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  14 <h1>Exception Handling in LLVM</h1>
  15
  16 <table class="layout" style="width:100%">
  17   <tr class="layout">
  18     <td class="left">
  19 <ul>
  20   <li><a href="#introduction">Introduction</a>
  21   <ol>
  22     <li><a href="#itanium">Itanium ABI Zero-cost Exception Handling</a></li>
  23     <li><a href="#sjlj">Setjmp/Longjmp Exception Handling</a></li>
  24     <li><a href="#overview">Overview</a></li>
  25   </ol></li>
  26   <li><a href="#codegen">LLVM Code Generation</a>
  27   <ol>
  28     <li><a href="#throw">Throw</a></li>
  29     <li><a href="#try_catch">Try/Catch</a></li>
  30     <li><a href="#cleanups">Cleanups</a></li>
  31     <li><a href="#throw_filters">Throw Filters</a></li>
  32     <li><a href="#restrictions">Restrictions</a></li>
  33   </ol></li>
  34   <li><a href="#format_common_intrinsics">Exception Handling Intrinsics</a>
  35   <ol>
  36         <li><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a></li>
  37         <li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a></li>
  38         <li><a href="#llvm_eh_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 <h2><a name="introduction">Introduction</a></h2>
  62 <!-- *********************************************************************** -->
  63
  64 <div>
  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 <!-- ======================================================================= -->
  74 <h3>
  75   <a name="itanium">Itanium ABI Zero-cost Exception Handling</a>
  76 </h3>
  77
  78 <div>
  79
  80 <p>Exception handling for most programming languages is designed to recover from
  81    conditions that rarely occur during general use of an application.  To that
  82    end, exception handling should not interfere with the main flow of an
  83    application's algorithm by performing checkpointing tasks, such as saving the
  84    current pc or register state.</p>
  85
  86 <p>The Itanium ABI Exception Handling Specification defines a methodology for
  87    providing outlying data in the form of exception tables without inlining
  88    speculative exception handling code in the flow of an application's main
  89    algorithm.  Thus, the specification is said to add "zero-cost" to the normal
  90    execution of an application.</p>
  91
  92 <p>A more complete description of the Itanium ABI exception handling runtime
  93    support of can be found at
  94    <a href="http://www.codesourcery.com/cxx-abi/abi-eh.html">Itanium C++ ABI:
  95    Exception Handling</a>. A description of the exception frame format can be
  96    found at
  97    <a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html">Exception
  98    Frames</a>, with details of the DWARF 3 specification at
  99    <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3 Standard</a>.
 100    A description for the C++ exception table formats can be found at
 101    <a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling
 102    Tables</a>.</p>
 103
 104 </div>
 105
 106 <!-- ======================================================================= -->
 107 <h3>
 108   <a name="sjlj">Setjmp/Longjmp Exception Handling</a>
 109 </h3>
 110
 111 <div>
 112
 113 <p>Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics
 114    <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a> and
 115    <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to
 116    handle control flow for exception handling.</p>
 117
 118 <p>For each function which does exception processing, be it try/catch blocks
 119    or cleanups, that function registers itself on a global frame list. When
 120    exceptions are being unwound, the runtime uses this list to identify which
 121    functions need processing.<p>
 122
 123 <p>Landing pad selection is encoded in the call site entry of the function
 124    context. The runtime returns to the function via
 125    <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>, where
 126    a switch table transfers control to the appropriate landing pad based on
 127    the index stored in the function context.</p>
 128
 129 <p>In contrast to DWARF exception handling, which encodes exception regions
 130    and frame information in out-of-line tables, SJLJ exception handling
 131    builds and removes the unwind frame context at runtime. This results in
 132    faster exception handling at the expense of slower execution when no
 133    exceptions are thrown. As exceptions are, by their nature, intended for
 134    uncommon code paths, DWARF exception handling is generally preferred to
 135    SJLJ.</p>
 136 </div>
 137
 138 <!-- ======================================================================= -->
 139 <h3>
 140   <a name="overview">Overview</a>
 141 </h3>
 142
 143 <div>
 144
 145 <p>When an exception is thrown in LLVM code, the runtime does its best to find a
 146    handler suited to processing the circumstance.</p>
 147
 148 <p>The runtime first attempts to find an <i>exception frame</i> corresponding to
 149    the function where the exception was thrown.  If the programming language
 150    (e.g. C++) supports exception handling, the exception frame contains a
 151    reference to an exception table describing how to process the exception.  If
 152    the language (e.g. C) does not support exception handling, or if the
 153    exception needs to be forwarded to a prior activation, the exception frame
 154    contains information about how to unwind the current activation and restore
 155    the state of the prior activation.  This process is repeated until the
 156    exception is handled.  If the exception is not handled and no activations
 157    remain, then the application is terminated with an appropriate error
 158    message.</p>
 159
 160 <p>Because different programming languages have different behaviors when
 161    handling exceptions, the exception handling ABI provides a mechanism for
 162    supplying <i>personalities.</i> An exception handling personality is defined
 163    by way of a <i>personality function</i> (e.g. <tt>__gxx_personality_v0</tt>
 164    in C++), which receives the context of the exception, an <i>exception
 165    structure</i> containing the exception object type and value, and a reference
 166    to the exception table for the current function.  The personality function
 167    for the current compile unit is specified in a <i>common exception
 168    frame</i>.</p>
 169
 170 <p>The organization of an exception table is language dependent.  For C++, an
 171    exception table is organized as a series of code ranges defining what to do
 172    if an exception occurs in that range.  Typically, the information associated
 173    with a range defines which types of exception objects (using C++ <i>type
 174    info</i>) that are handled in that range, and an associated action that
 175    should take place.  Actions typically pass control to a <i>landing
 176    pad</i>.</p>
 177
 178 <p>A landing pad corresponds to the code found in the <i>catch</i> portion of
 179    a <i>try</i>/<i>catch</i> sequence.  When execution resumes at a landing
 180    pad, it receives the exception structure and a selector corresponding to
 181    the <i>type</i> of exception thrown.  The selector is then used to determine
 182    which <i>catch</i> should actually process the exception.</p>
 183
 184 </div>
 185
 186 </div>
 187
 188 <!-- ======================================================================= -->
 189 <h2>
 190   <a name="codegen">LLVM Code Generation</a>
 191 </h2>
 192
 193 <div>
 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 <!-- ======================================================================= -->
 204 <h3>
 205   <a name="throw">Throw</a>
 206 </h3>
 207
 208 <div>
 209
 210 <p>Languages that support exception handling typically provide a <tt>throw</tt>
 211    operation to initiate the exception process.  Internally, a throw operation
 212    breaks down into two steps.  First, a request is made to allocate exception
 213    space for an exception structure.  This structure needs to survive beyond the
 214    current activation.  This structure will contain the type and value of the
 215    object being thrown.  Second, a call is made to the runtime to raise the
 216    exception, passing the exception structure as an argument.</p>
 217
 218 <p>In C++, the allocation of the exception structure is done by
 219    the <tt>__cxa_allocate_exception</tt> runtime function.  The exception
 220    raising is handled by <tt>__cxa_throw</tt>.  The type of the exception is
 221    represented using a C++ RTTI structure.</p>
 222
 223 </div>
 224
 225 <!-- ======================================================================= -->
 226 <h3>
 227   <a name="try_catch">Try/Catch</a>
 228 </h3>
 229
 230 <div>
 231
 232 <p>A call within the scope of a <i>try</i> statement can potentially raise an
 233    exception.  In those circumstances, the LLVM C++ front-end replaces the call
 234    with an <tt>invoke</tt> instruction.  Unlike a call, the <tt>invoke</tt> has
 235    two potential continuation points: where to continue when the call succeeds
 236    as per normal; and where to continue if the call raises an exception, either
 237    by a throw or the unwinding of a throw.</p>
 238
 239 <p>The term used to define a the place where an <tt>invoke</tt> continues after
 240    an exception is called a <i>landing pad</i>.  LLVM landing pads are
 241    conceptually alternative function entry points where an exception structure
 242    reference and a type info index are passed in as arguments.  The landing pad
 243    saves the exception structure reference and then proceeds to select the catch
 244    block that corresponds to the type info of the exception object.</p>
 245
 246 <p>Two LLVM intrinsic functions are used to convey information about the landing
 247    pad to the back end.</p>
 248
 249 <ol>
 250   <li><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a> takes no
 251       arguments and returns a pointer to the exception structure.  This only
 252       returns a sensible value if called after an <tt>invoke</tt> has branched
 253       to a landing pad.  Due to code generation limitations, it must currently
 254       be called in the landing pad itself.</li>
 255
 256   <li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum
 257       of three arguments.  The first argument is the reference to the exception
 258       structure. The second argument is a reference to the personality function
 259       to be used for this <tt>try</tt>/<tt>catch</tt> sequence. Each of the
 260       remaining arguments is either a reference to the type info for
 261       a <tt>catch</tt> statement, a <a href="#throw_filters">filter</a>
 262       expression, or the number zero (<tt>0</tt>) representing
 263       a <a href="#cleanups">cleanup</a>.  The exception is tested against the
 264       arguments sequentially from first to last.  The result of
 265       the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a
 266       positive number if the exception matched a type info, a negative number if
 267       it matched a filter, and zero if it matched a cleanup.  If nothing is
 268       matched, the behaviour of the program
 269       is <a href="#restrictions">undefined</a>.  This only returns a sensible
 270       value if called after an <tt>invoke</tt> has branched to a landing pad.
 271       Due to codegen limitations, it must currently be called in the landing pad
 272       itself.  If a type info matched, then the selector value is the index of
 273       the type info in the exception table, which can be obtained using the
 274       <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a>
 275       intrinsic.</li>
 276 </ol>
 277
 278 <p>Once the landing pad has the type info selector, the code branches to the
 279    code for the first catch.  The catch then checks the value of the type info
 280    selector against the index of type info for that catch.  Since the type info
 281    index is not known until all the type info have been gathered in the backend,
 282    the catch code will call the
 283    <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic
 284    to determine the index for a given type info.  If the catch fails to match
 285    the selector then control is passed on to the next catch. Note: Since the
 286    landing pad will not be used if there is no match in the list of type info on
 287    the call to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>, then
 288    neither the last catch nor <i>catch all</i> need to perform the check
 289    against the selector.</p>
 290
 291 <p>Finally, the entry and exit of catch code is bracketed with calls
 292    to <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p>
 293
 294 <ul>
 295   <li><tt>__cxa_begin_catch</tt> takes a exception structure reference as an
 296       argument and returns the value of the exception object.</li>
 297
 298   <li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br>
 299     <ol>
 300       <li>Locates the most recently caught exception and decrements its handler
 301           count,</li>
 302       <li>Removes the exception from the "caught" stack if the handler count
 303           goes to zero, and</li>
 304       <li>Destroys the exception if the handler count goes to zero, and the
 305           exception was not re-thrown by throw.</li>
 306     </ol>
 307     <p>Note: a rethrow from within the catch may replace this call with
 308        a <tt>__cxa_rethrow</tt>.</p></li>
 309 </ul>
 310
 311 </div>
 312
 313 <!-- ======================================================================= -->
 314 <h3>
 315   <a name="cleanups">Cleanups</a>
 316 </h3>
 317
 318 <div>
 319
 320 <p>To handle destructors and cleanups in <tt>try</tt> code, control may not run
 321    directly from a landing pad to the first catch.  Control may actually flow
 322    from the landing pad to clean up code and then to the first catch.  Since the
 323    required clean up for each <tt>invoke</tt> in a <tt>try</tt> may be different
 324    (e.g. intervening constructor), there may be several landing pads for a given
 325    try.  If cleanups need to be run, an <tt>i32 0</tt> should be passed as the
 326    last <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument.
 327    However, when using DWARF exception handling with C++, a <tt>i8* null</tt>
 328    <a href="#restrictions">must</a> be passed instead.</p>
 329
 330 </div>
 331
 332 <!-- ======================================================================= -->
 333 <h3>
 334   <a name="throw_filters">Throw Filters</a>
 335 </h3>
 336
 337 <div>
 338
 339 <p>C++ allows the specification of which exception types can be thrown from a
 340    function.  To represent this a top level landing pad may exist to filter out
 341    invalid types.  To express this in LLVM code the landing pad will
 342    call <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>.  The
 343    arguments are a reference to the exception structure, a reference to the
 344    personality function, the length of the filter expression (the number of type
 345    infos plus one), followed by the type infos themselves.
 346    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> will return a
 347    negative value if the exception does not match any of the type infos.  If no
 348    match is found then a call to <tt>__cxa_call_unexpected</tt> should be made,
 349    otherwise <tt>_Unwind_Resume</tt>.  Each of these functions requires a
 350    reference to the exception structure.  Note that the most general form of an
 351    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> call can contain
 352    any number of type infos, filter expressions and cleanups (though having more
 353    than one cleanup is pointless).  The LLVM C++ front-end can generate such
 354    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls due to
 355    inlining creating nested exception handling scopes.</p>
 356
 357 </div>
 358
 359 <!-- ======================================================================= -->
 360 <h3>
 361   <a name="restrictions">Restrictions</a>
 362 </h3>
 363
 364 <div>
 365
 366 <p>The semantics of the invoke instruction require that any exception that
 367    unwinds through an invoke call should result in a branch to the invoke's
 368    unwind label.  However such a branch will only happen if the
 369    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> matches. Thus in
 370    order to ensure correct operation, the front-end must only generate
 371    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls that are
 372    guaranteed to always match whatever exception unwinds through the invoke.
 373    For most languages it is enough to pass zero, indicating the presence of
 374    a <a href="#cleanups">cleanup</a>, as the
 375    last <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument.
 376    However for C++ this is not sufficient, because the C++ personality function
 377    will terminate the program if it detects that unwinding the exception only
 378    results in matches with cleanups.  For C++ a <tt>null i8*</tt> should be
 379    passed as the last <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>
 380    argument instead.  This is interpreted as a catch-all by the C++ personality
 381    function, and will always match.</p>
 382
 383 </div>
 384
 385 </div>
 386
 387 <!-- ======================================================================= -->
 388 <h2>
 389   <a name="format_common_intrinsics">Exception Handling Intrinsics</a>
 390 </h2>
 391
 392 <div>
 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 <!-- ======================================================================= -->
 399 <h4>
 400   <a name="llvm_eh_exception">llvm.eh.exception</a>
 401 </h4>
 402
 403 <div>
 404
 405 <pre>
 406   i8* %<a href="#llvm_eh_exception">llvm.eh.exception</a>()
 407 </pre>
 408
 409 <p>This intrinsic returns a pointer to the exception structure.</p>
 410
 411 </div>
 412
 413 <!-- ======================================================================= -->
 414 <h4>
 415   <a name="llvm_eh_selector">llvm.eh.selector</a>
 416 </h4>
 417
 418 <div>
 419
 420 <pre>
 421   i32 %<a href="#llvm_eh_selector">llvm.eh.selector</a>(i8*, i8*, ...)
 422 </pre>
 423
 424 <p>This intrinsic is used to compare the exception with the given type infos,
 425    filters and cleanups.</p>
 426
 427 <p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum of
 428    three arguments.  The first argument is the reference to the exception
 429    structure. The second argument is a reference to the personality function to
 430    be used for this try catch sequence. Each of the remaining arguments is
 431    either a reference to the type info for a catch statement,
 432    a <a href="#throw_filters">filter</a> expression, or the number zero
 433    representing a <a href="#cleanups">cleanup</a>.  The exception is tested
 434    against the arguments sequentially from first to last.  The result of
 435    the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a positive
 436    number if the exception matched a type info, a negative number if it matched
 437    a filter, and zero if it matched a cleanup.  If nothing is matched, the
 438    behaviour of the program is <a href="#restrictions">undefined</a>.  If a type
 439    info matched then the selector value is the index of the type info in the
 440    exception table, which can be obtained using the
 441    <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
 442
 443 </div>
 444
 445 <!-- ======================================================================= -->
 446 <h4>
 447   <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
 448 </h4>
 449
 450 <div>
 451
 452 <pre>
 453   i32 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for</a>(i8*)
 454 </pre>
 455
 456 <p>This intrinsic returns the type info index in the exception table of the
 457    current function.  This value can be used to compare against the result
 458    of <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>.  The single
 459    argument is a reference to a type info.</p>
 460
 461 </div>
 462
 463 <!-- ======================================================================= -->
 464 <h4>
 465   <a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>
 466 </h4>
 467
 468 <div>
 469
 470 <pre>
 471   i32 %<a href="#llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>(i8*)
 472 </pre>
 473
 474 <p>The SJLJ exception handling uses this intrinsic to force register saving for
 475    the current function and to store the address of the following instruction
 476    for use as a destination address by <a href="#llvm_eh_sjlj_longjmp">
 477    <tt>llvm.eh.sjlj.longjmp</tt></a>. The buffer format and the overall
 478    functioning of this intrinsic is compatible with the GCC
 479    <tt>__builtin_setjmp</tt> implementation, allowing code built with the
 480    two compilers to interoperate.</p>
 481
 482 <p>The single parameter is a pointer to a five word buffer in which the calling
 483    context is saved. The front end places the frame pointer in the first word,
 484    and the target implementation of this intrinsic should place the destination
 485    address for a
 486    <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> in the
 487    second word. The following three words are available for use in a
 488    target-specific manner.</p>
 489
 490 </div>
 491
 492 <!-- ======================================================================= -->
 493 <h4>
 494   <a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a>
 495 </h4>
 496
 497 <div>
 498
 499 <pre>
 500   void %<a href="#llvm_eh_sjlj_longjmp">llvm.eh.sjlj.setjmp</a>(i8*)
 501 </pre>
 502
 503 <p>The <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>
 504    intrinsic is used to implement <tt>__builtin_longjmp()</tt> for SJLJ
 505    style exception handling. The single parameter is a pointer to a
 506    buffer populated by <a href="#llvm_eh_sjlj_setjmp">
 507      <tt>llvm.eh.sjlj.setjmp</tt></a>. The frame pointer and stack pointer
 508    are restored from the buffer, then control is transferred to the
 509    destination address.</p>
 510
 511 </div>
 512 <!-- ======================================================================= -->
 513 <h4>
 514   <a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>
 515 </h4>
 516
 517 <div>
 518
 519 <pre>
 520   i8* %<a href="#llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>()
 521 </pre>
 522
 523 <p>Used for SJLJ based exception handling, the <a href="#llvm_eh_sjlj_lsda">
 524    <tt>llvm.eh.sjlj.lsda</tt></a> intrinsic returns the address of the Language
 525    Specific Data Area (LSDA) for the current function. The SJLJ front-end code
 526    stores this address in the exception handling function context for use by the
 527    runtime.</p>
 528
 529 </div>
 530
 531 <!-- ======================================================================= -->
 532 <h4>
 533   <a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>
 534 </h4>
 535
 536 <div>
 537
 538 <pre>
 539   void %<a href="#llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>(i32)
 540 </pre>
 541
 542 <p>For SJLJ based exception handling, the <a href="#llvm_eh_sjlj_callsite">
 543   <tt>llvm.eh.sjlj.callsite</tt></a> intrinsic identifies the callsite value
 544   associated with the following invoke instruction. This is used to ensure
 545   that landing pad entries in the LSDA are generated in the matching order.</p>
 546
 547 </div>
 548
 549 <!-- ======================================================================= -->
 550 <h4>
 551   <a name="llvm_eh_sjlj_dispatchsetup">llvm.eh.sjlj.dispatchsetup</a>
 552 </h4>
 553
 554 <div>
 555
 556 <pre>
 557   void %<a href="#llvm_eh_sjlj_dispatchsetup">llvm.eh.sjlj.dispatchsetup</a>(i32)
 558 </pre>
 559
 560 <p>For SJLJ based exception handling, the <a href="#llvm_eh_sjlj_dispatchsetup">
 561   <tt>llvm.eh.sjlj.dispatchsetup</tt></a> intrinsic is used by targets to do
 562   any unwind-edge setup they need. By default, no action is taken.  </p>
 563
 564 </div>
 565
 566 </div>
 567
 568 <!-- ======================================================================= -->
 569 <h2>
 570   <a name="asm">Asm Table Formats</a>
 571 </h2>
 572
 573 <div>
 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 <!-- ======================================================================= -->
 579 <h3>
 580   <a name="unwind_tables">Exception Handling Frame</a>
 581 </h3>
 582
 583 <div>
 584
 585 <p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind
 586    frame used by dwarf debug info.  The frame contains all the information
 587    necessary to tear down the current frame and restore the state of the prior
 588    frame.  There is an exception handling frame for each function in a compile
 589    unit, plus a common exception handling frame that defines information common
 590    to all functions in the unit.</p>
 591
 592 <p>Todo - Table details here.</p>
 593
 594 </div>
 595
 596 <!-- ======================================================================= -->
 597 <h3>
 598   <a name="exception_tables">Exception Tables</a>
 599 </h3>
 600
 601 <div>
 602
 603 <p>An exception table contains information about what actions to take when an
 604    exception is thrown in a particular part of a function's code.  There is one
 605    exception table per function except leaf routines and functions that have
 606    only calls to non-throwing functions will not need an exception table.</p>
 607
 608 <p>Todo - Table details here.</p>
 609
 610 </div>
 611
 612 </div>
 613
 614 <!-- ======================================================================= -->
 615 <h2>
 616   <a name="todo">ToDo</a>
 617 </h2>
 618
 619 <div>
 620
 621 <ol>
 622
 623   <li>Testing/Testing/Testing.</li>
 624
 625 </ol>
 626
 627 </div>
 628
 629 <!-- *********************************************************************** -->
 630
 631 <hr>
 632 <address>
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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: $Date$
 641 </address>
 642
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