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_exception"><tt>llvm.eh.exception</tt></a></li> 37 <li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a></li> 38 <li><a href="#llvm_eh_resume"><tt>llvm.eh.resume</tt></a></li> 39 <li><a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a></li> 40 <li><a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a></li> 41 <li><a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a></li> 42 <li><a href="#llvm_eh_sjlj_lsda"><tt>llvm.eh.sjlj.lsda</tt></a></li> 43 <li><a href="#llvm_eh_sjlj_callsite"><tt>llvm.eh.sjlj.callsite</tt></a></li> 44 <li><a href="#llvm_eh_sjlj_dispatchsetup"><tt>llvm.eh.sjlj.dispatchsetup</tt></a></li> 45 </ol></li> 46 <li><a href="#asm">Asm Table Formats</a> 47 <ol> 48 <li><a href="#unwind_tables">Exception Handling Frame</a></li> 49 <li><a href="#exception_tables">Exception Tables</a></li> 50 </ol></li> 51 <li><a href="#todo">ToDo</a></li> 52</ul> 53</td> 54</tr></table> 55 56<div class="doc_author"> 57 <p>Written by <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p> 58</div> 59 60 61<!-- *********************************************************************** --> 62<h2><a name="introduction">Introduction</a></h2> 63<!-- *********************************************************************** --> 64 65<div> 66 67<p>This document is the central repository for all information pertaining to 68 exception handling in LLVM. It describes the format that LLVM exception 69 handling information takes, which is useful for those interested in creating 70 front-ends or dealing directly with the information. Further, this document 71 provides specific examples of what exception handling information is used for 72 in C/C++.</p> 73 74<!-- ======================================================================= --> 75<h3> 76 <a name="itanium">Itanium ABI Zero-cost Exception Handling</a> 77</h3> 78 79<div> 80 81<p>Exception handling for most programming languages is designed to recover from 82 conditions that rarely occur during general use of an application. To that 83 end, exception handling should not interfere with the main flow of an 84 application's algorithm by performing checkpointing tasks, such as saving the 85 current pc or register state.</p> 86 87<p>The Itanium ABI Exception Handling Specification defines a methodology for 88 providing outlying data in the form of exception tables without inlining 89 speculative exception handling code in the flow of an application's main 90 algorithm. Thus, the specification is said to add "zero-cost" to the normal 91 execution of an application.</p> 92 93<p>A more complete description of the Itanium ABI exception handling runtime 94 support of can be found at 95 <a href="http://www.codesourcery.com/cxx-abi/abi-eh.html">Itanium C++ ABI: 96 Exception Handling</a>. A description of the exception frame format can be 97 found at 98 <a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html">Exception 99 Frames</a>, with details of the DWARF 3 specification at 100 <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3 Standard</a>. 101 A description for the C++ exception table formats can be found at 102 <a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling 103 Tables</a>.</p> 104 105</div> 106 107<!-- ======================================================================= --> 108<h3> 109 <a name="sjlj">Setjmp/Longjmp Exception Handling</a> 110</h3> 111 112<div> 113 114<p>Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics 115 <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a> and 116 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to 117 handle control flow for exception handling.</p> 118 119<p>For each function which does exception processing, be it try/catch blocks 120 or cleanups, that function registers itself on a global frame list. When 121 exceptions are being unwound, the runtime uses this list to identify which 122 functions need processing.<p> 123 124<p>Landing pad selection is encoded in the call site entry of the function 125 context. The runtime returns to the function via 126 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>, where 127 a switch table transfers control to the appropriate landing pad based on 128 the index stored in the function context.</p> 129 130<p>In contrast to DWARF exception handling, which encodes exception regions 131 and frame information in out-of-line tables, SJLJ exception handling 132 builds and removes the unwind frame context at runtime. This results in 133 faster exception handling at the expense of slower execution when no 134 exceptions are thrown. As exceptions are, by their nature, intended for 135 uncommon code paths, DWARF exception handling is generally preferred to 136 SJLJ.</p> 137</div> 138 139<!-- ======================================================================= --> 140<h3> 141 <a name="overview">Overview</a> 142</h3> 143 144<div> 145 146<p>When an exception is thrown in LLVM code, the runtime does its best to find a 147 handler suited to processing the circumstance.</p> 148 149<p>The runtime first attempts to find an <i>exception frame</i> corresponding to 150 the function where the exception was thrown. If the programming language 151 (e.g. C++) supports exception handling, the exception frame contains a 152 reference to an exception table describing how to process the exception. If 153 the language (e.g. C) does not support exception handling, or if the 154 exception needs to be forwarded to a prior activation, the exception frame 155 contains information about how to unwind the current activation and restore 156 the state of the prior activation. This process is repeated until the 157 exception is handled. If the exception is not handled and no activations 158 remain, then the application is terminated with an appropriate error 159 message.</p> 160 161<p>Because different programming languages have different behaviors when 162 handling exceptions, the exception handling ABI provides a mechanism for 163 supplying <i>personalities.</i> An exception handling personality is defined 164 by way of a <i>personality function</i> (e.g. <tt>__gxx_personality_v0</tt> 165 in C++), which receives the context of the exception, an <i>exception 166 structure</i> containing the exception object type and value, and a reference 167 to the exception table for the current function. The personality function 168 for the current compile unit is specified in a <i>common exception 169 frame</i>.</p> 170 171<p>The organization of an exception table is language dependent. For C++, an 172 exception table is organized as a series of code ranges defining what to do 173 if an exception occurs in that range. Typically, the information associated 174 with a range defines which types of exception objects (using C++ <i>type 175 info</i>) that are handled in that range, and an associated action that 176 should take place. Actions typically pass control to a <i>landing 177 pad</i>.</p> 178 179<p>A landing pad corresponds to the code found in the <i>catch</i> portion of 180 a <i>try</i>/<i>catch</i> sequence. When execution resumes at a landing 181 pad, it receives the exception structure and a selector corresponding to 182 the <i>type</i> of exception thrown. The selector is then used to determine 183 which <i>catch</i> should actually process the exception.</p> 184 185</div> 186 187</div> 188 189<!-- ======================================================================= --> 190<h2> 191 <a name="codegen">LLVM Code Generation</a> 192</h2> 193 194<div> 195 196<p>At the time of this writing, only C++ exception handling support is available 197 in LLVM. So the remainder of this document will be somewhat C++-centric.</p> 198 199<p>From the C++ developers perspective, exceptions are defined in terms of the 200 <tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements. In this section 201 we will describe the implementation of LLVM exception handling in terms of 202 C++ examples.</p> 203 204<!-- ======================================================================= --> 205<h3> 206 <a name="throw">Throw</a> 207</h3> 208 209<div> 210 211<p>Languages that support exception handling typically provide a <tt>throw</tt> 212 operation to initiate the exception process. Internally, a throw operation 213 breaks down into two steps. First, a request is made to allocate exception 214 space for an exception structure. This structure needs to survive beyond the 215 current activation. This structure will contain the type and value of the 216 object being thrown. Second, a call is made to the runtime to raise the 217 exception, passing the exception structure as an argument.</p> 218 219<p>In C++, the allocation of the exception structure is done by 220 the <tt>__cxa_allocate_exception</tt> runtime function. The exception 221 raising is handled by <tt>__cxa_throw</tt>. The type of the exception is 222 represented using a C++ RTTI structure.</p> 223 224</div> 225 226<!-- ======================================================================= --> 227<h3> 228 <a name="try_catch">Try/Catch</a> 229</h3> 230 231<div> 232 233<p>A call within the scope of a <i>try</i> statement can potentially raise an 234 exception. In those circumstances, the LLVM C++ front-end replaces the call 235 with an <tt>invoke</tt> instruction. Unlike a call, the <tt>invoke</tt> has 236 two potential continuation points: where to continue when the call succeeds 237 as per normal; and where to continue if the call raises an exception, either 238 by a throw or the unwinding of a throw.</p> 239 240<p>The term used to define a the place where an <tt>invoke</tt> continues after 241 an exception is called a <i>landing pad</i>. LLVM landing pads are 242 conceptually alternative function entry points where an exception structure 243 reference and a type info index are passed in as arguments. The landing pad 244 saves the exception structure reference and then proceeds to select the catch 245 block that corresponds to the type info of the exception object.</p> 246 247<p>Two LLVM intrinsic functions are used to convey information about the landing 248 pad to the back end.</p> 249 250<ol> 251 <li><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a> takes no 252 arguments and returns a pointer to the exception structure. This only 253 returns a sensible value if called after an <tt>invoke</tt> has branched 254 to a landing pad. Due to code generation limitations, it must currently 255 be called in the landing pad itself.</li> 256 257 <li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum 258 of three arguments. The first argument is the reference to the exception 259 structure. The second argument is a reference to the personality function 260 to be used for this <tt>try</tt>/<tt>catch</tt> sequence. Each of the 261 remaining arguments is either a reference to the type info for 262 a <tt>catch</tt> statement, a <a href="#throw_filters">filter</a> 263 expression, or the number zero (<tt>0</tt>) representing 264 a <a href="#cleanups">cleanup</a>. The exception is tested against the 265 arguments sequentially from first to last. The result of 266 the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a 267 positive number if the exception matched a type info, a negative number if 268 it matched a filter, and zero if it matched a cleanup. If nothing is 269 matched, the behaviour of the program 270 is <a href="#restrictions">undefined</a>. This only returns a sensible 271 value if called after an <tt>invoke</tt> has branched to a landing pad. 272 Due to codegen limitations, it must currently be called in the landing pad 273 itself. If a type info matched, then the selector value is the index of 274 the type info in the exception table, which can be obtained using the 275 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> 276 intrinsic.</li> 277</ol> 278 279<p>Once the landing pad has the type info selector, the code branches to the 280 code for the first catch. The catch then checks the value of the type info 281 selector against the index of type info for that catch. Since the type info 282 index is not known until all the type info have been gathered in the backend, 283 the catch code will call the 284 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic 285 to determine the index for a given type info. If the catch fails to match 286 the selector then control is passed on to the next catch. Note: Since the 287 landing pad will not be used if there is no match in the list of type info on 288 the call to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>, then 289 neither the last catch nor <i>catch all</i> need to perform the check 290 against the selector.</p> 291 292<p>Finally, the entry and exit of catch code is bracketed with calls 293 to <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p> 294 295<ul> 296 <li><tt>__cxa_begin_catch</tt> takes a exception structure reference as an 297 argument and returns the value of the exception object.</li> 298 299 <li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br> 300 <ol> 301 <li>Locates the most recently caught exception and decrements its handler 302 count,</li> 303 <li>Removes the exception from the "caught" stack if the handler count 304 goes to zero, and</li> 305 <li>Destroys the exception if the handler count goes to zero, and the 306 exception was not re-thrown by throw.</li> 307 </ol> 308 <p>Note: a rethrow from within the catch may replace this call with 309 a <tt>__cxa_rethrow</tt>.</p></li> 310</ul> 311 312</div> 313 314<!-- ======================================================================= --> 315<h3> 316 <a name="cleanups">Cleanups</a> 317</h3> 318 319<div> 320 321<p>A cleanup is extra code which needs to be run as part of unwinding 322 a scope. C++ destructors are a prominent example, but other 323 languages and language extensions provide a variety of different 324 kinds of cleanup. In general, a landing pad may need to run 325 arbitrary amounts of cleanup code before actually entering a catch 326 block. To indicate the presence of cleanups, a landing pad's call 327 to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> should 328 end with the argument <tt>i32 0</tt>; otherwise, the unwinder will 329 not stop at the landing pad if there are no catches or filters that 330 require it to.</p> 331 332<p>Do not allow a new exception to propagate out of the execution of a 333 cleanup. This can corrupt the internal state of the unwinder. 334 Different languages describe different high-level semantics for 335 these situations: for example, C++ requires that the process be 336 terminated, whereas Ada cancels both exceptions and throws a third.</p> 337 338<p>When all cleanups have completed, if the exception is not handled 339 by the current function, resume unwinding by calling the 340 <a href="#llvm_eh_resume"><tt>llvm.eh.resume</tt></a> intrinsic, 341 passing in the results of <tt>llvm.eh.exception</tt> and 342 <tt>llvm.eh.selector</tt> for the original landing pad.</p> 343 344</div> 345 346<!-- ======================================================================= --> 347<h3> 348 <a name="throw_filters">Throw Filters</a> 349</h3> 350 351<div> 352 353<p>C++ allows the specification of which exception types can be thrown from a 354 function. To represent this a top level landing pad may exist to filter out 355 invalid types. To express this in LLVM code the landing pad will 356 call <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>. The 357 arguments are a reference to the exception structure, a reference to the 358 personality function, the length of the filter expression (the number of type 359 infos plus one), followed by the type infos themselves. 360 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> will return a 361 negative value if the exception does not match any of the type infos. If no 362 match is found then a call to <tt>__cxa_call_unexpected</tt> should be made, 363 otherwise <tt>_Unwind_Resume</tt>. Each of these functions requires a 364 reference to the exception structure. Note that the most general form of an 365 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> call can contain 366 any number of type infos, filter expressions and cleanups (though having more 367 than one cleanup is pointless). The LLVM C++ front-end can generate such 368 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls due to 369 inlining creating nested exception handling scopes.</p> 370 371</div> 372 373<!-- ======================================================================= --> 374<h3> 375 <a name="restrictions">Restrictions</a> 376</h3> 377 378<div> 379 380<p>The unwinder delegates the decision of whether to stop in a call 381 frame to that call frame's language-specific personality function. 382 Not all personalities functions guarantee that they will stop to 383 perform cleanups: for example, the GNU C++ personality doesn't do 384 so unless the exception is actually caught somewhere further up the 385 stack. When using this personality to implement EH for a language 386 that guarantees that cleanups will always be run, be sure to 387 indicate a catch-all in the 388 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> call 389 rather than just cleanups.</p> 390 391<p>In order for inlining to behave correctly, landing pads must be 392 prepared to handle selector results that they did not originally 393 advertise. Suppose that a function catches exceptions of 394 type <tt>A</tt>, and it's inlined into a function that catches 395 exceptions of type <tt>B</tt>. The inliner will update the 396 selector for the inlined landing pad to include the fact 397 that <tt>B</tt> is caught. If that landing pad assumes that it 398 will only be entered to catch an <tt>A</tt>, it's in for a rude 399 surprise. Consequently, landing pads must test for the selector 400 results they understand and then resume exception propagation 401 with the <a href="#llvm_eh_resume"><tt>llvm.eh.resume</tt></a> 402 intrinsic if none of the conditions match.</p> 403 404</div> 405 406</div> 407 408<!-- ======================================================================= --> 409<h2> 410 <a name="format_common_intrinsics">Exception Handling Intrinsics</a> 411</h2> 412 413<div> 414 415<p>LLVM uses several intrinsic functions (name prefixed with "llvm.eh") to 416 provide exception handling information at various points in generated 417 code.</p> 418 419<!-- ======================================================================= --> 420<h4> 421 <a name="llvm_eh_exception">llvm.eh.exception</a> 422</h4> 423 424<div> 425 426<pre> 427 i8* %<a href="#llvm_eh_exception">llvm.eh.exception</a>() 428</pre> 429 430<p>This intrinsic returns a pointer to the exception structure.</p> 431 432</div> 433 434<!-- ======================================================================= --> 435<h4> 436 <a name="llvm_eh_selector">llvm.eh.selector</a> 437</h4> 438 439<div> 440 441<pre> 442 i32 %<a href="#llvm_eh_selector">llvm.eh.selector</a>(i8*, i8*, ...) 443</pre> 444 445<p>This intrinsic is used to compare the exception with the given type infos, 446 filters and cleanups.</p> 447 448<p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a 449 minimum of three arguments. The first argument is the reference to 450 the exception structure. The second argument is a reference to the 451 personality function to be used for this try catch sequence. Each 452 of the remaining arguments is either a reference to the type info 453 for a catch statement, a <a href="#throw_filters">filter</a> 454 expression, or the number zero representing 455 a <a href="#cleanups">cleanup</a>. The exception is tested against 456 the arguments sequentially from first to last. The result of 457 the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a 458 positive number if the exception matched a type info, a negative 459 number if it matched a filter, and zero if it matched a cleanup. 460 If nothing is matched, or if only a cleanup is matched, different 461 personality functions may or may not cause control to stop at the 462 landing pad; see <a href="#restrictions">the restrictions</a> for 463 more information. If a type info matched then the selector value 464 is the index of the type info in the exception table, which can be 465 obtained using the 466 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p> 467 468<p>If a landing pad containing a call to <tt>llvm.eh.selector</tt> is 469 inlined into an <tt>invoke</tt> instruction, the selector arguments 470 for the outer landing pad are appended to those of the inlined 471 landing pad. Consequently, landing pads must be written to ignore 472 selector values that they did not originally advertise.</p> 473 474</div> 475 476<!-- ======================================================================= --> 477<h4> 478 <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a> 479</h4> 480 481<div> 482 483<pre> 484 i32 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for</a>(i8*) 485</pre> 486 487<p>This intrinsic returns the type info index in the exception table of the 488 current function. This value can be used to compare against the result 489 of <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>. The single 490 argument is a reference to a type info.</p> 491 492</div> 493 494<!-- ======================================================================= --> 495<h4> 496 <a name="llvm_eh_resume">llvm.eh.resume</a> 497</h4> 498 499<div> 500 501<pre> 502 void %<a href="#llvm_eh_resume">llvm.eh.resume</a>(i8*, i32) noreturn 503</pre> 504 505<p>This intrinsic is used to resume propagation of an exception after 506 landing at a landing pad. The first argument should be the result 507 of <a href="#llvm_eh_exception">llvm.eh.exception</a> for that 508 landing pad, and the second argument should be the result of 509 <a href="#llvm_eh_selector">llvm.eh.selector</a>. When a call to 510 this intrinsic is inlined into an invoke, the call is transformed 511 into a branch to the invoke's unwind destination, using its 512 arguments in place of the calls 513 to <a href="#llvm_eh_exception">llvm.eh.exception</a> and 514 <a href="#llvm_eh_selector">llvm.eh.selector</a> there.</p> 515 516<p>This intrinsic is not implicitly <tt>nounwind</tt>; calls to it 517 will always throw. It may not be invoked.</p> 518 519</div> 520 521<!-- ======================================================================= --> 522<h4> 523 <a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a> 524</h4> 525 526<div> 527 528<pre> 529 i32 %<a href="#llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>(i8*) 530</pre> 531 532<p>The SJLJ exception handling uses this intrinsic to force register saving for 533 the current function and to store the address of the following instruction 534 for use as a destination address by <a href="#llvm_eh_sjlj_longjmp"> 535 <tt>llvm.eh.sjlj.longjmp</tt></a>. The buffer format and the overall 536 functioning of this intrinsic is compatible with the GCC 537 <tt>__builtin_setjmp</tt> implementation, allowing code built with the 538 two compilers to interoperate.</p> 539 540<p>The single parameter is a pointer to a five word buffer in which the calling 541 context is saved. The front end places the frame pointer in the first word, 542 and the target implementation of this intrinsic should place the destination 543 address for a 544 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> in the 545 second word. The following three words are available for use in a 546 target-specific manner.</p> 547 548</div> 549 550<!-- ======================================================================= --> 551<h4> 552 <a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a> 553</h4> 554 555<div> 556 557<pre> 558 void %<a href="#llvm_eh_sjlj_longjmp">llvm.eh.sjlj.setjmp</a>(i8*) 559</pre> 560 561<p>The <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> 562 intrinsic is used to implement <tt>__builtin_longjmp()</tt> for SJLJ 563 style exception handling. The single parameter is a pointer to a 564 buffer populated by <a href="#llvm_eh_sjlj_setjmp"> 565 <tt>llvm.eh.sjlj.setjmp</tt></a>. The frame pointer and stack pointer 566 are restored from the buffer, then control is transferred to the 567 destination address.</p> 568 569</div> 570<!-- ======================================================================= --> 571<h4> 572 <a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a> 573</h4> 574 575<div> 576 577<pre> 578 i8* %<a href="#llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>() 579</pre> 580 581<p>Used for SJLJ based exception handling, the <a href="#llvm_eh_sjlj_lsda"> 582 <tt>llvm.eh.sjlj.lsda</tt></a> intrinsic returns the address of the Language 583 Specific Data Area (LSDA) for the current function. The SJLJ front-end code 584 stores this address in the exception handling function context for use by the 585 runtime.</p> 586 587</div> 588 589<!-- ======================================================================= --> 590<h4> 591 <a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a> 592</h4> 593 594<div> 595 596<pre> 597 void %<a href="#llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>(i32) 598</pre> 599 600<p>For SJLJ based exception handling, the <a href="#llvm_eh_sjlj_callsite"> 601 <tt>llvm.eh.sjlj.callsite</tt></a> intrinsic identifies the callsite value 602 associated with the following invoke instruction. This is used to ensure 603 that landing pad entries in the LSDA are generated in the matching order.</p> 604 605</div> 606 607<!-- ======================================================================= --> 608<h4> 609 <a name="llvm_eh_sjlj_dispatchsetup">llvm.eh.sjlj.dispatchsetup</a> 610</h4> 611 612<div> 613 614<pre> 615 void %<a href="#llvm_eh_sjlj_dispatchsetup">llvm.eh.sjlj.dispatchsetup</a>(i32) 616</pre> 617 618<p>For SJLJ based exception handling, the <a href="#llvm_eh_sjlj_dispatchsetup"> 619 <tt>llvm.eh.sjlj.dispatchsetup</tt></a> intrinsic is used by targets to do 620 any unwind-edge setup they need. By default, no action is taken. </p> 621 622</div> 623 624</div> 625 626<!-- ======================================================================= --> 627<h2> 628 <a name="asm">Asm Table Formats</a> 629</h2> 630 631<div> 632 633<p>There are two tables that are used by the exception handling runtime to 634 determine which actions should take place when an exception is thrown.</p> 635 636<!-- ======================================================================= --> 637<h3> 638 <a name="unwind_tables">Exception Handling Frame</a> 639</h3> 640 641<div> 642 643<p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind 644 frame used by dwarf debug info. The frame contains all the information 645 necessary to tear down the current frame and restore the state of the prior 646 frame. There is an exception handling frame for each function in a compile 647 unit, plus a common exception handling frame that defines information common 648 to all functions in the unit.</p> 649 650<p>Todo - Table details here.</p> 651 652</div> 653 654<!-- ======================================================================= --> 655<h3> 656 <a name="exception_tables">Exception Tables</a> 657</h3> 658 659<div> 660 661<p>An exception table contains information about what actions to take when an 662 exception is thrown in a particular part of a function's code. There is one 663 exception table per function except leaf routines and functions that have 664 only calls to non-throwing functions will not need an exception table.</p> 665 666<p>Todo - Table details here.</p> 667 668</div> 669 670</div> 671 672<!-- ======================================================================= --> 673<h2> 674 <a name="todo">ToDo</a> 675</h2> 676 677<div> 678 679<ol> 680 681 <li>Testing/Testing/Testing.</li> 682 683</ol> 684 685</div> 686 687<!-- *********************************************************************** --> 688 689<hr> 690<address> 691 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 692 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 693 <a href="http://validator.w3.org/check/referer"><img 694 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 695 696 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br> 697 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> 698 Last modified: $Date$ 699</address> 700 701</body> 702</html> 703