ --> 764<h4> 765 <a name="doInitialization_loop"> 766 The <tt>doInitialization(Loop *,LPPassManager &)</tt> method 767 </a> 768</h4> 769 770<div> 771 772<div class="doc_code"><pre> 773 <b>virtual bool</b> doInitialization(Loop *, LPPassManager &LPM); 774</pre></div> 775 776<p>The <tt>doInitialization</tt> method is designed to do simple initialization 777type of stuff that does not depend on the functions being processed. The 778<tt>doInitialization</tt> method call is not scheduled to overlap with any 779other pass executions (thus it should be very fast). LPPassManager 780interface should be used to access Function or Module level analysis 781information.</p> 782 783</div> 784 785 786<!-- _______________________________________________________________________ --> 787<h4> 788 <a name="runOnLoop">The <tt>runOnLoop</tt> method</a> 789</h4> 790 791<div> 792 793<div class="doc_code"><pre> 794 <b>virtual bool</b> runOnLoop(Loop *, LPPassManager &LPM) = 0; 795</pre></div><p> 796 797<p>The <tt>runOnLoop</tt> method must be implemented by your subclass to do 798the transformation or analysis work of your pass. As usual, a true value should 799be returned if the function is modified. <tt>LPPassManager</tt> interface 800should be used to update loop nest.</p> 801 802</div> 803 804<!-- _______________________________________________________________________ --> 805<h4> 806 <a name="doFinalization_loop">The <tt>doFinalization()</tt> method</a> 807</h4> 808 809<div> 810 811<div class="doc_code"><pre> 812 <b>virtual bool</b> doFinalization(); 813</pre></div> 814 815<p>The <tt>doFinalization</tt> method is an infrequently used method that is 816called when the pass framework has finished calling <a 817href="#runOnLoop"><tt>runOnLoop</tt></a> for every loop in the 818program being compiled. </p> 819 820</div> 821 822</div> 823 824<!-- ======================================================================= --> 825<h3> 826 <a name="RegionPass">The <tt>RegionPass</tt> class </a> 827</h3> 828 829<div> 830 831<p> <tt>RegionPass</tt> is similar to <a href="#LoopPass"><tt>LoopPass</tt></a>, 832but executes on each single entry single exit region in the function. 833<tt>RegionPass</tt> processes regions in nested order such that the outer most 834region is processed last. </p> 835 836<p> <tt>RegionPass</tt> subclasses are allowed to update the region tree by using 837the <tt>RGPassManager</tt> interface. You may overload three virtual methods of 838<tt>RegionPass</tt> to implement your own region pass. All these 839methods should return true if they modified the program, or false if they didn not. 840</p> 841 842<!-- _______________________________________________________________________ --> 843<h4> 844 <a name="doInitialization_region"> 845 The <tt>doInitialization(Region *, RGPassManager &)</tt> method 846 </a> 847</h4> 848 849<div> 850 851<div class="doc_code"><pre> 852 <b>virtual bool</b> doInitialization(Region *, RGPassManager &RGM); 853</pre></div> 854 855<p>The <tt>doInitialization</tt> method is designed to do simple initialization 856type of stuff that does not depend on the functions being processed. The 857<tt>doInitialization</tt> method call is not scheduled to overlap with any 858other pass executions (thus it should be very fast). RPPassManager 859interface should be used to access Function or Module level analysis 860information.</p> 861 862</div> 863 864 865<!-- _______________________________________________________________________ --> 866<h4> 867 <a name="runOnRegion">The <tt>runOnRegion</tt> method</a> 868</h4> 869 870<div> 871 872<div class="doc_code"><pre> 873 <b>virtual bool</b> runOnRegion(Region *, RGPassManager &RGM) = 0; 874</pre></div><p> 875 876<p>The <tt>runOnRegion</tt> method must be implemented by your subclass to do 877the transformation or analysis work of your pass. As usual, a true value should 878be returned if the region is modified. <tt>RGPassManager</tt> interface 879should be used to update region tree.</p> 880 881</div> 882 883<!-- _______________________________________________________________________ --> 884<h4> 885 <a name="doFinalization_region">The <tt>doFinalization()</tt> method</a> 886</h4> 887 888<div> 889 890<div class="doc_code"><pre> 891 <b>virtual bool</b> doFinalization(); 892</pre></div> 893 894<p>The <tt>doFinalization</tt> method is an infrequently used method that is 895called when the pass framework has finished calling <a 896href="#runOnRegion"><tt>runOnRegion</tt></a> for every region in the 897program being compiled. </p> 898 899</div> 900 901</div> 902 903<!-- ======================================================================= --> 904<h3> 905 <a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a> 906</h3> 907 908<div> 909 910<p><tt>BasicBlockPass</tt>'s are just like <a 911href="#FunctionPass"><tt>FunctionPass</tt></a>'s, except that they must limit 912their scope of inspection and modification to a single basic block at a time. 913As such, they are <b>not</b> allowed to do any of the following:</p> 914 915<ol> 916<li>Modify or inspect any basic blocks outside of the current one</li> 917<li>Maintain state across invocations of 918 <a href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a></li> 919<li>Modify the control flow graph (by altering terminator instructions)</li> 920<li>Any of the things forbidden for 921 <a href="#FunctionPass"><tt>FunctionPass</tt></a>es.</li> 922</ol> 923 924<p><tt>BasicBlockPass</tt>es are useful for traditional local and "peephole" 925optimizations. They may override the same <a 926href="#doInitialization_mod"><tt>doInitialization(Module &)</tt></a> and <a 927href="#doFinalization_mod"><tt>doFinalization(Module &)</tt></a> methods that <a 928href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have the following virtual methods that may also be implemented:</p> 929 930<!-- _______________________________________________________________________ --> 931<h4> 932 <a name="doInitialization_fn"> 933 The <tt>doInitialization(Function &)</tt> method 934 </a> 935</h4> 936 937<div> 938 939<div class="doc_code"><pre> 940 <b>virtual bool</b> doInitialization(Function &F); 941</pre></div> 942 943<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that 944<tt>BasicBlockPass</tt>'s are not allowed to do, but that 945<tt>FunctionPass</tt>'s can. The <tt>doInitialization</tt> method is designed 946to do simple initialization that does not depend on the 947BasicBlocks being processed. The <tt>doInitialization</tt> method call is not 948scheduled to overlap with any other pass executions (thus it should be very 949fast).</p> 950 951</div> 952 953<!-- _______________________________________________________________________ --> 954<h4> 955 <a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a> 956</h4> 957 958<div> 959 960<div class="doc_code"><pre> 961 <b>virtual bool</b> runOnBasicBlock(BasicBlock &BB) = 0; 962</pre></div> 963 964<p>Override this function to do the work of the <tt>BasicBlockPass</tt>. This 965function is not allowed to inspect or modify basic blocks other than the 966parameter, and are not allowed to modify the CFG. A true value must be returned 967if the basic block is modified.</p> 968 969</div> 970 971<!-- _______________________________________________________________________ --> 972<h4> 973 <a name="doFinalization_fn"> 974 The <tt>doFinalization(Function &)</tt> method 975 </a> 976</h4> 977 978<div> 979 980<div class="doc_code"><pre> 981 <b>virtual bool</b> doFinalization(Function &F); 982</pre></div> 983 984<p>The <tt>doFinalization</tt> method is an infrequently used method that is 985called when the pass framework has finished calling <a 986href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a> for every BasicBlock in the 987program being compiled. This can be used to perform per-function 988finalization.</p> 989 990</div> 991 992</div> 993 994<!-- ======================================================================= --> 995<h3> 996 <a name="MachineFunctionPass">The <tt>MachineFunctionPass</tt> class</a> 997</h3> 998 999<div> 1000 1001<p>A <tt>MachineFunctionPass</tt> is a part of the LLVM code generator that 1002executes on the machine-dependent representation of each LLVM function in the 1003program.</p> 1004 1005<p>Code generator passes are registered and initialized specially by 1006<tt>TargetMachine::addPassesToEmitFile</tt> and similar routines, so they 1007cannot generally be run from the <tt>opt</tt> or <tt>bugpoint</tt> 1008commands.</p> 1009 1010<p>A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all 1011the restrictions that apply to a <tt>FunctionPass</tt> also apply to it. 1012<tt>MachineFunctionPass</tt>es also have additional restrictions. In particular, 1013<tt>MachineFunctionPass</tt>es are not allowed to do any of the following:</p> 1014 1015<ol> 1016<li>Modify or create any LLVM IR Instructions, BasicBlocks, Arguments, 1017 Functions, GlobalVariables, GlobalAliases, or Modules.</li> 1018<li>Modify a MachineFunction other than the one currently being processed.</li> 1019<li>Maintain state across invocations of <a 1020href="#runOnMachineFunction"><tt>runOnMachineFunction</tt></a> (including global 1021data)</li> 1022</ol> 1023 1024<!-- _______________________________________________________________________ --> 1025<h4> 1026 <a name="runOnMachineFunction"> 1027 The <tt>runOnMachineFunction(MachineFunction &MF)</tt> method 1028 </a> 1029</h4> 1030 1031<div> 1032 1033<div class="doc_code"><pre> 1034 <b>virtual bool</b> runOnMachineFunction(MachineFunction &MF) = 0; 1035</pre></div> 1036 1037<p><tt>runOnMachineFunction</tt> can be considered the main entry point of a 1038<tt>MachineFunctionPass</tt>; that is, you should override this method to do the 1039work of your <tt>MachineFunctionPass</tt>.</p> 1040 1041<p>The <tt>runOnMachineFunction</tt> method is called on every 1042<tt>MachineFunction</tt> in a <tt>Module</tt>, so that the 1043<tt>MachineFunctionPass</tt> may perform optimizations on the machine-dependent 1044representation of the function. If you want to get at the LLVM <tt>Function</tt> 1045for the <tt>MachineFunction</tt> you're working on, use 1046<tt>MachineFunction</tt>'s <tt>getFunction()</tt> accessor method -- but 1047remember, you may not modify the LLVM <tt>Function</tt> or its contents from a 1048<tt>MachineFunctionPass</tt>.</p> 1049 1050</div> 1051 1052</div> 1053 1054</div> 1055 1056<!-- *********************************************************************** --> 1057<h2> 1058 <a name="registration">Pass registration</a> 1059</h2> 1060<!-- *********************************************************************** --> 1061 1062<div> 1063 1064<p>In the <a href="#basiccode">Hello World</a> example pass we illustrated how 1065pass registration works, and discussed some of the reasons that it is used and 1066what it does. Here we discuss how and why passes are registered.</p> 1067 1068<p>As we saw above, passes are registered with the <b><tt>RegisterPass</tt></b> 1069template. The template parameter is the name of the pass that is to be used on 1070the command line to specify that the pass should be added to a program (for 1071example, with <tt>opt</tt> or <tt>bugpoint</tt>). The first argument is the 1072name of the pass, which is to be used for the <tt>-help</tt> output of 1073programs, as 1074well as for debug output generated by the <tt>--debug-pass</tt> option.</p> 1075 1076<p>If you want your pass to be easily dumpable, you should 1077implement the virtual <tt>print</tt> method:</p> 1078 1079<!-- _______________________________________________________________________ --> 1080<h4> 1081 <a name="print">The <tt>print</tt> method</a> 1082</h4> 1083 1084<div> 1085 1086<div class="doc_code"><pre> 1087 <b>virtual void</b> print(std::ostream &O, <b>const</b> Module *M) <b>const</b>; 1088</pre></div> 1089 1090<p>The <tt>print</tt> method must be implemented by "analyses" in order to print 1091a human readable version of the analysis results. This is useful for debugging 1092an analysis itself, as well as for other people to figure out how an analysis 1093works. Use the <tt>opt -analyze</tt> argument to invoke this method.</p> 1094 1095<p>The <tt>llvm::OStream</tt> parameter specifies the stream to write the results on, 1096and the <tt>Module</tt> parameter gives a pointer to the top level module of the 1097program that has been analyzed. Note however that this pointer may be null in 1098certain circumstances (such as calling the <tt>Pass::dump()</tt> from a 1099debugger), so it should only be used to enhance debug output, it should not be 1100depended on.</p> 1101 1102</div> 1103 1104</div> 1105 1106<!-- *********************************************************************** --> 1107<h2> 1108 <a name="interaction">Specifying interactions between passes</a> 1109</h2> 1110<!-- *********************************************************************** --> 1111 1112<div> 1113 1114<p>One of the main responsibilities of the <tt>PassManager</tt> is to make sure 1115that passes interact with each other correctly. Because <tt>PassManager</tt> 1116tries to <a href="#passmanager">optimize the execution of passes</a> it must 1117know how the passes interact with each other and what dependencies exist between 1118the various passes. To track this, each pass can declare the set of passes that 1119are required to be executed before the current pass, and the passes which are 1120invalidated by the current pass.</p> 1121 1122<p>Typically this functionality is used to require that analysis results are 1123computed before your pass is run. Running arbitrary transformation passes can 1124invalidate the computed analysis results, which is what the invalidation set 1125specifies. If a pass does not implement the <tt><a 1126href="#getAnalysisUsage">getAnalysisUsage</a></tt> method, it defaults to not 1127having any prerequisite passes, and invalidating <b>all</b> other passes.</p> 1128 1129<!-- _______________________________________________________________________ --> 1130<h4> 1131 <a name="getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a> 1132</h4> 1133 1134<div> 1135 1136<div class="doc_code"><pre> 1137 <b>virtual void</b> getAnalysisUsage(AnalysisUsage &Info) <b>const</b>; 1138</pre></div> 1139 1140<p>By implementing the <tt>getAnalysisUsage</tt> method, the required and 1141invalidated sets may be specified for your transformation. The implementation 1142should fill in the <tt><a 1143href="http://llvm.org/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt> 1144object with information about which passes are required and not invalidated. To 1145do this, a pass may call any of the following methods on the AnalysisUsage 1146object:</p> 1147</div> 1148 1149<!-- _______________________________________________________________________ --> 1150<h4> 1151 <a name="AU::addRequired"> 1152 The <tt>AnalysisUsage::addRequired<></tt> 1153 and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods 1154 </a> 1155</h4> 1156 1157<div> 1158<p> 1159If your pass requires a previous pass to be executed (an analysis for example), 1160it can use one of these methods to arrange for it to be run before your pass. 1161LLVM has many different types of analyses and passes that can be required, 1162spanning the range from <tt>DominatorSet</tt> to <tt>BreakCriticalEdges</tt>. 1163Requiring <tt>BreakCriticalEdges</tt>, for example, guarantees that there will 1164be no critical edges in the CFG when your pass has been run. 1165</p> 1166 1167<p> 1168Some analyses chain to other analyses to do their job. For example, an <a 1169href="AliasAnalysis.html">AliasAnalysis</a> implementation is required to <a 1170href="AliasAnalysis.html#chaining">chain</a> to other alias analysis passes. In 1171cases where analyses chain, the <tt>addRequiredTransitive</tt> method should be 1172used instead of the <tt>addRequired</tt> method. This informs the PassManager 1173that the transitively required pass should be alive as long as the requiring 1174pass is. 1175</p> 1176</div> 1177 1178<!-- _______________________________________________________________________ --> 1179<h4> 1180 <a name="AU::addPreserved"> 1181 The <tt>AnalysisUsage::addPreserved<></tt> method 1182 </a> 1183</h4> 1184 1185<div> 1186<p> 1187One of the jobs of the PassManager is to optimize how and when analyses are run. 1188In particular, it attempts to avoid recomputing data unless it needs to. For 1189this reason, passes are allowed to declare that they preserve (i.e., they don't 1190invalidate) an existing analysis if it's available. For example, a simple 1191constant folding pass would not modify the CFG, so it can't possibly affect the 1192results of dominator analysis. By default, all passes are assumed to invalidate 1193all others. 1194</p> 1195 1196<p> 1197The <tt>AnalysisUsage</tt> class provides several methods which are useful in 1198certain circumstances that are related to <tt>addPreserved</tt>. In particular, 1199the <tt>setPreservesAll</tt> method can be called to indicate that the pass does 1200not modify the LLVM program at all (which is true for analyses), and the 1201<tt>setPreservesCFG</tt> method can be used by transformations that change 1202instructions in the program but do not modify the CFG or terminator instructions 1203(note that this property is implicitly set for <a 1204href="#BasicBlockPass">BasicBlockPass</a>'s). 1205</p> 1206 1207<p> 1208<tt>addPreserved</tt> is particularly useful for transformations like 1209<tt>BreakCriticalEdges</tt>. This pass knows how to update a small set of loop 1210and dominator related analyses if they exist, so it can preserve them, despite 1211the fact that it hacks on the CFG. 1212</p> 1213</div> 1214 1215<!-- _______________________________________________________________________ --> 1216<h4> 1217 <a name="AU::examples"> 1218 Example implementations of <tt>getAnalysisUsage</tt> 1219 </a> 1220</h4> 1221 1222<div> 1223 1224<div class="doc_code"><pre> 1225 <i>// This example modifies the program, but does not modify the CFG</i> 1226 <b>void</b> <a href="http://llvm.org/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> { 1227 AU.setPreservesCFG(); 1228 AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1LoopInfo.html">LoopInfo</a>>(); 1229 } 1230</pre></div> 1231 1232</div> 1233 1234<!-- _______________________________________________________________________ --> 1235<h4> 1236 <a name="getAnalysis"> 1237 The <tt>getAnalysis<></tt> and 1238 <tt>getAnalysisIfAvailable<></tt> methods 1239 </a> 1240</h4> 1241 1242<div> 1243 1244<p>The <tt>Pass::getAnalysis<></tt> method is automatically inherited by 1245your class, providing you with access to the passes that you declared that you 1246required with the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> 1247method. It takes a single template argument that specifies which pass class you 1248want, and returns a reference to that pass. For example:</p> 1249 1250<div class="doc_code"><pre> 1251 bool LICM::runOnFunction(Function &F) { 1252 LoopInfo &LI = getAnalysis<LoopInfo>(); 1253 ... 1254 } 1255</pre></div> 1256 1257<p>This method call returns a reference to the pass desired. You may get a 1258runtime assertion failure if you attempt to get an analysis that you did not 1259declare as required in your <a 1260href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> implementation. This 1261method can be called by your <tt>run*</tt> method implementation, or by any 1262other local method invoked by your <tt>run*</tt> method. 1263 1264A module level pass can use function level analysis info using this interface. 1265For example:</p> 1266 1267<div class="doc_code"><pre> 1268 bool ModuleLevelPass::runOnModule(Module &M) { 1269 ... 1270 DominatorTree &DT = getAnalysis<DominatorTree>(Func); 1271 ... 1272 } 1273</pre></div> 1274 1275<p>In above example, runOnFunction for DominatorTree is called by pass manager 1276before returning a reference to the desired pass.</p> 1277 1278<p> 1279If your pass is capable of updating analyses if they exist (e.g., 1280<tt>BreakCriticalEdges</tt>, as described above), you can use the 1281<tt>getAnalysisIfAvailable</tt> method, which returns a pointer to the analysis 1282if it is active. For example:</p> 1283 1284<div class="doc_code"><pre> 1285 ... 1286 if (DominatorSet *DS = getAnalysisIfAvailable<DominatorSet>()) { 1287 <i>// A DominatorSet is active. This code will update it.</i> 1288 } 1289 ... 1290</pre></div> 1291 1292</div> 1293 1294</div> 1295 1296<!-- *********************************************************************** --> 1297<h2> 1298 <a name="analysisgroup">Implementing Analysis Groups</a> 1299</h2> 1300<!-- *********************************************************************** --> 1301 1302<div> 1303 1304<p>Now that we understand the basics of how passes are defined, how they are 1305used, and how they are required from other passes, it's time to get a little bit 1306fancier. All of the pass relationships that we have seen so far are very 1307simple: one pass depends on one other specific pass to be run before it can run. 1308For many applications, this is great, for others, more flexibility is 1309required.</p> 1310 1311<p>In particular, some analyses are defined such that there is a single simple 1312interface to the analysis results, but multiple ways of calculating them. 1313Consider alias analysis for example. The most trivial alias analysis returns 1314"may alias" for any alias query. The most sophisticated analysis a 1315flow-sensitive, context-sensitive interprocedural analysis that can take a 1316significant amount of time to execute (and obviously, there is a lot of room 1317between these two extremes for other implementations). To cleanly support 1318situations like this, the LLVM Pass Infrastructure supports the notion of 1319Analysis Groups.</p> 1320 1321<!-- _______________________________________________________________________ --> 1322<h4> 1323 <a name="agconcepts">Analysis Group Concepts</a> 1324</h4> 1325 1326<div> 1327 1328<p>An Analysis Group is a single simple interface that may be implemented by 1329multiple different passes. Analysis Groups can be given human readable names 1330just like passes, but unlike passes, they need not derive from the <tt>Pass</tt> 1331class. An analysis group may have one or more implementations, one of which is 1332the "default" implementation.</p> 1333 1334<p>Analysis groups are used by client passes just like other passes are: the 1335<tt>AnalysisUsage::addRequired()</tt> and <tt>Pass::getAnalysis()</tt> methods. 1336In order to resolve this requirement, the <a href="#passmanager">PassManager</a> 1337scans the available passes to see if any implementations of the analysis group 1338are available. If none is available, the default implementation is created for 1339the pass to use. All standard rules for <A href="#interaction">interaction 1340between passes</a> still apply.</p> 1341 1342<p>Although <a href="#registration">Pass Registration</a> is optional for normal 1343passes, all analysis group implementations must be registered, and must use the 1344<A href="#registerag"><tt>INITIALIZE_AG_PASS</tt></a> template to join the 1345implementation pool. Also, a default implementation of the interface 1346<b>must</b> be registered with <A 1347href="#registerag"><tt>RegisterAnalysisGroup</tt></a>.</p> 1348 1349<p>As a concrete example of an Analysis Group in action, consider the <a 1350href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a> 1351analysis group. The default implementation of the alias analysis interface (the 1352<tt><a 1353href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">basicaa</a></tt> 1354pass) just does a few simple checks that don't require significant analysis to 1355compute (such as: two different globals can never alias each other, etc). 1356Passes that use the <tt><a 1357href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt> 1358interface (for example the <tt><a 1359href="http://llvm.org/doxygen/structGCSE.html">gcse</a></tt> pass), do 1360not care which implementation of alias analysis is actually provided, they just 1361use the designated interface.</p> 1362 1363<p>From the user's perspective, commands work just like normal. Issuing the 1364command '<tt>opt -gcse ...</tt>' will cause the <tt>basicaa</tt> class to be 1365instantiated and added to the pass sequence. Issuing the command '<tt>opt 1366-somefancyaa -gcse ...</tt>' will cause the <tt>gcse</tt> pass to use the 1367<tt>somefancyaa</tt> alias analysis (which doesn't actually exist, it's just a 1368hypothetical example) instead.</p> 1369 1370</div> 1371 1372<!-- _______________________________________________________________________ --> 1373<h4> 1374 <a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a> 1375</h4> 1376 1377<div> 1378 1379<p>The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis 1380group itself, while the <tt>INITIALIZE_AG_PASS</tt> is used to add pass 1381implementations to the analysis group. First, 1382an analysis group should be registered, with a human readable name 1383provided for it. 1384Unlike registration of passes, there is no command line argument to be specified 1385for the Analysis Group Interface itself, because it is "abstract":</p> 1386 1387<div class="doc_code"><pre> 1388 <b>static</b> RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> A("<i>Alias Analysis</i>"); 1389</pre></div> 1390 1391<p>Once the analysis is registered, passes can declare that they are valid 1392implementations of the interface by using the following code:</p> 1393 1394<div class="doc_code"><pre> 1395<b>namespace</b> { 1396 //<i> Declare that we implement the AliasAnalysis interface</i> 1397 INITIALIZE_AG_PASS(FancyAA, <a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, "<i>somefancyaa</i>", 1398 "<i>A more complex alias analysis implementation</i>", 1399 false, // <i>Is CFG Only?</i> 1400 true, // <i>Is Analysis?</i> 1401 false, // <i>Is default Analysis Group implementation?</i> 1402 ); 1403} 1404</pre></div> 1405 1406<p>This just shows a class <tt>FancyAA</tt> that 1407uses the <tt>INITIALIZE_AG_PASS</tt> macro both to register and 1408to "join" the <tt><a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt> 1409analysis group. Every implementation of an analysis group should join using 1410this macro.</p> 1411 1412<div class="doc_code"><pre> 1413<b>namespace</b> { 1414 //<i> Declare that we implement the AliasAnalysis interface</i> 1415 INITIALIZE_AG_PASS(BasicAA, <a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, "<i>basicaa</i>", 1416 "<i>Basic Alias Analysis (default AA impl)</i>", 1417 false, // <i>Is CFG Only?</i> 1418 true, // <i>Is Analysis?</i> 1419 true, // <i>Is default Analysis Group implementation?</i> 1420 ); 1421} 1422</pre></div> 1423 1424<p>Here we show how the default implementation is specified (using the final 1425argument to the <tt>INITIALIZE_AG_PASS</tt> template). There must be exactly 1426one default implementation available at all times for an Analysis Group to be 1427used. Only default implementation can derive from <tt>ImmutablePass</tt>. 1428Here we declare that the 1429 <tt><a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a></tt> 1430pass is the default implementation for the interface.</p> 1431 1432</div> 1433 1434</div> 1435 1436<!-- *********************************************************************** --> 1437<h2> 1438 <a name="passStatistics">Pass Statistics</a> 1439</h2> 1440<!-- *********************************************************************** --> 1441 1442<div> 1443<p>The <a 1444href="http://llvm.org/doxygen/Statistic_8h-source.html"><tt>Statistic</tt></a> 1445class is designed to be an easy way to expose various success 1446metrics from passes. These statistics are printed at the end of a 1447run, when the -stats command line option is enabled on the command 1448line. See the <a href="http://llvm.org/docs/ProgrammersManual.html#Statistic">Statistics section</a> in the Programmer's Manual for details. 1449 1450</div> 1451 1452 1453<!-- *********************************************************************** --> 1454<h2> 1455 <a name="passmanager">What PassManager does</a> 1456</h2> 1457<!-- *********************************************************************** --> 1458 1459<div> 1460 1461<p>The <a 1462href="http://llvm.org/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a> 1463<a 1464href="http://llvm.org/doxygen/classllvm_1_1PassManager.html">class</a> 1465takes a list of passes, ensures their <a href="#interaction">prerequisites</a> 1466are set up correctly, and then schedules passes to run efficiently. All of the 1467LLVM tools that run passes use the <tt>PassManager</tt> for execution of these 1468passes.</p> 1469 1470<p>The <tt>PassManager</tt> does two main things to try to reduce the execution 1471time of a series of passes:</p> 1472 1473<ol> 1474<li><b>Share analysis results</b> - The PassManager attempts to avoid 1475recomputing analysis results as much as possible. This means keeping track of 1476which analyses are available already, which analyses get invalidated, and which 1477analyses are needed to be run for a pass. An important part of work is that the 1478<tt>PassManager</tt> tracks the exact lifetime of all analysis results, allowing 1479it to <a href="#releaseMemory">free memory</a> allocated to holding analysis 1480results as soon as they are no longer needed.</li> 1481 1482<li><b>Pipeline the execution of passes on the program</b> - The 1483<tt>PassManager</tt> attempts to get better cache and memory usage behavior out 1484of a series of passes by pipelining the passes together. This means that, given 1485a series of consecutive <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s, it 1486will execute all of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s on 1487the first function, then all of the <a 1488href="#FunctionPass"><tt>FunctionPass</tt></a>es on the second function, 1489etc... until the entire program has been run through the passes. 1490 1491<p>This improves the cache behavior of the compiler, because it is only touching 1492the LLVM program representation for a single function at a time, instead of 1493traversing the entire program. It reduces the memory consumption of compiler, 1494because, for example, only one <a 1495href="http://llvm.org/doxygen/classllvm_1_1DominatorSet.html"><tt>DominatorSet</tt></a> 1496needs to be calculated at a time. This also makes it possible to implement 1497some <a 1498href="#SMP">interesting enhancements</a> in the future.</p></li> 1499 1500</ol> 1501 1502<p>The effectiveness of the <tt>PassManager</tt> is influenced directly by how 1503much information it has about the behaviors of the passes it is scheduling. For 1504example, the "preserved" set is intentionally conservative in the face of an 1505unimplemented <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method. 1506Not implementing when it should be implemented will have the effect of not 1507allowing any analysis results to live across the execution of your pass.</p> 1508 1509<p>The <tt>PassManager</tt> class exposes a <tt>--debug-pass</tt> command line 1510options that is useful for debugging pass execution, seeing how things work, and 1511diagnosing when you should be preserving more analyses than you currently are 1512(To get information about all of the variants of the <tt>--debug-pass</tt> 1513option, just type '<tt>opt -help-hidden</tt>').</p> 1514 1515<p>By using the <tt>--debug-pass=Structure</tt> option, for example, we can see 1516how our <a href="#basiccode">Hello World</a> pass interacts with other passes. 1517Lets try it out with the <tt>gcse</tt> and <tt>licm</tt> passes:</p> 1518 1519<div class="doc_code"><pre> 1520$ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null 1521Module Pass Manager 1522 Function Pass Manager 1523 Dominator Set Construction 1524 Immediate Dominators Construction 1525 Global Common Subexpression Elimination 1526-- Immediate Dominators Construction 1527-- Global Common Subexpression Elimination 1528 Natural Loop Construction 1529 Loop Invariant Code Motion 1530-- Natural Loop Construction 1531-- Loop Invariant Code Motion 1532 Module Verifier 1533-- Dominator Set Construction 1534-- Module Verifier 1535 Bitcode Writer 1536--Bitcode Writer 1537</pre></div> 1538 1539<p>This output shows us when passes are constructed and when the analysis 1540results are known to be dead (prefixed with '<tt>--</tt>'). Here we see that 1541GCSE uses dominator and immediate dominator information to do its job. The LICM 1542pass uses natural loop information, which uses dominator sets, but not immediate 1543dominators. Because immediate dominators are no longer useful after the GCSE 1544pass, it is immediately destroyed. The dominator sets are then reused to 1545compute natural loop information, which is then used by the LICM pass.</p> 1546 1547<p>After the LICM pass, the module verifier runs (which is automatically added 1548by the '<tt>opt</tt>' tool), which uses the dominator set to check that the 1549resultant LLVM code is well formed. After it finishes, the dominator set 1550information is destroyed, after being computed once, and shared by three 1551passes.</p> 1552 1553<p>Lets see how this changes when we run the <a href="#basiccode">Hello 1554World</a> pass in between the two passes:</p> 1555 1556<div class="doc_code"><pre> 1557$ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null 1558Module Pass Manager 1559 Function Pass Manager 1560 Dominator Set Construction 1561 Immediate Dominators Construction 1562 Global Common Subexpression Elimination 1563<b>-- Dominator Set Construction</b> 1564-- Immediate Dominators Construction 1565-- Global Common Subexpression Elimination 1566<b> Hello World Pass 1567-- Hello World Pass 1568 Dominator Set Construction</b> 1569 Natural Loop Construction 1570 Loop Invariant Code Motion 1571-- Natural Loop Construction 1572-- Loop Invariant Code Motion 1573 Module Verifier 1574-- Dominator Set Construction 1575-- Module Verifier 1576 Bitcode Writer 1577--Bitcode Writer 1578Hello: __main 1579Hello: puts 1580Hello: main 1581</pre></div> 1582 1583<p>Here we see that the <a href="#basiccode">Hello World</a> pass has killed the 1584Dominator Set pass, even though it doesn't modify the code at all! To fix this, 1585we need to add the following <a 1586href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method to our pass:</p> 1587 1588<div class="doc_code"><pre> 1589 <i>// We don't modify the program, so we preserve all analyses</i> 1590 <b>virtual void</b> getAnalysisUsage(AnalysisUsage &AU) <b>const</b> { 1591 AU.setPreservesAll(); 1592 } 1593</pre></div> 1594 1595<p>Now when we run our pass, we get this output:</p> 1596 1597<div class="doc_code"><pre> 1598$ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null 1599Pass Arguments: -gcse -hello -licm 1600Module Pass Manager 1601 Function Pass Manager 1602 Dominator Set Construction 1603 Immediate Dominators Construction 1604 Global Common Subexpression Elimination 1605-- Immediate Dominators Construction 1606-- Global Common Subexpression Elimination 1607 Hello World Pass 1608-- Hello World Pass 1609 Natural Loop Construction 1610 Loop Invariant Code Motion 1611-- Loop Invariant Code Motion 1612-- Natural Loop Construction 1613 Module Verifier 1614-- Dominator Set Construction 1615-- Module Verifier 1616 Bitcode Writer 1617--Bitcode Writer 1618Hello: __main 1619Hello: puts 1620Hello: main 1621</pre></div> 1622 1623<p>Which shows that we don't accidentally invalidate dominator information 1624anymore, and therefore do not have to compute it twice.</p> 1625 1626<!-- _______________________________________________________________________ --> 1627<h4> 1628 <a name="releaseMemory">The <tt>releaseMemory</tt> method</a> 1629</h4> 1630 1631<div> 1632 1633<div class="doc_code"><pre> 1634 <b>virtual void</b> releaseMemory(); 1635</pre></div> 1636 1637<p>The <tt>PassManager</tt> automatically determines when to compute analysis 1638results, and how long to keep them around for. Because the lifetime of the pass 1639object itself is effectively the entire duration of the compilation process, we 1640need some way to free analysis results when they are no longer useful. The 1641<tt>releaseMemory</tt> virtual method is the way to do this.</p> 1642 1643<p>If you are writing an analysis or any other pass that retains a significant 1644amount of state (for use by another pass which "requires" your pass and uses the 1645<a href="#getAnalysis">getAnalysis</a> method) you should implement 1646<tt>releaseMemory</tt> to, well, release the memory allocated to maintain this 1647internal state. This method is called after the <tt>run*</tt> method for the 1648class, before the next call of <tt>run*</tt> in your pass.</p> 1649 1650</div> 1651 1652</div> 1653 1654<!-- *********************************************************************** --> 1655<h2> 1656 <a name="registering">Registering dynamically loaded passes</a> 1657</h2> 1658<!-- *********************************************************************** --> 1659 1660<div> 1661 1662<p><i>Size matters</i> when constructing production quality tools using llvm, 1663both for the purposes of distribution, and for regulating the resident code size 1664when running on the target system. Therefore, it becomes desirable to 1665selectively use some passes, while omitting others and maintain the flexibility 1666to change configurations later on. You want to be able to do all this, and, 1667provide feedback to the user. This is where pass registration comes into 1668play.</p> 1669 1670<p>The fundamental mechanisms for pass registration are the 1671<tt>MachinePassRegistry</tt> class and subclasses of 1672<tt>MachinePassRegistryNode</tt>.</p> 1673 1674<p>An instance of <tt>MachinePassRegistry</tt> is used to maintain a list of 1675<tt>MachinePassRegistryNode</tt> objects. This instance maintains the list and 1676communicates additions and deletions to the command line interface.</p> 1677 1678<p>An instance of <tt>MachinePassRegistryNode</tt> subclass is used to maintain 1679information provided about a particular pass. This information includes the 1680command line name, the command help string and the address of the function used 1681to create an instance of the pass. A global static constructor of one of these 1682instances <i>registers</i> with a corresponding <tt>MachinePassRegistry</tt>, 1683the static destructor <i>unregisters</i>. Thus a pass that is statically linked 1684in the tool will be registered at start up. A dynamically loaded pass will 1685register on load and unregister at unload.</p> 1686 1687<!-- _______________________________________________________________________ --> 1688<h3> 1689 <a name="registering_existing">Using existing registries</a> 1690</h3> 1691 1692<div> 1693 1694<p>There are predefined registries to track instruction scheduling 1695(<tt>RegisterScheduler</tt>) and register allocation (<tt>RegisterRegAlloc</tt>) 1696machine passes. Here we will describe how to <i>register</i> a register 1697allocator machine pass.</p> 1698 1699<p>Implement your register allocator machine pass. In your register allocator 1700.cpp file add the following include;</p> 1701 1702<div class="doc_code"><pre> 1703 #include "llvm/CodeGen/RegAllocRegistry.h" 1704</pre></div> 1705 1706<p>Also in your register allocator .cpp file, define a creator function in the 1707form; </p> 1708 1709<div class="doc_code"><pre> 1710 FunctionPass *createMyRegisterAllocator() { 1711 return new MyRegisterAllocator(); 1712 } 1713</pre></div> 1714 1715<p>Note that the signature of this function should match the type of 1716<tt>RegisterRegAlloc::FunctionPassCtor</tt>. In the same file add the 1717"installing" declaration, in the form;</p> 1718 1719<div class="doc_code"><pre> 1720 static RegisterRegAlloc myRegAlloc("myregalloc", 1721 " my register allocator help string", 1722 createMyRegisterAllocator); 1723</pre></div> 1724 1725<p>Note the two spaces prior to the help string produces a tidy result on the 1726-help query.</p> 1727 1728<div class="doc_code"><pre> 1729$ llc -help 1730 ... 1731 -regalloc - Register allocator to use (default=linearscan) 1732 =linearscan - linear scan register allocator 1733 =local - local register allocator 1734 =simple - simple register allocator 1735 =myregalloc - my register allocator help string 1736 ... 1737</pre></div> 1738 1739<p>And that's it. The user is now free to use <tt>-regalloc=myregalloc</tt> as 1740an option. Registering instruction schedulers is similar except use the 1741<tt>RegisterScheduler</tt> class. Note that the 1742<tt>RegisterScheduler::FunctionPassCtor</tt> is significantly different from 1743<tt>RegisterRegAlloc::FunctionPassCtor</tt>.</p> 1744 1745<p>To force the load/linking of your register allocator into the llc/lli tools, 1746add your creator function's global declaration to "Passes.h" and add a "pseudo" 1747call line to <tt>llvm/Codegen/LinkAllCodegenComponents.h</tt>.</p> 1748 1749</div> 1750 1751 1752<!-- _______________________________________________________________________ --> 1753<h3> 1754 <a name="registering_new">Creating new registries</a> 1755</h3> 1756 1757<div> 1758 1759<p>The easiest way to get started is to clone one of the existing registries; we 1760recommend <tt>llvm/CodeGen/RegAllocRegistry.h</tt>. The key things to modify 1761are the class name and the <tt>FunctionPassCtor</tt> type.</p> 1762 1763<p>Then you need to declare the registry. Example: if your pass registry is 1764<tt>RegisterMyPasses</tt> then define;</p> 1765 1766<div class="doc_code"><pre> 1767MachinePassRegistry RegisterMyPasses::Registry; 1768</pre></div> 1769 1770<p>And finally, declare the command line option for your passes. Example:</p> 1771 1772<div class="doc_code"><pre> 1773 cl::opt<RegisterMyPasses::FunctionPassCtor, false, 1774 RegisterPassParser<RegisterMyPasses> > 1775 MyPassOpt("mypass", 1776 cl::init(&createDefaultMyPass), 1777 cl::desc("my pass option help")); 1778</pre></div> 1779 1780<p>Here the command option is "mypass", with createDefaultMyPass as the default 1781creator.</p> 1782 1783</div> 1784 1785</div> 1786 1787<!-- *********************************************************************** --> 1788<h2> 1789 <a name="debughints">Using GDB with dynamically loaded passes</a> 1790</h2> 1791<!-- *********************************************************************** --> 1792 1793<div> 1794 1795<p>Unfortunately, using GDB with dynamically loaded passes is not as easy as it 1796should be. First of all, you can't set a breakpoint in a shared object that has 1797not been loaded yet, and second of all there are problems with inlined functions 1798in shared objects. Here are some suggestions to debugging your pass with 1799GDB.</p> 1800 1801<p>For sake of discussion, I'm going to assume that you are debugging a 1802transformation invoked by <tt>opt</tt>, although nothing described here depends 1803on that.</p> 1804 1805<!-- _______________________________________________________________________ --> 1806<h4> 1807 <a name="breakpoint">Setting a breakpoint in your pass</a> 1808</h4> 1809 1810<div> 1811 1812<p>First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:</p> 1813 1814<div class="doc_code"><pre> 1815$ <b>gdb opt</b> 1816GNU gdb 5.0 1817Copyright 2000 Free Software Foundation, Inc. 1818GDB is free software, covered by the GNU General Public License, and you are 1819welcome to change it and/or distribute copies of it under certain conditions. 1820Type "show copying" to see the conditions. 1821There is absolutely no warranty for GDB. Type "show warranty" for details. 1822This GDB was configured as "sparc-sun-solaris2.6"... 1823(gdb) 1824</pre></div> 1825 1826<p>Note that <tt>opt</tt> has a lot of debugging information in it, so it takes 1827time to load. Be patient. Since we cannot set a breakpoint in our pass yet 1828(the shared object isn't loaded until runtime), we must execute the process, and 1829have it stop before it invokes our pass, but after it has loaded the shared 1830object. The most foolproof way of doing this is to set a breakpoint in 1831<tt>PassManager::run</tt> and then run the process with the arguments you 1832want:</p> 1833 1834<div class="doc_code"><pre> 1835(gdb) <b>break llvm::PassManager::run</b> 1836Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70. 1837(gdb) <b>run test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption]</b> 1838Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption] 1839Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70 184070 bool PassManager::run(Module &M) { return PM->run(M); } 1841(gdb) 1842</pre></div> 1843 1844<p>Once the <tt>opt</tt> stops in the <tt>PassManager::run</tt> method you are 1845now free to set breakpoints in your pass so that you can trace through execution 1846or do other standard debugging stuff.</p> 1847 1848</div> 1849 1850<!-- _______________________________________________________________________ --> 1851<h4> 1852 <a name="debugmisc">Miscellaneous Problems</a> 1853</h4> 1854 1855<div> 1856 1857<p>Once you have the basics down, there are a couple of problems that GDB has, 1858some with solutions, some without.</p> 1859 1860<ul> 1861<li>Inline functions have bogus stack information. In general, GDB does a 1862pretty good job getting stack traces and stepping through inline functions. 1863When a pass is dynamically loaded however, it somehow completely loses this 1864capability. The only solution I know of is to de-inline a function (move it 1865from the body of a class to a .cpp file).</li> 1866 1867<li>Restarting the program breaks breakpoints. After following the information 1868above, you have succeeded in getting some breakpoints planted in your pass. Nex 1869thing you know, you restart the program (i.e., you type '<tt>run</tt>' again), 1870and you start getting errors about breakpoints being unsettable. The only way I 1871have found to "fix" this problem is to <tt>delete</tt> the breakpoints that are 1872already set in your pass, run the program, and re-set the breakpoints once 1873execution stops in <tt>PassManager::run</tt>.</li> 1874 1875</ul> 1876 1877<p>Hopefully these tips will help with common case debugging situations. If 1878you'd like to contribute some tips of your own, just contact <a 1879href="mailto:sabre@nondot.org">Chris</a>.</p> 1880 1881</div> 1882 1883</div> 1884 1885<!-- *********************************************************************** --> 1886<h2> 1887 <a name="future">Future extensions planned</a> 1888</h2> 1889<!-- *********************************************************************** --> 1890 1891<div> 1892 1893<p>Although the LLVM Pass Infrastructure is very capable as it stands, and does 1894some nifty stuff, there are things we'd like to add in the future. Here is 1895where we are going:</p> 1896 1897<!-- _______________________________________________________________________ --> 1898<h4> 1899 <a name="SMP">Multithreaded LLVM</a> 1900</h4> 1901 1902<div> 1903 1904<p>Multiple CPU machines are becoming more common and compilation can never be 1905fast enough: obviously we should allow for a multithreaded compiler. Because of 1906the semantics defined for passes above (specifically they cannot maintain state 1907across invocations of their <tt>run*</tt> methods), a nice clean way to 1908implement a multithreaded compiler would be for the <tt>PassManager</tt> class 1909to create multiple instances of each pass object, and allow the separate 1910instances to be hacking on different parts of the program at the same time.</p> 1911 1912<p>This implementation would prevent each of the passes from having to implement 1913multithreaded constructs, requiring only the LLVM core to have locking in a few 1914places (for global resources). Although this is a simple extension, we simply 1915haven't had time (or multiprocessor machines, thus a reason) to implement this. 1916Despite that, we have kept the LLVM passes SMP ready, and you should too.</p> 1917 1918</div> 1919 1920</div> 1921 1922<!-- *********************************************************************** --> 1923<hr> 1924<address> 1925 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 1926 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 1927 <a href="http://validator.w3.org/check/referer"><img 1928 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 1929 1930 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br> 1931 <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br> 1932 Last modified: $Date$ 1933</address> 1934 1935</body> 1936</html> 1937