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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 <li><a href="#llvm_eh_sjlj_dispatchsetup"><tt>llvm.eh.sjlj.dispatchsetup</tt></a></li> 42 </ol></li> 43 <li><a href="#asm">Asm Table Formats</a> 44 <ol> 45 <li><a href="#unwind_tables">Exception Handling Frame</a></li> 46 <li><a href="#exception_tables">Exception Tables</a></li> 47 </ol></li> 48</ul> 49</td> 50</tr></table> 51 52<div class="doc_author"> 53 <p>Written by <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p> 54</div> 55 56 57<!-- *********************************************************************** --> 58<h2><a name="introduction">Introduction</a></h2> 59<!-- *********************************************************************** --> 60 61<div> 62 63<p>This document is the central repository for all information pertaining to 64 exception handling in LLVM. It describes the format that LLVM exception 65 handling information takes, which is useful for those interested in creating 66 front-ends or dealing directly with the information. Further, this document 67 provides specific examples of what exception handling information is used for 68 in C and C++.</p> 69 70<!-- ======================================================================= --> 71<h3> 72 <a name="itanium">Itanium ABI Zero-cost Exception Handling</a> 73</h3> 74 75<div> 76 77<p>Exception handling for most programming languages is designed to recover from 78 conditions that rarely occur during general use of an application. To that 79 end, exception handling should not interfere with the main flow of an 80 application's algorithm by performing checkpointing tasks, such as saving the 81 current pc or register state.</p> 82 83<p>The Itanium ABI Exception Handling Specification defines a methodology for 84 providing outlying data in the form of exception tables without inlining 85 speculative exception handling code in the flow of an application's main 86 algorithm. Thus, the specification is said to add "zero-cost" to the normal 87 execution of an application.</p> 88 89<p>A more complete description of the Itanium ABI exception handling runtime 90 support of can be found at 91 <a href="http://www.codesourcery.com/cxx-abi/abi-eh.html">Itanium C++ ABI: 92 Exception Handling</a>. A description of the exception frame format can be 93 found at 94 <a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html">Exception 95 Frames</a>, with details of the DWARF 4 specification at 96 <a href="http://dwarfstd.org/Dwarf4Std.php">DWARF 4 Standard</a>. 97 A description for the C++ exception table formats can be found at 98 <a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling 99 Tables</a>.</p> 100 101</div> 102 103<!-- ======================================================================= --> 104<h3> 105 <a name="sjlj">Setjmp/Longjmp Exception Handling</a> 106</h3> 107 108<div> 109 110<p>Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics 111 <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a> and 112 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to 113 handle control flow for exception handling.</p> 114 115<p>For each function which does exception processing — be 116 it <tt>try</tt>/<tt>catch</tt> blocks or cleanups — that function 117 registers itself on a global frame list. When exceptions are unwinding, the 118 runtime uses this list to identify which functions need processing.<p> 119 120<p>Landing pad selection is encoded in the call site entry of the function 121 context. The runtime returns to the function via 122 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>, where 123 a switch table transfers control to the appropriate landing pad based on 124 the index stored in the function context.</p> 125 126<p>In contrast to DWARF exception handling, which encodes exception regions 127 and frame information in out-of-line tables, SJLJ exception handling 128 builds and removes the unwind frame context at runtime. This results in 129 faster exception handling at the expense of slower execution when no 130 exceptions are thrown. As exceptions are, by their nature, intended for 131 uncommon code paths, DWARF exception handling is generally preferred to 132 SJLJ.</p> 133 134</div> 135 136<!-- ======================================================================= --> 137<h3> 138 <a name="overview">Overview</a> 139</h3> 140 141<div> 142 143<p>When an exception is thrown in LLVM code, the runtime does its best to find a 144 handler suited to processing the circumstance.</p> 145 146<p>The runtime first attempts to find an <i>exception frame</i> corresponding to 147 the function where the exception was thrown. If the programming language 148 supports exception handling (e.g. C++), the exception frame contains a 149 reference to an exception table describing how to process the exception. If 150 the language does not support exception handling (e.g. C), or if the 151 exception needs to be forwarded to a prior activation, the exception frame 152 contains information about how to unwind the current activation and restore 153 the state of the prior activation. This process is repeated until the 154 exception is handled. If the exception is not handled and no activations 155 remain, then the application is terminated with an appropriate error 156 message.</p> 157 158<p>Because different programming languages have different behaviors when 159 handling exceptions, the exception handling ABI provides a mechanism for 160 supplying <i>personalities</i>. An exception handling personality is defined 161 by way of a <i>personality function</i> (e.g. <tt>__gxx_personality_v0</tt> 162 in C++), which receives the context of the exception, an <i>exception 163 structure</i> containing the exception object type and value, and a reference 164 to the exception table for the current function. The personality function 165 for the current compile unit is specified in a <i>common exception 166 frame</i>.</p> 167 168<p>The organization of an exception table is language dependent. For C++, an 169 exception table is organized as a series of code ranges defining what to do 170 if an exception occurs in that range. Typically, the information associated 171 with a range defines which types of exception objects (using C++ <i>type 172 info</i>) that are handled in that range, and an associated action that 173 should take place. Actions typically pass control to a <i>landing 174 pad</i>.</p> 175 176<p>A landing pad corresponds roughly to the code found in the <tt>catch</tt> 177 portion of a <tt>try</tt>/<tt>catch</tt> sequence. When execution resumes at 178 a landing pad, it receives an <i>exception structure</i> and a 179 <i>selector value</i> corresponding to the <i>type</i> of exception 180 thrown. The selector is then used to determine which <i>catch</i> should 181 actually process the exception.</p> 182 183</div> 184 185</div> 186 187<!-- ======================================================================= --> 188<h2> 189 <a name="codegen">LLVM Code Generation</a> 190</h2> 191 192<div> 193 194<p>From a C++ developer's perspective, exceptions are defined in terms of the 195 <tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements. In this section 196 we will describe the implementation of LLVM exception handling in terms of 197 C++ examples.</p> 198 199<!-- ======================================================================= --> 200<h3> 201 <a name="throw">Throw</a> 202</h3> 203 204<div> 205 206<p>Languages that support exception handling typically provide a <tt>throw</tt> 207 operation to initiate the exception process. Internally, a <tt>throw</tt> 208 operation breaks down into two steps.</p> 209 210<ol> 211 <li>A request is made to allocate exception space for an exception structure. 212 This structure needs to survive beyond the current activation. This 213 structure will contain the type and value of the object being thrown.</li> 214 215 <li>A call is made to the runtime to raise the exception, passing the 216 exception structure as an argument.</li> 217</ol> 218 219<p>In C++, the allocation of the exception structure is done by the 220 <tt>__cxa_allocate_exception</tt> runtime function. The exception raising is 221 handled by <tt>__cxa_throw</tt>. The type of the exception is represented 222 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:</p> 237 238<ol> 239 <li>where to continue when the call succeeds as per normal, and</li> 240 241 <li>where to continue if the call raises an exception, either by a throw or 242 the unwinding of a throw</li> 243</ol> 244 245<p>The term used to define a the place where an <tt>invoke</tt> continues after 246 an exception is called a <i>landing pad</i>. LLVM landing pads are 247 conceptually alternative function entry points where an exception structure 248 reference and a type info index are passed in as arguments. The landing pad 249 saves the exception structure reference and then proceeds to select the catch 250 block that corresponds to the type info of the exception object.</p> 251 252<p>The LLVM <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> 253 instruction</a> is used to convey information about the landing pad to the 254 back end. For C++, the <tt>landingpad</tt> instruction returns a pointer and 255 integer pair corresponding to the pointer to the <i>exception structure</i> 256 and the <i>selector value</i> respectively.</p> 257 258<p>The <tt>landingpad</tt> instruction takes a reference to the personality 259 function to be used for this <tt>try</tt>/<tt>catch</tt> sequence. The 260 remainder of the instruction is a list of <i>cleanup</i>, <i>catch</i>, 261 and <i>filter</i> clauses. The exception is tested against the clauses 262 sequentially from first to last. The selector value is a positive number if 263 the exception matched a type info, a negative number if it matched a filter, 264 and zero if it matched a cleanup. If nothing is matched, the behavior of 265 the program is <a href="#restrictions">undefined</a>. If a type info matched, 266 then the selector value is the index of the type info in the exception table, 267 which can be obtained using the 268 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p> 269 270<p>Once the landing pad has the type info selector, the code branches to the 271 code for the first catch. The catch then checks the value of the type info 272 selector against the index of type info for that catch. Since the type info 273 index is not known until all the type infos have been gathered in the 274 backend, the catch code must call the 275 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic to 276 determine the index for a given type info. If the catch fails to match the 277 selector then control is passed on to the next catch.</p> 278 279<p>Finally, the entry and exit of catch code is bracketed with calls to 280 <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p> 281 282<ul> 283 <li><tt>__cxa_begin_catch</tt> takes an exception structure reference as an 284 argument and returns the value of the exception object.</li> 285 286 <li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br> 287 <ol> 288 <li>Locates the most recently caught exception and decrements its handler 289 count,</li> 290 <li>Removes the exception from the <i>caught</i> stack if the handler 291 count goes to zero, and</li> 292 <li>Destroys the exception if the handler count goes to zero and the 293 exception was not re-thrown by throw.</li> 294 </ol> 295 <p><b>Note:</b> a rethrow from within the catch may replace this call with 296 a <tt>__cxa_rethrow</tt>.</p></li> 297</ul> 298 299</div> 300 301<!-- ======================================================================= --> 302<h3> 303 <a name="cleanups">Cleanups</a> 304</h3> 305 306<div> 307 308<p>A cleanup is extra code which needs to be run as part of unwinding a scope. 309 C++ destructors are a typical example, but other languages and language 310 extensions provide a variety of different kinds of cleanups. In general, a 311 landing pad may need to run arbitrary amounts of cleanup code before actually 312 entering a catch block. To indicate the presence of cleanups, a 313 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> 314 should have a <i>cleanup</i> clause. Otherwise, the unwinder will not stop at 315 the landing pad if there are no catches or filters that require it to.</p> 316 317<p><b>Note:</b> Do not allow a new exception to propagate out of the execution 318 of a cleanup. This can corrupt the internal state of the unwinder. 319 Different languages describe different high-level semantics for these 320 situations: for example, C++ requires that the process be terminated, whereas 321 Ada cancels both exceptions and throws a third.</p> 322 323<p>When all cleanups are finished, if the exception is not handled by the 324 current function, resume unwinding by calling the 325 <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a>, passing in 326 the result of the <tt>landingpad</tt> instruction for the original landing 327 pad.</p> 328 329</div> 330 331<!-- ======================================================================= --> 332<h3> 333 <a name="throw_filters">Throw Filters</a> 334</h3> 335 336<div> 337 338<p>C++ allows the specification of which exception types may be thrown from a 339 function. To represent this, a top level landing pad may exist to filter out 340 invalid types. To express this in LLVM code the 341 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> will 342 have a filter clause. The clause consists of an array of type infos. 343 <tt>landingpad</tt> will return a negative value if the exception does not 344 match any of the type infos. If no match is found then a call 345 to <tt>__cxa_call_unexpected</tt> should be made, otherwise 346 <tt>_Unwind_Resume</tt>. Each of these functions requires a reference to the 347 exception structure. Note that the most general form of a 348 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> can 349 have any number of catch, cleanup, and filter clauses (though having more 350 than one cleanup is pointless). The LLVM C++ front-end can generate such 351 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instructions</a> due 352 to inlining creating nested exception handling scopes.</p> 353 354</div> 355 356<!-- ======================================================================= --> 357<h3> 358 <a name="restrictions">Restrictions</a> 359</h3> 360 361<div> 362 363<p>The unwinder delegates the decision of whether to stop in a call frame to 364 that call frame's language-specific personality function. Not all unwinders 365 guarantee that they will stop to perform cleanups. For example, the GNU C++ 366 unwinder doesn't do so unless the exception is actually caught somewhere 367 further up the stack.</p> 368 369<p>In order for inlining to behave correctly, landing pads must be prepared to 370 handle selector results that they did not originally advertise. Suppose that 371 a function catches exceptions of type <tt>A</tt>, and it's inlined into a 372 function that catches exceptions of type <tt>B</tt>. The inliner will update 373 the <tt>landingpad</tt> instruction for the inlined landing pad to include 374 the fact that <tt>B</tt> is also caught. If that landing pad assumes that it 375 will only be entered to catch an <tt>A</tt>, it's in for a rude awakening. 376 Consequently, landing pads must test for the selector results they understand 377 and then resume exception propagation with the 378 <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a> if none of 379 the conditions match.</p> 380 381</div> 382 383</div> 384 385<!-- ======================================================================= --> 386<h2> 387 <a name="format_common_intrinsics">Exception Handling Intrinsics</a> 388</h2> 389 390<div> 391 392<p>In addition to the 393 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt></a> and 394 <a href="LangRef.html#i_resume"><tt>resume</tt></a> instructions, LLVM uses 395 several intrinsic functions (name prefixed with <i><tt>llvm.eh</tt></i>) to 396 provide exception handling information at various points in generated 397 code.</p> 398 399<!-- ======================================================================= --> 400<h4> 401 <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a> 402</h4> 403 404<div> 405 406<pre> 407 i32 @llvm.eh.typeid.for(i8* %type_info) 408</pre> 409 410<p>This intrinsic returns the type info index in the exception table of the 411 current function. This value can be used to compare against the result 412 of <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>. 413 The single argument is a reference to a type info.</p> 414 415</div> 416 417<!-- ======================================================================= --> 418<h4> 419 <a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a> 420</h4> 421 422<div> 423 424<pre> 425 i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf) 426</pre> 427 428<p>For SJLJ based exception handling, this intrinsic forces register saving for 429 the current function and stores the address of the following instruction for 430 use as a destination address 431 by <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>. The 432 buffer format and the overall functioning of this intrinsic is compatible 433 with the GCC <tt>__builtin_setjmp</tt> implementation allowing code built 434 with the clang and GCC to interoperate.</p> 435 436<p>The single parameter is a pointer to a five word buffer in which the calling 437 context is saved. The front end places the frame pointer in the first word, 438 and the target implementation of this intrinsic should place the destination 439 address for a 440 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> in the 441 second word. The following three words are available for use in a 442 target-specific manner.</p> 443 444</div> 445 446<!-- ======================================================================= --> 447<h4> 448 <a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a> 449</h4> 450 451<div> 452 453<pre> 454 void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf) 455</pre> 456 457<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.longjmp</tt> 458 intrinsic is used to implement <tt>__builtin_longjmp()</tt>. The single 459 parameter is a pointer to a buffer populated 460 by <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a>. The frame 461 pointer and stack pointer are restored from the buffer, then control is 462 transferred to the destination address.</p> 463 464</div> 465<!-- ======================================================================= --> 466<h4> 467 <a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a> 468</h4> 469 470<div> 471 472<pre> 473 i8* @llvm.eh.sjlj.lsda() 474</pre> 475 476<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.lsda</tt> intrinsic 477 returns the address of the Language Specific Data Area (LSDA) for the current 478 function. The SJLJ front-end code stores this address in the exception 479 handling function context for use by the runtime.</p> 480 481</div> 482 483<!-- ======================================================================= --> 484<h4> 485 <a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a> 486</h4> 487 488<div> 489 490<pre> 491 void @llvm.eh.sjlj.callsite(i32 %call_site_num) 492</pre> 493 494<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.callsite</tt> 495 intrinsic identifies the callsite value associated with the 496 following <tt>invoke</tt> instruction. This is used to ensure that landing 497 pad entries in the LSDA are generated in matching order.</p> 498 499</div> 500 501<!-- ======================================================================= --> 502<h4> 503 <a name="llvm_eh_sjlj_dispatchsetup">llvm.eh.sjlj.dispatchsetup</a> 504</h4> 505 506<div> 507 508<pre> 509 void @llvm.eh.sjlj.dispatchsetup(i32 %dispatch_value) 510</pre> 511 512<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.dispatchsetup</tt> 513 intrinsic is used by targets to do any unwind edge setup they need. By 514 default, no action is taken.</p> 515 516</div> 517 518</div> 519 520<!-- ======================================================================= --> 521<h2> 522 <a name="asm">Asm Table Formats</a> 523</h2> 524 525<div> 526 527<p>There are two tables that are used by the exception handling runtime to 528 determine which actions should be taken when an exception is thrown.</p> 529 530<!-- ======================================================================= --> 531<h3> 532 <a name="unwind_tables">Exception Handling Frame</a> 533</h3> 534 535<div> 536 537<p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind 538 frame used by DWARF debug info. The frame contains all the information 539 necessary to tear down the current frame and restore the state of the prior 540 frame. There is an exception handling frame for each function in a compile 541 unit, plus a common exception handling frame that defines information common 542 to all functions in the unit.</p> 543 544<!-- Todo - Table details here. --> 545 546</div> 547 548<!-- ======================================================================= --> 549<h3> 550 <a name="exception_tables">Exception Tables</a> 551</h3> 552 553<div> 554 555<p>An exception table contains information about what actions to take when an 556 exception is thrown in a particular part of a function's code. There is one 557 exception table per function, except leaf functions and functions that have 558 calls only to non-throwing functions. They do not need an exception 559 table.</p> 560 561<!-- Todo - Table details here. --> 562 563</div> 564 565</div> 566 567<!-- *********************************************************************** --> 568 569<hr> 570<address> 571 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 572 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 573 <a href="http://validator.w3.org/check/referer"><img 574 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 575 576 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br> 577 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> 578 Last modified: $Date: 2011-09-28 04:16:57 +0800 (Wed, 28 Sep 2011) $ 579</address> 580 581</body> 582</html>