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