docs/ExceptionHandling.html

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  10 <div class="doc_title">Exception Handling in LLVM</div>
  11
  12 <table class="layout" style="width:100%">
  13   <tr class="layout">
  14     <td class="left">
  15 <ul>
  16   <li><a href="#introduction">Introduction</a>
  17   <ol>
  18     <li><a href="#itanium">Itanium ABI Zero-cost Exception Handling</a></li>
  19     <li><a href="#overview">Overview</a></li>
  20   </ol></li>
  21   <li><a href="#codegen">LLVM Code Generation</a>
  22   <ol>
  23     <li><a href="#throw">Throw</a></li>
  24     <li><a href="#try_catch">Try/Catch</a></li>
  25     <li><a href="#cleanups">Cleanups</a></li>
  26     <li><a href="#throw_filters">Throw Filters</a></li>
  27     <li><a href="#restrictions">Restrictions</a></li>
  28   </ol></li>
  29   <li><a href="#format_common_intrinsics">Exception Handling Intrinsics</a>
  30   <ol>
  31         <li><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a></li>
  32         <li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a></li>
  33         <li><a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a></li>
  34   </ol></li>
  35   <li><a href="#asm">Asm Table Formats</a>
  36   <ol>
  37     <li><a href="#unwind_tables">Exception Handling Frame</a></li>
  38     <li><a href="#exception_tables">Exception Tables</a></li>
  39   </ol></li>
  40   <li><a href="#todo">ToDo</a></li>
  41 </ul>
  42 </td>
  43 </tr></table>
  44
  45 <div class="doc_author">
  46   <p>Written by <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
  47 </div>
  48
  49
  50 <!-- *********************************************************************** -->
  51 <div class="doc_section"><a name="introduction">Introduction</a></div>
  52 <!-- *********************************************************************** -->
  53
  54 <div class="doc_text">
  55
  56 <p>This document is the central repository for all information pertaining to
  57 exception handling in LLVM.  It describes the format that LLVM exception
  58 handling information takes, which is useful for those interested in creating
  59 front-ends or dealing directly with the information.  Further, this document
  60 provides specific examples of what exception handling information is used for
  61 C/C++.</p>
  62
  63 </div>
  64
  65 <!-- ======================================================================= -->
  66 <div class="doc_subsection">
  67   <a name="itanium">Itanium ABI Zero-cost Exception Handling</a>
  68 </div>
  69
  70 <div class="doc_text">
  71
  72 <p>Exception handling for most programming languages is designed to recover from
  73 conditions that rarely occur during general use of an application.  To that end,
  74 exception handling should not interfere with the main flow of an
  75 application's algorithm by performing checkpointing tasks such as saving
  76 the current pc or register state.</p>
  77
  78 <p>The Itanium ABI Exception Handling Specification defines a methodology for
  79 providing outlying data in the form of exception tables without inlining
  80 speculative exception handling code in the flow of an application's main
  81 algorithm.  Thus, the specification is said to add "zero-cost" to the normal
  82 execution of an application.</p>
  83
  84 <p>A more complete description of the Itanium ABI exception handling runtime
  85 support of can be found at <a
  86 href="http://www.codesourcery.com/cxx-abi/abi-eh.html">Itanium C++ ABI:
  87 Exception Handling.</a> A description of the exception frame format can be found
  88 at <a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-
  89 Core-generic/ehframechpt.html">Exception Frames</a>, with details of the Dwarf
  90 specification at <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">Dwarf 3
  91 Standard.</a> A description for the C++ exception table formats can be found at
  92 <a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling
  93 Tables.</a></p>
  94
  95 </div>
  96
  97 <!-- ======================================================================= -->
  98 <div class="doc_subsection">
  99   <a name="overview">Overview</a>
 100 </div>
 101
 102 <div class="doc_text">
 103
 104 <p>When an exception is thrown in llvm code, the runtime does a best effort to
 105 find a handler suited to process the circumstance.</p>
 106
 107 <p>The runtime first attempts to find an <i>exception frame</i> corresponding to
 108 the function where the exception was thrown.  If the programming language (ex.
 109 C++) supports exception handling, the exception frame contains a reference to an
 110 exception table describing how to process the exception.  If the language (ex.
 111 C) does not support exception handling or if the exception needs to be forwarded
 112 to a prior activation, the exception frame contains information about how to
 113 unwind the current activation and restore the state of the prior activation.
 114 This process is repeated until the exception is handled.  If the exception is
 115 not handled and no activations remain, then the application is terminated with
 116 an appropriate error message.</p>
 117
 118 <p>Since different programming languages have different behaviors when handling
 119 exceptions, the exception handling ABI provides a mechanism for supplying
 120 <i>personalities.</i> An exception handling personality is defined by way of a
 121 <i>personality function</i> (ex. for C++ <tt>__gxx_personality_v0</tt>) which
 122 receives the context of the exception, an <i>exception structure</i> containing
 123 the exception object type and value, and a reference to the exception table for
 124 the current function.  The personality function for the current compile unit is
 125 specified in a <i>common exception frame</i>.</p>
 126
 127 <p>The organization of an exception table is language dependent.  For C++, an
 128 exception table is organized as a series of code ranges defining what to do if
 129 an exception occurs in that range.  Typically, the information associated with a
 130 range defines which types of exception objects (using C++ <i>type info</i>) that
 131 are handled in that range, and an associated action that should take place.
 132 Actions typically pass control to a <i>landing pad</i>.</p>
 133
 134 <p>A landing pad corresponds to the code found in the catch portion of a
 135 try/catch sequence.  When execution resumes at a landing pad, it receives the
 136 exception structure and a selector corresponding to the <i>type</i> of exception
 137 thrown.  The selector is then used to determine which catch should actually
 138 process the exception.</p>
 139
 140 </div>
 141
 142 <!-- ======================================================================= -->
 143 <div class="doc_section">
 144   <a name="codegen">LLVM Code Generation</a>
 145 </div>
 146
 147 <div class="doc_text">
 148
 149 <p>At the time of this writing, only C++ exception handling support is available
 150 in LLVM.  So the remainder of this document will be somewhat C++-centric.</p>
 151
 152 <p>From the C++ developers perspective, exceptions are defined in terms of the
 153 <tt>throw</tt> and <tt>try/catch</tt> statements.  In this section we will
 154 describe the implementation of llvm exception handling in terms of C++
 155 examples.</p>
 156
 157 </div>
 158
 159 <!-- ======================================================================= -->
 160 <div class="doc_subsection">
 161   <a name="throw">Throw</a>
 162 </div>
 163
 164 <div class="doc_text">
 165
 166 <p>Languages that support exception handling typically provide a <tt>throw</tt>
 167 operation to initiate the exception process.  Internally, a throw operation
 168 breaks down into two steps.  First, a request is made to allocate exception
 169 space for an exception structure.  This structure needs to survive beyond the
 170 current activation.  This structure will contain the type and value of the
 171 object being thrown.  Second, a call is made to the runtime to raise the
 172 exception, passing the exception structure as an argument.</p>
 173
 174 <p>In C++, the allocation of the exception structure is done by the
 175 <tt>__cxa_allocate_exception</tt> runtime function.  The exception raising is
 176 handled by <tt>__cxa_throw</tt>.  The type of the exception is represented using
 177 a C++ RTTI type info structure.</p>
 178
 179 </div>
 180
 181 <!-- ======================================================================= -->
 182 <div class="doc_subsection">
 183   <a name="try_catch">Try/Catch</a>
 184 </div>
 185
 186 <div class="doc_text">
 187
 188 <p>A call within the scope of a try statement can potentially raise an exception.
 189 In those circumstances, the LLVM C++ front-end replaces the call with an
 190 <tt>invoke</tt> instruction.  Unlike a call, the invoke has two potential
 191 continuation points; where to continue when the call succeeds as per normal, and
 192 where to continue if the call raises an exception, either by a throw or the
 193 unwinding of a throw.</p>
 194
 195 <p>The term used to define a the place where an invoke continues after an
 196 exception is called a <i>landing pad</i>.  LLVM landing pads are conceptually
 197 alternative function entry points where a exception structure reference and a type
 198 info index are passed in as arguments.  The landing pad saves the exception
 199 structure reference and then proceeds to select the catch block that corresponds
 200 to the type info of the exception object.</p>
 201
 202 <p>Two llvm intrinsic functions are used convey information about the landing
 203 pad to the back end.</p>
 204
 205 <p><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a> takes no
 206 arguments and returns a pointer to the exception structure.  This only returns a
 207 sensible value if called after an invoke has branched to a landing pad.  Due to
 208 codegen limitations, it must currently be called in the landing pad itself.</p>
 209
 210 <p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum of
 211 three arguments.  The first argument is the reference to the exception
 212 structure. The second argument is a reference to the personality function to be
 213 used for this try catch sequence. Each of the remaining arguments is either a
 214 reference to the type info for a catch statement,
 215 a <a href="#throw_filters">filter</a> expression,
 216 or the number zero representing a <a href="#cleanups">cleanup</a>.
 217 The exception is tested against the arguments sequentially from first to last.
 218 The result of the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a
 219 positive number if the exception matched a type info, a negative number if it matched
 220 a filter, and zero if it matched a cleanup.  If nothing is matched, the behaviour of
 221 the program is <a href="#restrictions">undefined</a>.
 222 This only returns a sensible value if called after an invoke has branched to a
 223 landing pad.  Due to codegen limitations, it must currently be called in the
 224 landing pad itself.
 225 If a type info matched then the selector value is the index of the type info in
 226 the exception table, which can be obtained using the
 227 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
 228
 229 <p>Once the landing pad has the type info selector, the code branches to the
 230 code for the first catch.  The catch then checks the value of the type info
 231 selector against the index of type info for that catch.  Since the type info
 232 index is not known until all the type info have been gathered in the backend,
 233 the catch code will call the <a
 234 href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic to
 235 determine the index for a given type info.  If the catch fails to match the
 236 selector then control is passed on to the next catch. Note: Since the landing
 237 pad will not be used if there is no match in the list of type info on the call
 238 to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>, then neither the
 239 last catch nor <i>catch all</i> need to perform the the check against the
 240 selector.</p>
 241
 242 <p>Finally, the entry and exit of catch code is bracketed with calls to
 243 <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.
 244 <tt>__cxa_begin_catch</tt> takes a exception structure reference as an argument
 245 and returns the value of the exception object. <tt>__cxa_end_catch</tt>
 246 takes a exception structure reference as an argument. This function clears the
 247 exception from the exception space.  Note: a rethrow from within the catch may
 248 replace this call with a <tt>__cxa_rethrow</tt>.</p>
 249
 250 </div>
 251
 252 <!-- ======================================================================= -->
 253 <div class="doc_subsection">
 254   <a name="cleanups">Cleanups</a>
 255 </div>
 256
 257 <div class="doc_text">
 258
 259 <p>To handle destructors and cleanups in try code, control may not run directly
 260 from a landing pad to the first catch.  Control may actually flow from the
 261 landing pad to clean up code and then to the first catch.  Since the required
 262 clean up for each invoke in a try may be different (ex., intervening
 263 constructor), there may be several landing pads for a given try.  If cleanups
 264 need to be run, the number zero should be passed as the last
 265 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument.
 266 However for C++ a <tt>null i8*</tt> <a href="#restrictions">must</a> be passed
 267 instead.
 268 </p>
 269
 270 </div>
 271
 272 <!-- ======================================================================= -->
 273 <div class="doc_subsection">
 274   <a name="throw_filters">Throw Filters</a>
 275 </div>
 276
 277 <div class="doc_text">
 278
 279 <p>C++ allows the specification of which exception types can be thrown from
 280 a function.  To represent this a top level landing pad may exist to filter out
 281 invalid types.  To express this in LLVM code the landing pad will call <a
 282 href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>.  The arguments are a
 283 reference to the exception structure, a reference to the personality function,
 284 the length of the filter expression (the number of type infos plus one),
 285 followed by the type infos themselves.
 286 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> will return a negative
 287 value if the exception does not match any of the type infos.  If no match is
 288 found then a call to <tt>__cxa_call_unexpected</tt> should be made, otherwise
 289 <tt>_Unwind_Resume</tt>.  Each of these functions requires a reference to the
 290 exception structure.  Note that the most general form of an
 291 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> call can contain
 292 any number of type infos, filter expressions and cleanups (though having more
 293 than one cleanup is pointless).  The LLVM C++ front-end can generate such
 294 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls due to inlining
 295 creating nested exception handling scopes.</p>
 296
 297 </div>
 298
 299 <!-- ======================================================================= -->
 300 <div class="doc_subsection">
 301   <a name="restrictions">Restrictions</a>
 302 </div>
 303
 304 <div class="doc_text">
 305
 306 <p>The semantics of the invoke instruction require that any exception that
 307 unwinds through an invoke call should result in a branch to the invoke's unwind
 308 label.  However such a branch will only happen if the
 309 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> matches.
 310 Thus in order to ensure correct operation, the front-end must only generate
 311 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls that are
 312 guaranteed to always match whatever exception unwinds through the invoke.
 313 For most languages it is enough to pass zero, indicating the presence of
 314 a <a href="#cleanups">cleanup</a>, as the last
 315 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument.
 316 However for C++ this is not sufficient, because the C++ personality function
 317 will terminate the program if it detects that unwinding the exception only
 318 results in matches with cleanups.  For C++ a <tt>null i8*</tt> should
 319 be passed as the last
 320 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument instead.
 321 This is interpreted as a catch-all by the C++ personality function, and will
 322 always match.
 323 </p>
 324
 325 </div>
 326
 327 <!-- ======================================================================= -->
 328 <div class="doc_section">
 329   <a name="format_common_intrinsics">Exception Handling Intrinsics</a>
 330 </div>
 331
 332 <div class="doc_text">
 333
 334 <p>LLVM uses several intrinsic functions (name prefixed with "llvm.eh") to
 335 provide exception handling information at various points in generated code.</p>
 336
 337 </div>
 338
 339 <!-- ======================================================================= -->
 340 <div class="doc_subsubsection">
 341   <a name="llvm_eh_exception">llvm.eh.exception</a>
 342 </div>
 343
 344 <div class="doc_text">
 345 <pre>
 346   i8* %<a href="#llvm_eh_exception">llvm.eh.exception</a>( )
 347 </pre>
 348
 349 <p>This intrinsic returns a pointer to the exception structure.</p>
 350
 351 </div>
 352
 353 <!-- ======================================================================= -->
 354 <div class="doc_subsubsection">
 355   <a name="llvm_eh_selector">llvm.eh.selector</a>
 356 </div>
 357
 358 <div class="doc_text">
 359 <pre>
 360   i32 %<a href="#llvm_eh_selector">llvm.eh.selector.i32</a>(i8*, i8*, i8*, ...)
 361   i64 %<a href="#llvm_eh_selector">llvm.eh.selector.i64</a>(i8*, i8*, i8*, ...)
 362 </pre>
 363
 364 <p>This intrinsic is used to compare the exception with the given type infos,
 365 filters and cleanups.</p>
 366
 367 <p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum of
 368 three arguments.  The first argument is the reference to the exception
 369 structure. The second argument is a reference to the personality function to be
 370 used for this try catch sequence. Each of the remaining arguments is either a
 371 reference to the type info for a catch statement,
 372 a <a href="#throw_filters">filter</a> expression,
 373 or the number zero representing a <a href="#cleanups">cleanup</a>.
 374 The exception is tested against the arguments sequentially from first to last.
 375 The result of the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a
 376 positive number if the exception matched a type info, a negative number if it matched
 377 a filter, and zero if it matched a cleanup.  If nothing is matched, the behaviour of
 378 the program is <a href="#restrictions">undefined</a>.
 379 If a type info matched then the selector value is the index of the type info in
 380 the exception table, which can be obtained using the
 381 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
 382
 383 </div>
 384
 385 <!-- ======================================================================= -->
 386 <div class="doc_subsubsection">
 387   <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
 388 </div>
 389
 390 <div class="doc_text">
 391 <pre>
 392   i32 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for.i32</a>(i8*)
 393   i64 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for.i64</a>(i8*)
 394 </pre>
 395
 396 <p>This intrinsic returns the type info index in the exception table of the
 397 current function.  This value can be used to compare against the result of <a
 398 href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>.  The single argument is
 399 a reference to a type info.</p>
 400
 401 </div>
 402
 403 <!-- ======================================================================= -->
 404 <div class="doc_section">
 405   <a name="asm">Asm Table Formats</a>
 406 </div>
 407
 408 <div class="doc_text">
 409
 410 <p>There are two tables that are used by the exception handling runtime to
 411 determine which actions should take place when an exception is thrown.</p>
 412
 413 </div>
 414
 415 <!-- ======================================================================= -->
 416 <div class="doc_subsection">
 417   <a name="unwind_tables">Exception Handling Frame</a>
 418 </div>
 419
 420 <div class="doc_text">
 421
 422 <p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind
 423 frame used by dwarf debug info.  The frame contains all the information
 424 necessary to tear down the current frame and restore the state of the prior
 425 frame.  There is an exception handling frame for each function in a compile
 426 unit, plus a common exception handling frame that defines information common to
 427 all functions in the unit.</p>
 428
 429 <p>Todo - Table details here.</p>
 430
 431 </div>
 432
 433 <!-- ======================================================================= -->
 434 <div class="doc_subsection">
 435   <a name="exception_tables">Exception Tables</a>
 436 </div>
 437
 438 <div class="doc_text">
 439
 440 <p>An exception table contains information about what actions to take when an
 441 exception is thrown in a particular part of a function's code.  There is
 442 one exception table per function except leaf routines and functions that have
 443 only calls to non-throwing functions will not need an exception table.</p>
 444
 445 <p>Todo - Table details here.</p>
 446
 447 </div>
 448
 449 <!-- ======================================================================= -->
 450 <div class="doc_section">
 451   <a name="todo">ToDo</a>
 452 </div>
 453
 454 <div class="doc_text">
 455
 456 <ol>
 457
 458 <li><p>Testing/Testing/Testing.</p></li>
 459
 460 </ol>
 461
 462 </div>
 463
 464 <!-- *********************************************************************** -->
 465
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 473   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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