1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3<html> 4<head> 5 <title>Exception Handling in LLVM</title> 6 <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 7 <meta name="description" 8 content="Exception Handling in LLVM."> 9 <link rel="stylesheet" href="llvm.css" type="text/css"> 10</head> 11 12<body> 13 14<h1>Exception Handling in LLVM</h1> 15 16<table class="layout" style="width:100%"> 17 <tr class="layout"> 18 <td class="left"> 19<ul> 20 <li><a href="#introduction">Introduction</a> 21 <ol> 22 <li><a href="#itanium">Itanium ABI Zero-cost Exception Handling</a></li> 23 <li><a href="#sjlj">Setjmp/Longjmp Exception Handling</a></li> 24 <li><a href="#overview">Overview</a></li> 25 </ol></li> 26 <li><a href="#codegen">LLVM Code Generation</a> 27 <ol> 28 <li><a href="#throw">Throw</a></li> 29 <li><a href="#try_catch">Try/Catch</a></li> 30 <li><a href="#cleanups">Cleanups</a></li> 31 <li><a href="#throw_filters">Throw Filters</a></li> 32 <li><a href="#restrictions">Restrictions</a></li> 33 </ol></li> 34 <li><a href="#format_common_intrinsics">Exception Handling Intrinsics</a> 35 <ol> 36 <li><a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a></li> 37 <li><a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a></li> 38 <li><a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a></li> 39 <li><a href="#llvm_eh_sjlj_lsda"><tt>llvm.eh.sjlj.lsda</tt></a></li> 40 <li><a href="#llvm_eh_sjlj_callsite"><tt>llvm.eh.sjlj.callsite</tt></a></li> 41 </ol></li> 42 <li><a href="#asm">Asm Table Formats</a> 43 <ol> 44 <li><a href="#unwind_tables">Exception Handling Frame</a></li> 45 <li><a href="#exception_tables">Exception Tables</a></li> 46 </ol></li> 47</ul> 48</td> 49</tr></table> 50 51<div class="doc_author"> 52 <p>Written by the <a href="http://llvm.org/">LLVM Team</a></p> 53</div> 54 55 56<!-- *********************************************************************** --> 57<h2><a name="introduction">Introduction</a></h2> 58<!-- *********************************************************************** --> 59 60<div> 61 62<p>This document is the central repository for all information pertaining to 63 exception handling in LLVM. It describes the format that LLVM exception 64 handling information takes, which is useful for those interested in creating 65 front-ends or dealing directly with the information. Further, this document 66 provides specific examples of what exception handling information is used for 67 in C and C++.</p> 68 69<!-- ======================================================================= --> 70<h3> 71 <a name="itanium">Itanium ABI Zero-cost Exception Handling</a> 72</h3> 73 74<div> 75 76<p>Exception handling for most programming languages is designed to recover from 77 conditions that rarely occur during general use of an application. To that 78 end, exception handling should not interfere with the main flow of an 79 application's algorithm by performing checkpointing tasks, such as saving the 80 current pc or register state.</p> 81 82<p>The Itanium ABI Exception Handling Specification defines a methodology for 83 providing outlying data in the form of exception tables without inlining 84 speculative exception handling code in the flow of an application's main 85 algorithm. Thus, the specification is said to add "zero-cost" to the normal 86 execution of an application.</p> 87 88<p>A more complete description of the Itanium ABI exception handling runtime 89 support of can be found at 90 <a href="http://www.codesourcery.com/cxx-abi/abi-eh.html">Itanium C++ ABI: 91 Exception Handling</a>. A description of the exception frame format can be 92 found at 93 <a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html">Exception 94 Frames</a>, with details of the DWARF 4 specification at 95 <a href="http://dwarfstd.org/Dwarf4Std.php">DWARF 4 Standard</a>. 96 A description for the C++ exception table formats can be found at 97 <a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling 98 Tables</a>.</p> 99 100</div> 101 102<!-- ======================================================================= --> 103<h3> 104 <a name="sjlj">Setjmp/Longjmp Exception Handling</a> 105</h3> 106 107<div> 108 109<p>Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics 110 <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a> and 111 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to 112 handle control flow for exception handling.</p> 113 114<p>For each function which does exception processing — be 115 it <tt>try</tt>/<tt>catch</tt> blocks or cleanups — that function 116 registers itself on a global frame list. When exceptions are unwinding, the 117 runtime uses this list to identify which functions need processing.<p> 118 119<p>Landing pad selection is encoded in the call site entry of the function 120 context. The runtime returns to the function via 121 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>, where 122 a switch table transfers control to the appropriate landing pad based on 123 the index stored in the function context.</p> 124 125<p>In contrast to DWARF exception handling, which encodes exception regions 126 and frame information in out-of-line tables, SJLJ exception handling 127 builds and removes the unwind frame context at runtime. This results in 128 faster exception handling at the expense of slower execution when no 129 exceptions are thrown. As exceptions are, by their nature, intended for 130 uncommon code paths, DWARF exception handling is generally preferred to 131 SJLJ.</p> 132 133</div> 134 135<!-- ======================================================================= --> 136<h3> 137 <a name="overview">Overview</a> 138</h3> 139 140<div> 141 142<p>When an exception is thrown in LLVM code, the runtime does its best to find a 143 handler suited to processing the circumstance.</p> 144 145<p>The runtime first attempts to find an <i>exception frame</i> corresponding to 146 the function where the exception was thrown. If the programming language 147 supports exception handling (e.g. C++), the exception frame contains a 148 reference to an exception table describing how to process the exception. If 149 the language does not support exception handling (e.g. C), or if the 150 exception needs to be forwarded to a prior activation, the exception frame 151 contains information about how to unwind the current activation and restore 152 the state of the prior activation. This process is repeated until the 153 exception is handled. If the exception is not handled and no activations 154 remain, then the application is terminated with an appropriate error 155 message.</p> 156 157<p>Because different programming languages have different behaviors when 158 handling exceptions, the exception handling ABI provides a mechanism for 159 supplying <i>personalities</i>. An exception handling personality is defined 160 by way of a <i>personality function</i> (e.g. <tt>__gxx_personality_v0</tt> 161 in C++), which receives the context of the exception, an <i>exception 162 structure</i> containing the exception object type and value, and a reference 163 to the exception table for the current function. The personality function 164 for the current compile unit is specified in a <i>common exception 165 frame</i>.</p> 166 167<p>The organization of an exception table is language dependent. For C++, an 168 exception table is organized as a series of code ranges defining what to do 169 if an exception occurs in that range. Typically, the information associated 170 with a range defines which types of exception objects (using C++ <i>type 171 info</i>) that are handled in that range, and an associated action that 172 should take place. Actions typically pass control to a <i>landing 173 pad</i>.</p> 174 175<p>A landing pad corresponds roughly to the code found in the <tt>catch</tt> 176 portion of a <tt>try</tt>/<tt>catch</tt> sequence. When execution resumes at 177 a landing pad, it receives an <i>exception structure</i> and a 178 <i>selector value</i> corresponding to the <i>type</i> of exception 179 thrown. The selector is then used to determine which <i>catch</i> should 180 actually process the exception.</p> 181 182</div> 183 184</div> 185 186<!-- ======================================================================= --> 187<h2> 188 <a name="codegen">LLVM Code Generation</a> 189</h2> 190 191<div> 192 193<p>From a C++ developer's perspective, exceptions are defined in terms of the 194 <tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements. In this section 195 we will describe the implementation of LLVM exception handling in terms of 196 C++ examples.</p> 197 198<!-- ======================================================================= --> 199<h3> 200 <a name="throw">Throw</a> 201</h3> 202 203<div> 204 205<p>Languages that support exception handling typically provide a <tt>throw</tt> 206 operation to initiate the exception process. Internally, a <tt>throw</tt> 207 operation breaks down into two steps.</p> 208 209<ol> 210 <li>A request is made to allocate exception space for an exception structure. 211 This structure needs to survive beyond the current activation. This 212 structure will contain the type and value of the object being thrown.</li> 213 214 <li>A call is made to the runtime to raise the exception, passing the 215 exception structure as an argument.</li> 216</ol> 217 218<p>In C++, the allocation of the exception structure is done by the 219 <tt>__cxa_allocate_exception</tt> runtime function. The exception raising is 220 handled by <tt>__cxa_throw</tt>. The type of the exception is represented 221 using a C++ RTTI structure.</p> 222 223</div> 224 225<!-- ======================================================================= --> 226<h3> 227 <a name="try_catch">Try/Catch</a> 228</h3> 229 230<div> 231 232<p>A call within the scope of a <i>try</i> statement can potentially raise an 233 exception. In those circumstances, the LLVM C++ front-end replaces the call 234 with an <tt>invoke</tt> instruction. Unlike a call, the <tt>invoke</tt> has 235 two potential continuation points:</p> 236 237<ol> 238 <li>where to continue when the call succeeds as per normal, and</li> 239 240 <li>where to continue if the call raises an exception, either by a throw or 241 the unwinding of a throw</li> 242</ol> 243 244<p>The term used to define a the place where an <tt>invoke</tt> continues after 245 an exception is called a <i>landing pad</i>. LLVM landing pads are 246 conceptually alternative function entry points where an exception structure 247 reference and a type info index are passed in as arguments. The landing pad 248 saves the exception structure reference and then proceeds to select the catch 249 block that corresponds to the type info of the exception object.</p> 250 251<p>The LLVM <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> 252 instruction</a> is used to convey information about the landing pad to the 253 back end. For C++, the <tt>landingpad</tt> instruction returns a pointer and 254 integer pair corresponding to the pointer to the <i>exception structure</i> 255 and the <i>selector value</i> respectively.</p> 256 257<p>The <tt>landingpad</tt> instruction takes a reference to the personality 258 function to be used for this <tt>try</tt>/<tt>catch</tt> sequence. The 259 remainder of the instruction is a list of <i>cleanup</i>, <i>catch</i>, 260 and <i>filter</i> clauses. The exception is tested against the clauses 261 sequentially from first to last. The selector value is a positive number if 262 the exception matched a type info, a negative number if it matched a filter, 263 and zero if it matched a cleanup. If nothing is matched, the behavior of 264 the program is <a href="#restrictions">undefined</a>. If a type info matched, 265 then the selector value is the index of the type info in the exception table, 266 which can be obtained using the 267 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p> 268 269<p>Once the landing pad has the type info selector, the code branches to the 270 code for the first catch. The catch then checks the value of the type info 271 selector against the index of type info for that catch. Since the type info 272 index is not known until all the type infos have been gathered in the 273 backend, the catch code must call the 274 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic to 275 determine the index for a given type info. If the catch fails to match the 276 selector then control is passed on to the next catch.</p> 277 278<p>Finally, the entry and exit of catch code is bracketed with calls to 279 <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p> 280 281<ul> 282 <li><tt>__cxa_begin_catch</tt> takes an exception structure reference as an 283 argument and returns the value of the exception object.</li> 284 285 <li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br> 286 <ol> 287 <li>Locates the most recently caught exception and decrements its handler 288 count,</li> 289 <li>Removes the exception from the <i>caught</i> stack if the handler 290 count goes to zero, and</li> 291 <li>Destroys the exception if the handler count goes to zero and the 292 exception was not re-thrown by throw.</li> 293 </ol> 294 <p><b>Note:</b> a rethrow from within the catch may replace this call with 295 a <tt>__cxa_rethrow</tt>.</p></li> 296</ul> 297 298</div> 299 300<!-- ======================================================================= --> 301<h3> 302 <a name="cleanups">Cleanups</a> 303</h3> 304 305<div> 306 307<p>A cleanup is extra code which needs to be run as part of unwinding a scope. 308 C++ destructors are a typical example, but other languages and language 309 extensions provide a variety of different kinds of cleanups. In general, a 310 landing pad may need to run arbitrary amounts of cleanup code before actually 311 entering a catch block. To indicate the presence of cleanups, a 312 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> 313 should have a <i>cleanup</i> clause. Otherwise, the unwinder will not stop at 314 the landing pad if there are no catches or filters that require it to.</p> 315 316<p><b>Note:</b> Do not allow a new exception to propagate out of the execution 317 of a cleanup. This can corrupt the internal state of the unwinder. 318 Different languages describe different high-level semantics for these 319 situations: for example, C++ requires that the process be terminated, whereas 320 Ada cancels both exceptions and throws a third.</p> 321 322<p>When all cleanups are finished, if the exception is not handled by the 323 current function, resume unwinding by calling the 324 <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a>, passing in 325 the result of the <tt>landingpad</tt> instruction for the original landing 326 pad.</p> 327 328</div> 329 330<!-- ======================================================================= --> 331<h3> 332 <a name="throw_filters">Throw Filters</a> 333</h3> 334 335<div> 336 337<p>C++ allows the specification of which exception types may be thrown from a 338 function. To represent this, a top level landing pad may exist to filter out 339 invalid types. To express this in LLVM code the 340 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> will 341 have a filter clause. The clause consists of an array of type infos. 342 <tt>landingpad</tt> will return a negative value if the exception does not 343 match any of the type infos. If no match is found then a call 344 to <tt>__cxa_call_unexpected</tt> should be made, otherwise 345 <tt>_Unwind_Resume</tt>. Each of these functions requires a reference to the 346 exception structure. Note that the most general form of a 347 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> can 348 have any number of catch, cleanup, and filter clauses (though having more 349 than one cleanup is pointless). The LLVM C++ front-end can generate such 350 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instructions</a> due 351 to inlining creating nested exception handling scopes.</p> 352 353</div> 354 355<!-- ======================================================================= --> 356<h3> 357 <a name="restrictions">Restrictions</a> 358</h3> 359 360<div> 361 362<p>The unwinder delegates the decision of whether to stop in a call frame to 363 that call frame's language-specific personality function. Not all unwinders 364 guarantee that they will stop to perform cleanups. For example, the GNU C++ 365 unwinder doesn't do so unless the exception is actually caught somewhere 366 further up the stack.</p> 367 368<p>In order for inlining to behave correctly, landing pads must be prepared to 369 handle selector results that they did not originally advertise. Suppose that 370 a function catches exceptions of type <tt>A</tt>, and it's inlined into a 371 function that catches exceptions of type <tt>B</tt>. The inliner will update 372 the <tt>landingpad</tt> instruction for the inlined landing pad to include 373 the fact that <tt>B</tt> is also caught. If that landing pad assumes that it 374 will only be entered to catch an <tt>A</tt>, it's in for a rude awakening. 375 Consequently, landing pads must test for the selector results they understand 376 and then resume exception propagation with the 377 <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a> if none of 378 the conditions match.</p> 379 380</div> 381 382</div> 383 384<!-- ======================================================================= --> 385<h2> 386 <a name="format_common_intrinsics">Exception Handling Intrinsics</a> 387</h2> 388 389<div> 390 391<p>In addition to the 392 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt></a> and 393 <a href="LangRef.html#i_resume"><tt>resume</tt></a> instructions, LLVM uses 394 several intrinsic functions (name prefixed with <i><tt>llvm.eh</tt></i>) to 395 provide exception handling information at various points in generated 396 code.</p> 397 398<!-- ======================================================================= --> 399<h4> 400 <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a> 401</h4> 402 403<div> 404 405<pre> 406 i32 @llvm.eh.typeid.for(i8* %type_info) 407</pre> 408 409<p>This intrinsic returns the type info index in the exception table of the 410 current function. This value can be used to compare against the result 411 of <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>. 412 The single argument is a reference to a type info.</p> 413 414</div> 415 416<!-- ======================================================================= --> 417<h4> 418 <a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a> 419</h4> 420 421<div> 422 423<pre> 424 i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf) 425</pre> 426 427<p>For SJLJ based exception handling, this intrinsic forces register saving for 428 the current function and stores the address of the following instruction for 429 use as a destination address 430 by <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>. The 431 buffer format and the overall functioning of this intrinsic is compatible 432 with the GCC <tt>__builtin_setjmp</tt> implementation allowing code built 433 with the clang and GCC to interoperate.</p> 434 435<p>The single parameter is a pointer to a five word buffer in which the calling 436 context is saved. The front end places the frame pointer in the first word, 437 and the target implementation of this intrinsic should place the destination 438 address for a 439 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> in the 440 second word. The following three words are available for use in a 441 target-specific manner.</p> 442 443</div> 444 445<!-- ======================================================================= --> 446<h4> 447 <a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a> 448</h4> 449 450<div> 451 452<pre> 453 void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf) 454</pre> 455 456<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.longjmp</tt> 457 intrinsic is used to implement <tt>__builtin_longjmp()</tt>. The single 458 parameter is a pointer to a buffer populated 459 by <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a>. The frame 460 pointer and stack pointer are restored from the buffer, then control is 461 transferred to the destination address.</p> 462 463</div> 464<!-- ======================================================================= --> 465<h4> 466 <a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a> 467</h4> 468 469<div> 470 471<pre> 472 i8* @llvm.eh.sjlj.lsda() 473</pre> 474 475<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.lsda</tt> intrinsic 476 returns the address of the Language Specific Data Area (LSDA) for the current 477 function. The SJLJ front-end code stores this address in the exception 478 handling function context for use by the runtime.</p> 479 480</div> 481 482<!-- ======================================================================= --> 483<h4> 484 <a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a> 485</h4> 486 487<div> 488 489<pre> 490 void @llvm.eh.sjlj.callsite(i32 %call_site_num) 491</pre> 492 493<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.callsite</tt> 494 intrinsic identifies the callsite value associated with the 495 following <tt>invoke</tt> instruction. This is used to ensure that landing 496 pad entries in the LSDA are generated in matching order.</p> 497 498</div> 499 500</div> 501 502<!-- ======================================================================= --> 503<h2> 504 <a name="asm">Asm Table Formats</a> 505</h2> 506 507<div> 508 509<p>There are two tables that are used by the exception handling runtime to 510 determine which actions should be taken when an exception is thrown.</p> 511 512<!-- ======================================================================= --> 513<h3> 514 <a name="unwind_tables">Exception Handling Frame</a> 515</h3> 516 517<div> 518 519<p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind 520 frame used by DWARF debug info. The frame contains all the information 521 necessary to tear down the current frame and restore the state of the prior 522 frame. There is an exception handling frame for each function in a compile 523 unit, plus a common exception handling frame that defines information common 524 to all functions in the unit.</p> 525 526<!-- Todo - Table details here. --> 527 528</div> 529 530<!-- ======================================================================= --> 531<h3> 532 <a name="exception_tables">Exception Tables</a> 533</h3> 534 535<div> 536 537<p>An exception table contains information about what actions to take when an 538 exception is thrown in a particular part of a function's code. There is one 539 exception table per function, except leaf functions and functions that have 540 calls only to non-throwing functions. They do not need an exception 541 table.</p> 542 543<!-- Todo - Table details here. --> 544 545</div> 546 547</div> 548 549<!-- *********************************************************************** --> 550 551<hr> 552<address> 553 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 554 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 555 <a href="http://validator.w3.org/check/referer"><img 556 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 557 558 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> 559 Last modified: $Date$ 560</address> 561 562</body> 563</html> 564