docs/ExceptionHandling.html

   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   <link rel="stylesheet" href="llvm.css" type="text/css">
   7 </head>
   8 <body>
   9
  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="#finallys">Finallys</a></li>
  26     <li><a href="#throw_filters">Throw Filters</a></li>
  27   </ol></li>
  28   <li><a href="#format_common_intrinsics">Exception Handling Intrinsics</a>
  29   <ol>
  30         <li><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a></li>
  31         <li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a></li>
  32         <li><a href="#llvm_eh_filter"><tt>llvm.eh.filter</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&apos;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&apos;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
  88 found at <a
  89 href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-
  90 Core-generic/ehframechpt.html">Exception Frames</a>, with details of the Dwarf
  91 specification at <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">Dwarf 3
  92 Standard.</a> A description for the C++ exception table formats can be found at
  93 <a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling
  94 Tables.</a></p>
  95
  96 </div>
  97
  98 <!-- ======================================================================= -->
  99 <div class="doc_subsection">
 100   <a name="overview">Overview</a>
 101 </div>
 102
 103 <div class="doc_text">
 104
 105 <p>When an exception is thrown in llvm code, the runtime does a best effort to
 106 find a handler suited to process the circumstance.</p>
 107
 108 <p>The runtime first attempts to find an <i>exception frame</i> corresponding to
 109 the function where the exception was thrown.  If the programming language (ex.
 110 C++) supports exception handling, the exception frame contains a reference to an
 111 exception table describing how to process the exception.  If the language (ex.
 112 C) does not support exception handling or if the exception needs to be forwarded
 113 to a prior activation, the exception frame contains information about how to
 114 unwind the current activation and restore the state of the prior activation.
 115 This process is repeated until the exception is handled.  If the exception is
 116 not handled and no activations remain, then the application is terminated with
 117 an appropriate error message.</p>
 118
 119 <p>Since different programming languages have different behaviors when handling
 120 exceptions, the exception handling ABI provides a mechanism for supplying
 121 <i>personalities.</i> An exception handling personality is defined by way of a
 122 <i>personality function</i> (ex. for C++ <tt>__gxx_personality_v0</tt>) which
 123 receives the context of the exception, an <i>exception structure</i> containing
 124 the exception object type and value, and a reference to the exception table for
 125 the current function.  The personality function for the current compile unit is
 126 specified in a <i>common exception frame</i>.</p>
 127
 128 <p>The organization of an exception table is language dependent.  For C++, an
 129 exception table is organized as a series of code ranges defining what to do if
 130 an exception occurs in that range.  Typically, the information associated with a
 131 range defines which types of exception objects (using C++ <i>type info</i>) that
 132 are handled in that range, and an associated action that should take place.
 133 Actions typically pass control to a <i>landing pad</i>.</p>
 134
 135 <p>A landing pad corresponds to the code found in the catch portion of a
 136 try/catch sequence.  When execution resumes at a landing pad, it receives the
 137 exception structure and a selector corresponding to the <i>type</i> of exception
 138 thrown.  The selector is then used to determine which catch should actually
 139 process the exception.</p>
 140
 141 </div>
 142
 143 <!-- ======================================================================= -->
 144 <div class="doc_section">
 145   <a name="codegen">LLVM Code Generation</a>
 146 </div>
 147
 148 <div class="doc_text">
 149
 150 <p>At the time of this writing, only C++ exception handling support is available
 151 in LLVM.  So the remainder of this document will be somewhat C++-centric.</p>
 152
 153 <p>From the C++ developers perspective, exceptions are defined in terms of the
 154 <tt>throw</tt> and <tt>try/catch</tt> statements.  In this section we will
 155 describe the implementation of llvm exception handling in terms of C++
 156 examples.</p>
 157
 158 </div>
 159
 160 <!-- ======================================================================= -->
 161 <div class="doc_subsection">
 162   <a name="throw">Throw</a>
 163 </div>
 164
 165 <div class="doc_text">
 166
 167 <p>Languages that support exception handling typically provide a <tt>throw</tt>
 168 operation to initiate the exception process.  Internally, a throw operation
 169 breaks down into two steps.  First, a request is made to allocate exception
 170 space for an exception structure.  This structure needs to survive beyond the
 171 current activation.  This structure will contain the type and value of the
 172 object being thrown.  Second, a call is made to the runtime to raise the
 173 exception, passing the exception structure as an argument.</p>
 174
 175 <p>In C++, the allocation of the exception structure is done by the
 176 <tt>__cxa_allocate_exception</tt> runtime function.  The exception raising is
 177 handled by <tt>__cxa_throw</tt>.  The type of the exception is represented using
 178 a C++ RTTI type info structure.</p>
 179
 180 </div>
 181
 182 <!-- ======================================================================= -->
 183 <div class="doc_subsection">
 184   <a name="try_catch">Try/Catch</a>
 185 </div>
 186
 187 <div class="doc_text">
 188
 189 <p>A call within the scope of a try statement can potentially raise an exception.
 190 In those circumstances, the LLVM C++ front-end replaces the call with an
 191 <tt>invoke</tt> instruction.  Unlike a call, the invoke has two potential
 192 continuation points; where to continue when the call succeeds as per normal, and
 193 where to continue if the call raises an exception, either by a throw or the
 194 unwinding of a throw.</p>
 195
 196 <p>The term used to define a the place where an invoke continues after an
 197 exception is called a <i>landing pad</i>.  LLVM landing pads are conceptually
 198 alternative function entry points where a exception structure reference and a type
 199 info index are passed in as arguments.  The landing pad saves the exception
 200 structure reference and then proceeds to select the catch block that corresponds
 201 to the type info of the exception object.</p>
 202
 203 <p>Two llvm intrinsic functions are used convey information about the landing
 204 pad to the back end.</p>
 205
 206 <p><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a> takes no
 207 arguments and returns the exception structure reference.  The backend replaces
 208 this intrinsic with the code that accesses the first argument of a call.  The
 209 LLVM C++ front end generates code to save this value in an alloca location for
 210 further use in the landing pad and catch code.</p>
 211
 212 <p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum of
 213 three arguments.  The first argument is the reference to the exception
 214 structure. The second argument is a reference to the personality function to be
 215 used for this try catch sequence. The remaining arguments are references to the
 216 type infos for each of the catch statements in the order they should be tested.
 217 The <i>catch all</i> (...) is represented with a <tt>null i8*</tt>.  The result
 218 of the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is the index of
 219 the type info in the corresponding exception table. The LLVM C++ front end
 220 generates code to save this value in an alloca location for further use in the
 221 landing pad and catch code.</p>
 222
 223 <p>Once the landing pad has the type info selector, the code branches to the
 224 code for the first catch.  The catch then checks the value of the type info
 225 selector against the index of type info for that catch.  Since the type info
 226 index is not known until all the type info have been gathered in the backend,
 227 the catch code will call the <a
 228 href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic to
 229 determine the index for a given type info.  If the catch fails to match the
 230 selector then control is passed on to the next catch. Note: Since the landing
 231 pad will not be used if there is no match in the list of type info on the call
 232 to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>, then neither the
 233 last catch nor <i>catch all</i> need to perform the the check against the
 234 selector.</p>
 235
 236 <p>Finally, the entry and exit of catch code is bracketed with calls to
 237 <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.
 238 <tt>__cxa_begin_catch</tt> takes a exception structure reference as an argument
 239 and returns the value of the exception object.</tt>  <tt>__cxa_end_catch</tt>
 240 takes a exception structure reference as an argument. This function clears the
 241 exception from the exception space.  Note: a rethrow from within the catch may
 242 replace this call with a <tt>__cxa_rethrow</tt>.</p>
 243
 244 </div>
 245
 246 <!-- ======================================================================= -->
 247 <div class="doc_subsection">
 248   <a name="finallys">Finallys</a>
 249 </div>
 250
 251 <div class="doc_text">
 252
 253 <p>To handle destructors and cleanups in try code, control may not run directly
 254 from a landing pad to the first catch.  Control may actually flow from the
 255 landing pad to clean up code and then to the first catch.  Since the required
 256 clean up for each invoke in a try may be different (ex., intervening
 257 constructor), there may be several landing pads for a given try.</p>
 258
 259 </div>
 260
 261 <!-- ======================================================================= -->
 262 <div class="doc_subsection">
 263   <a name="throw_filters">Throw Filters</a>
 264 </div>
 265
 266 <div class="doc_text">
 267
 268 <p>C++ allows the specification of which exception types that can be thrown from
 269 a function.  To represent this a top level landing pad may exist to filter out
 270 invalid types.  To express this in LLVM code the landing pad will call <a
 271 href="#llvm_eh_filter"><tt>llvm.eh.filter</tt></a> instead of <a
 272 href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>.  The arguments are the
 273 same, but what gets created in the exception table is different. <a
 274 href="#llvm_eh_filter"><tt>llvm.eh.filter</tt></a>  will return a negative value
 275 if it doesn't find a match. If no match is found then a call to
 276 <tt>__cxa_call_unexpected</tt> should be made, otherwise
 277 <tt>_Unwind_Resume</tt>.  Each of these functions require a reference to the
 278 exception structure.</p>
 279
 280 </div>
 281
 282 <!-- ======================================================================= -->
 283 <div class="doc_section">
 284   <a name="format_common_intrinsics">Exception Handling Intrinsics</a>
 285 </div>
 286
 287 <div class="doc_text">
 288
 289 <p>LLVM uses several intrinsic functions (name prefixed with "llvm.eh") to
 290 provide exception handling information at various points in generated code.</p>
 291
 292 </div>
 293
 294 <!-- ======================================================================= -->
 295 <div class="doc_subsubsection">
 296   <a name="llvm_eh_exception">llvm.eh.exception</a>
 297 </div>
 298
 299 <div class="doc_text">
 300 <pre>
 301   i8* %<a href="#llvm_eh_exception">llvm.eh.exception</a>( )
 302 </pre>
 303
 304 <p>This intrinsic indicates that the exception structure is available at this
 305 point in the code.  The backend will replace this intrinsic with code to fetch
 306 the first argument of a call.  The effect is that the intrinsic result is the
 307 exception structure reference.</p>
 308
 309 </div>
 310
 311 <!-- ======================================================================= -->
 312 <div class="doc_subsubsection">
 313   <a name="llvm_eh_selector">llvm.eh.selector</a>
 314 </div>
 315
 316 <div class="doc_text">
 317 <pre>
 318   i32 %<a href="#llvm_eh_selector">llvm.eh.selector</a>(i8*, i8*, i8*, ...)
 319 </pre>
 320
 321 <p>This intrinsic indicates that the exception selector is available at this
 322 point in the code.  The backend will replace this intrinsic with code to fetch
 323 the second argument of a call.  The effect is that the intrinsic result is the
 324 exception selector.</p>
 325
 326 <p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum of
 327 three arguments.  The first argument is the reference to the exception
 328 structure. The second argument is a reference to the personality function to be
 329 used for this try catch sequence. The remaining arguments are references to the
 330 type infos for each of the catch statements in the order they should be tested.
 331 The <i>catch all</i> (...) is represented with a <tt>null i8*</tt>.</p>
 332
 333 </div>
 334
 335 <!-- ======================================================================= -->
 336 <div class="doc_subsubsection">
 337   <a name="llvm_eh_filter">llvm.eh.filter</a>
 338 </div>
 339
 340 <div class="doc_text">
 341 <pre>
 342   i32 %<a href="#llvm_eh_filter">llvm.eh.filter</a>(i8*, i8*, i8*, ...)
 343 </pre>
 344
 345 <p>This intrinsic indicates that the exception selector is available at this
 346 point in the code.  The backend will replace this intrinsic with code to fetch
 347 the second argument of a call.  The effect is that the intrinsic result is the
 348 exception selector.</p>
 349
 350 <p><a href="#llvm_eh_filter"><tt>llvm.eh.filter</tt></a> takes a minimum of
 351 three arguments.  The first argument is the reference to the exception
 352 structure. The second argument is a reference to the personality function to be
 353 used for this function. The remaining arguments are references to the type infos
 354 for each type that can be thrown by the current function.</p>
 355
 356 </div>
 357
 358 <!-- ======================================================================= -->
 359 <div class="doc_subsubsection">
 360   <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
 361 </div>
 362
 363 <div class="doc_text">
 364 <pre>
 365   i32 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for</a>(i8*)
 366 </pre>
 367
 368 <p>This intrinsic returns the type info index in the exception table of the
 369 current function.  This value can be used to compare against the result of <a
 370 href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>.  The single argument is
 371 a reference to a type info.</p>
 372
 373 </div>
 374
 375 <!-- ======================================================================= -->
 376 <div class="doc_section">
 377   <a name="asm">Asm Table Formats</a>
 378 </div>
 379
 380 <div class="doc_text">
 381
 382 <p>There are two tables that are used by the exception handling runtime to
 383 determine which actions should take place when an exception is thrown.</p>
 384
 385 </div>
 386
 387 <!-- ======================================================================= -->
 388 <div class="doc_subsection">
 389   <a name="unwind_tables">Exception Handling Frame</a>
 390 </div>
 391
 392 <div class="doc_text">
 393
 394 <p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind
 395 frame used by dwarf debug info.  The frame contains all the information
 396 necessary to tear down the current frame and restore the state of the prior
 397 frame.  There is an exception handling frame for each function in a compile
 398 unit, plus a common exception handling frame that defines information common to
 399 all functions in the unit.</p>
 400
 401 <p>Todo - Table details here.</p>
 402
 403 </div>
 404
 405 <!-- ======================================================================= -->
 406 <div class="doc_subsection">
 407   <a name="exception_tables">Exception Tables</a>
 408 </div>
 409
 410 <div class="doc_text">
 411
 412 <p>An exception table contains information about what actions to take when an
 413 exception is thrown in a particular part of a function&apos;s code.  There is
 414 one exception table per function except leaf routines and functions that have
 415 only calls to non-throwing functions will not need an exception table.</p>
 416
 417 <p>Todo - Table details here.</p>
 418
 419 </div>
 420
 421 <!-- ======================================================================= -->
 422 <div class="doc_section">
 423   <a name="todo">ToDo</a>
 424 </div>
 425
 426 <div class="doc_text">
 427
 428 <ol>
 429
 430 <li><p>Need to create landing pads for code in between explicit landing pads.
 431 The landing pads will have a zero action and a NULL landing pad address and are
 432 used to inform the runtime that the exception should be rethrown.</li></p>
 433
 434 <li><p>Actions for a given function should be folded to save space.</p></li>
 435
 436 <li><p>Filters for inlined functions need to be handled more extensively.
 437 Currently it&apos;s hardwired for one filter per function.</li></p>
 438
 439 <li><p>Testing/Testing/Testing.</li></p>
 440
 441 </ol>
 442
 443 </div>
 444
 445 <!-- *********************************************************************** -->
 446
 447 <hr>
 448 <address>
 449   <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
 450   src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
 451   <a href="http://validator.w3.org/check/referer"><img
 452   src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
 453
 454   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
 455   <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
 456   Last modified: $Date$
 457 </address>
 458
 459 </body>
 460 </html>