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  13
  14 <h1>Exception Handling in LLVM</h1>
  15
  16 <table class="layout" style="width:100%">
  17   <tr class="layout">
  18     <td class="left">
  19 <ul>
  20   <li><a href="#introduction">Introduction</a>
  21   <ol>
  22     <li><a href="#itanium">Itanium ABI Zero-cost Exception Handling</a></li>
  23     <li><a href="#sjlj">Setjmp/Longjmp Exception Handling</a></li>
  24     <li><a href="#overview">Overview</a></li>
  25   </ol></li>
  26   <li><a href="#codegen">LLVM Code Generation</a>
  27   <ol>
  28     <li><a href="#throw">Throw</a></li>
  29     <li><a href="#try_catch">Try/Catch</a></li>
  30     <li><a href="#cleanups">Cleanups</a></li>
  31     <li><a href="#throw_filters">Throw Filters</a></li>
  32     <li><a href="#restrictions">Restrictions</a></li>
  33   </ol></li>
  34   <li><a href="#format_common_intrinsics">Exception Handling Intrinsics</a>
  35   <ol>
  36         <li><a href="#llvm_eh_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_resume"><tt>llvm.eh.resume</tt></a></li>
  39         <li><a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a></li>
  40         <li><a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a></li>
  41         <li><a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a></li>
  42         <li><a href="#llvm_eh_sjlj_lsda"><tt>llvm.eh.sjlj.lsda</tt></a></li>
  43         <li><a href="#llvm_eh_sjlj_callsite"><tt>llvm.eh.sjlj.callsite</tt></a></li>
  44         <li><a href="#llvm_eh_sjlj_dispatchsetup"><tt>llvm.eh.sjlj.dispatchsetup</tt></a></li>
  45   </ol></li>
  46   <li><a href="#asm">Asm Table Formats</a>
  47   <ol>
  48     <li><a href="#unwind_tables">Exception Handling Frame</a></li>
  49     <li><a href="#exception_tables">Exception Tables</a></li>
  50   </ol></li>
  51   <li><a href="#todo">ToDo</a></li>
  52 </ul>
  53 </td>
  54 </tr></table>
  55
  56 <div class="doc_author">
  57   <p>Written by <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
  58 </div>
  59
  60
  61 <!-- *********************************************************************** -->
  62 <h2><a name="introduction">Introduction</a></h2>
  63 <!-- *********************************************************************** -->
  64
  65 <div>
  66
  67 <p>This document is the central repository for all information pertaining to
  68    exception handling in LLVM.  It describes the format that LLVM exception
  69    handling information takes, which is useful for those interested in creating
  70    front-ends or dealing directly with the information.  Further, this document
  71    provides specific examples of what exception handling information is used for
  72    in C/C++.</p>
  73
  74 <!-- ======================================================================= -->
  75 <h3>
  76   <a name="itanium">Itanium ABI Zero-cost Exception Handling</a>
  77 </h3>
  78
  79 <div>
  80
  81 <p>Exception handling for most programming languages is designed to recover from
  82    conditions that rarely occur during general use of an application.  To that
  83    end, exception handling should not interfere with the main flow of an
  84    application's algorithm by performing checkpointing tasks, such as saving the
  85    current pc or register state.</p>
  86
  87 <p>The Itanium ABI Exception Handling Specification defines a methodology for
  88    providing outlying data in the form of exception tables without inlining
  89    speculative exception handling code in the flow of an application's main
  90    algorithm.  Thus, the specification is said to add "zero-cost" to the normal
  91    execution of an application.</p>
  92
  93 <p>A more complete description of the Itanium ABI exception handling runtime
  94    support of can be found at
  95    <a href="http://www.codesourcery.com/cxx-abi/abi-eh.html">Itanium C++ ABI:
  96    Exception Handling</a>. A description of the exception frame format can be
  97    found at
  98    <a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html">Exception
  99    Frames</a>, with details of the DWARF 3 specification at
 100    <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3 Standard</a>.
 101    A description for the C++ exception table formats can be found at
 102    <a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling
 103    Tables</a>.</p>
 104
 105 </div>
 106
 107 <!-- ======================================================================= -->
 108 <h3>
 109   <a name="sjlj">Setjmp/Longjmp Exception Handling</a>
 110 </h3>
 111
 112 <div>
 113
 114 <p>Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics
 115    <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a> and
 116    <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to
 117    handle control flow for exception handling.</p>
 118
 119 <p>For each function which does exception processing, be it try/catch blocks
 120    or cleanups, that function registers itself on a global frame list. When
 121    exceptions are being unwound, the runtime uses this list to identify which
 122    functions need processing.<p>
 123
 124 <p>Landing pad selection is encoded in the call site entry of the function
 125    context. The runtime returns to the function via
 126    <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>, where
 127    a switch table transfers control to the appropriate landing pad based on
 128    the index stored in the function context.</p>
 129
 130 <p>In contrast to DWARF exception handling, which encodes exception regions
 131    and frame information in out-of-line tables, SJLJ exception handling
 132    builds and removes the unwind frame context at runtime. This results in
 133    faster exception handling at the expense of slower execution when no
 134    exceptions are thrown. As exceptions are, by their nature, intended for
 135    uncommon code paths, DWARF exception handling is generally preferred to
 136    SJLJ.</p>
 137 </div>
 138
 139 <!-- ======================================================================= -->
 140 <h3>
 141   <a name="overview">Overview</a>
 142 </h3>
 143
 144 <div>
 145
 146 <p>When an exception is thrown in LLVM code, the runtime does its best to find a
 147    handler suited to processing the circumstance.</p>
 148
 149 <p>The runtime first attempts to find an <i>exception frame</i> corresponding to
 150    the function where the exception was thrown.  If the programming language
 151    (e.g. C++) supports exception handling, the exception frame contains a
 152    reference to an exception table describing how to process the exception.  If
 153    the language (e.g. C) does not support exception handling, or if the
 154    exception needs to be forwarded to a prior activation, the exception frame
 155    contains information about how to unwind the current activation and restore
 156    the state of the prior activation.  This process is repeated until the
 157    exception is handled.  If the exception is not handled and no activations
 158    remain, then the application is terminated with an appropriate error
 159    message.</p>
 160
 161 <p>Because different programming languages have different behaviors when
 162    handling exceptions, the exception handling ABI provides a mechanism for
 163    supplying <i>personalities.</i> An exception handling personality is defined
 164    by way of a <i>personality function</i> (e.g. <tt>__gxx_personality_v0</tt>
 165    in C++), which receives the context of the exception, an <i>exception
 166    structure</i> containing the exception object type and value, and a reference
 167    to the exception table for the current function.  The personality function
 168    for the current compile unit is specified in a <i>common exception
 169    frame</i>.</p>
 170
 171 <p>The organization of an exception table is language dependent.  For C++, an
 172    exception table is organized as a series of code ranges defining what to do
 173    if an exception occurs in that range.  Typically, the information associated
 174    with a range defines which types of exception objects (using C++ <i>type
 175    info</i>) that are handled in that range, and an associated action that
 176    should take place.  Actions typically pass control to a <i>landing
 177    pad</i>.</p>
 178
 179 <p>A landing pad corresponds to the code found in the <i>catch</i> portion of
 180    a <i>try</i>/<i>catch</i> sequence.  When execution resumes at a landing
 181    pad, it receives the exception structure and a selector corresponding to
 182    the <i>type</i> of exception thrown.  The selector is then used to determine
 183    which <i>catch</i> should actually process the exception.</p>
 184
 185 </div>
 186
 187 </div>
 188
 189 <!-- ======================================================================= -->
 190 <h2>
 191   <a name="codegen">LLVM Code Generation</a>
 192 </h2>
 193
 194 <div>
 195
 196 <p>At the time of this writing, only C++ exception handling support is available
 197    in LLVM.  So the remainder of this document will be somewhat C++-centric.</p>
 198
 199 <p>From the C++ developers perspective, exceptions are defined in terms of the
 200    <tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements.  In this section
 201    we will describe the implementation of LLVM exception handling in terms of
 202    C++ examples.</p>
 203
 204 <!-- ======================================================================= -->
 205 <h3>
 206   <a name="throw">Throw</a>
 207 </h3>
 208
 209 <div>
 210
 211 <p>Languages that support exception handling typically provide a <tt>throw</tt>
 212    operation to initiate the exception process.  Internally, a throw operation
 213    breaks down into two steps.  First, a request is made to allocate exception
 214    space for an exception structure.  This structure needs to survive beyond the
 215    current activation.  This structure will contain the type and value of the
 216    object being thrown.  Second, a call is made to the runtime to raise the
 217    exception, passing the exception structure as an argument.</p>
 218
 219 <p>In C++, the allocation of the exception structure is done by
 220    the <tt>__cxa_allocate_exception</tt> runtime function.  The exception
 221    raising is handled by <tt>__cxa_throw</tt>.  The type of the exception is
 222    represented using a C++ RTTI structure.</p>
 223
 224 </div>
 225
 226 <!-- ======================================================================= -->
 227 <h3>
 228   <a name="try_catch">Try/Catch</a>
 229 </h3>
 230
 231 <div>
 232
 233 <p>A call within the scope of a <i>try</i> statement can potentially raise an
 234    exception.  In those circumstances, the LLVM C++ front-end replaces the call
 235    with an <tt>invoke</tt> instruction.  Unlike a call, the <tt>invoke</tt> has
 236    two potential continuation points: where to continue when the call succeeds
 237    as per normal; and where to continue if the call raises an exception, either
 238    by a throw or the unwinding of a throw.</p>
 239
 240 <p>The term used to define a the place where an <tt>invoke</tt> continues after
 241    an exception is called a <i>landing pad</i>.  LLVM landing pads are
 242    conceptually alternative function entry points where an exception structure
 243    reference and a type info index are passed in as arguments.  The landing pad
 244    saves the exception structure reference and then proceeds to select the catch
 245    block that corresponds to the type info of the exception object.</p>
 246
 247 <p>Two LLVM intrinsic functions are used to convey information about the landing
 248    pad to the back end.</p>
 249
 250 <ol>
 251   <li><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a> takes no
 252       arguments and returns a pointer to the exception structure.  This only
 253       returns a sensible value if called after an <tt>invoke</tt> has branched
 254       to a landing pad.  Due to code generation limitations, it must currently
 255       be called in the landing pad itself.</li>
 256
 257   <li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum
 258       of three arguments.  The first argument is the reference to the exception
 259       structure. The second argument is a reference to the personality function
 260       to be used for this <tt>try</tt>/<tt>catch</tt> sequence. Each of the
 261       remaining arguments is either a reference to the type info for
 262       a <tt>catch</tt> statement, a <a href="#throw_filters">filter</a>
 263       expression, or the number zero (<tt>0</tt>) representing
 264       a <a href="#cleanups">cleanup</a>.  The exception is tested against the
 265       arguments sequentially from first to last.  The result of
 266       the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a
 267       positive number if the exception matched a type info, a negative number if
 268       it matched a filter, and zero if it matched a cleanup.  If nothing is
 269       matched, the behaviour of the program
 270       is <a href="#restrictions">undefined</a>.  This only returns a sensible
 271       value if called after an <tt>invoke</tt> has branched to a landing pad.
 272       Due to codegen limitations, it must currently be called in the landing pad
 273       itself.  If a type info matched, then the selector value is the index of
 274       the type info in the exception table, which can be obtained using the
 275       <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a>
 276       intrinsic.</li>
 277 </ol>
 278
 279 <p>Once the landing pad has the type info selector, the code branches to the
 280    code for the first catch.  The catch then checks the value of the type info
 281    selector against the index of type info for that catch.  Since the type info
 282    index is not known until all the type info have been gathered in the backend,
 283    the catch code will call the
 284    <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic
 285    to determine the index for a given type info.  If the catch fails to match
 286    the selector then control is passed on to the next catch. Note: Since the
 287    landing pad will not be used if there is no match in the list of type info on
 288    the call to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>, then
 289    neither the last catch nor <i>catch all</i> need to perform the check
 290    against the selector.</p>
 291
 292 <p>Finally, the entry and exit of catch code is bracketed with calls
 293    to <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p>
 294
 295 <ul>
 296   <li><tt>__cxa_begin_catch</tt> takes a exception structure reference as an
 297       argument and returns the value of the exception object.</li>
 298
 299   <li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br>
 300     <ol>
 301       <li>Locates the most recently caught exception and decrements its handler
 302           count,</li>
 303       <li>Removes the exception from the "caught" stack if the handler count
 304           goes to zero, and</li>
 305       <li>Destroys the exception if the handler count goes to zero, and the
 306           exception was not re-thrown by throw.</li>
 307     </ol>
 308     <p>Note: a rethrow from within the catch may replace this call with
 309        a <tt>__cxa_rethrow</tt>.</p></li>
 310 </ul>
 311
 312 </div>
 313
 314 <!-- ======================================================================= -->
 315 <h3>
 316   <a name="cleanups">Cleanups</a>
 317 </h3>
 318
 319 <div>
 320
 321 <p>A cleanup is extra code which needs to be run as part of unwinding
 322    a scope.  C++ destructors are a prominent example, but other
 323    languages and language extensions provide a variety of different
 324    kinds of cleanup.  In general, a landing pad may need to run
 325    arbitrary amounts of cleanup code before actually entering a catch
 326    block.  To indicate the presence of cleanups, a landing pad's call
 327    to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> should
 328    end with the argument <tt>i32 0</tt>; otherwise, the unwinder will
 329    not stop at the landing pad if there are no catches or filters that
 330    require it to.</p>
 331
 332 <p>Do not allow a new exception to propagate out of the execution of a
 333    cleanup.  This can corrupt the internal state of the unwinder.
 334    Different languages describe different high-level semantics for
 335    these situations: for example, C++ requires that the process be
 336    terminated, whereas Ada cancels both exceptions and throws a third.</p>
 337
 338 <p>When all cleanups have completed, if the exception is not handled
 339    by the current function, resume unwinding by calling the
 340    <a href="#llvm_eh_resume"><tt>llvm.eh.resume</tt></a> intrinsic,
 341    passing in the results of <tt>llvm.eh.exception</tt> and
 342    <tt>llvm.eh.selector</tt> for the original landing pad.</p>
 343
 344 </div>
 345
 346 <!-- ======================================================================= -->
 347 <h3>
 348   <a name="throw_filters">Throw Filters</a>
 349 </h3>
 350
 351 <div>
 352
 353 <p>C++ allows the specification of which exception types can be thrown from a
 354    function.  To represent this a top level landing pad may exist to filter out
 355    invalid types.  To express this in LLVM code the landing pad will
 356    call <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>.  The
 357    arguments are a reference to the exception structure, a reference to the
 358    personality function, the length of the filter expression (the number of type
 359    infos plus one), followed by the type infos themselves.
 360    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> will return a
 361    negative value if the exception does not match any of the type infos.  If no
 362    match is found then a call to <tt>__cxa_call_unexpected</tt> should be made,
 363    otherwise <tt>_Unwind_Resume</tt>.  Each of these functions requires a
 364    reference to the exception structure.  Note that the most general form of an
 365    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> call can contain
 366    any number of type infos, filter expressions and cleanups (though having more
 367    than one cleanup is pointless).  The LLVM C++ front-end can generate such
 368    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls due to
 369    inlining creating nested exception handling scopes.</p>
 370
 371 </div>
 372
 373 <!-- ======================================================================= -->
 374 <h3>
 375   <a name="restrictions">Restrictions</a>
 376 </h3>
 377
 378 <div>
 379
 380 <p>The unwinder delegates the decision of whether to stop in a call
 381    frame to that call frame's language-specific personality function.
 382    Not all personalities functions guarantee that they will stop to
 383    perform cleanups: for example, the GNU C++ personality doesn't do
 384    so unless the exception is actually caught somewhere further up the
 385    stack.  When using this personality to implement EH for a language
 386    that guarantees that cleanups will always be run, be sure to
 387    indicate a catch-all in the
 388    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> call
 389    rather than just cleanups.</p>
 390
 391 <p>In order for inlining to behave correctly, landing pads must be
 392    prepared to handle selector results that they did not originally
 393    advertise.  Suppose that a function catches exceptions of
 394    type <tt>A</tt>, and it's inlined into a function that catches
 395    exceptions of type <tt>B</tt>.  The inliner will update the
 396    selector for the inlined landing pad to include the fact
 397    that <tt>B</tt> is caught.  If that landing pad assumes that it
 398    will only be entered to catch an <tt>A</tt>, it's in for a rude
 399    surprise.  Consequently, landing pads must test for the selector
 400    results they understand and then resume exception propagation
 401    with the <a href="#llvm_eh_resume"><tt>llvm.eh.resume</tt></a>
 402    intrinsic if none of the conditions match.</p>
 403
 404 </div>
 405
 406 </div>
 407
 408 <!-- ======================================================================= -->
 409 <h2>
 410   <a name="format_common_intrinsics">Exception Handling Intrinsics</a>
 411 </h2>
 412
 413 <div>
 414
 415 <p>LLVM uses several intrinsic functions (name prefixed with "llvm.eh") to
 416    provide exception handling information at various points in generated
 417    code.</p>
 418
 419 <!-- ======================================================================= -->
 420 <h4>
 421   <a name="llvm_eh_exception">llvm.eh.exception</a>
 422 </h4>
 423
 424 <div>
 425
 426 <pre>
 427   i8* %<a href="#llvm_eh_exception">llvm.eh.exception</a>()
 428 </pre>
 429
 430 <p>This intrinsic returns a pointer to the exception structure.</p>
 431
 432 </div>
 433
 434 <!-- ======================================================================= -->
 435 <h4>
 436   <a name="llvm_eh_selector">llvm.eh.selector</a>
 437 </h4>
 438
 439 <div>
 440
 441 <pre>
 442   i32 %<a href="#llvm_eh_selector">llvm.eh.selector</a>(i8*, i8*, ...)
 443 </pre>
 444
 445 <p>This intrinsic is used to compare the exception with the given type infos,
 446    filters and cleanups.</p>
 447
 448 <p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a
 449    minimum of three arguments.  The first argument is the reference to
 450    the exception structure. The second argument is a reference to the
 451    personality function to be used for this try catch sequence. Each
 452    of the remaining arguments is either a reference to the type info
 453    for a catch statement, a <a href="#throw_filters">filter</a>
 454    expression, or the number zero representing
 455    a <a href="#cleanups">cleanup</a>.  The exception is tested against
 456    the arguments sequentially from first to last.  The result of
 457    the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a
 458    positive number if the exception matched a type info, a negative
 459    number if it matched a filter, and zero if it matched a cleanup.
 460    If nothing is matched, or if only a cleanup is matched, different
 461    personality functions may or may not cause control to stop at the
 462    landing pad; see <a href="#restrictions">the restrictions</a> for
 463    more information.  If a type info matched then the selector value
 464    is the index of the type info in the exception table, which can be
 465    obtained using the
 466    <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
 467
 468 <p>If a landing pad containing a call to <tt>llvm.eh.selector</tt> is
 469    inlined into an <tt>invoke</tt> instruction, the selector arguments
 470    for the outer landing pad are appended to those of the inlined
 471    landing pad.  Consequently, landing pads must be written to ignore
 472    selector values that they did not originally advertise.</p>
 473
 474 </div>
 475
 476 <!-- ======================================================================= -->
 477 <h4>
 478   <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
 479 </h4>
 480
 481 <div>
 482
 483 <pre>
 484   i32 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for</a>(i8*)
 485 </pre>
 486
 487 <p>This intrinsic returns the type info index in the exception table of the
 488    current function.  This value can be used to compare against the result
 489    of <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>.  The single
 490    argument is a reference to a type info.</p>
 491
 492 </div>
 493
 494 <!-- ======================================================================= -->
 495 <h4>
 496   <a name="llvm_eh_resume">llvm.eh.resume</a>
 497 </h4>
 498
 499 <div>
 500
 501 <pre>
 502   void %<a href="#llvm_eh_resume">llvm.eh.resume</a>(i8*, i32) noreturn
 503 </pre>
 504
 505 <p>This intrinsic is used to resume propagation of an exception after
 506    landing at a landing pad.  The first argument should be the result
 507    of <a href="#llvm_eh_exception">llvm.eh.exception</a> for that
 508    landing pad, and the second argument should be the result of
 509    <a href="#llvm_eh_selector">llvm.eh.selector</a>.  When a call to
 510    this intrinsic is inlined into an invoke, the call is transformed
 511    into a branch to the invoke's unwind destination, using its
 512    arguments in place of the calls
 513    to <a href="#llvm_eh_exception">llvm.eh.exception</a> and
 514    <a href="#llvm_eh_selector">llvm.eh.selector</a> there.</p>
 515
 516 <p>This intrinsic is not implicitly <tt>nounwind</tt>; calls to it
 517    will always throw.  It may not be invoked.</p>
 518
 519 </div>
 520
 521 <!-- ======================================================================= -->
 522 <h4>
 523   <a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>
 524 </h4>
 525
 526 <div>
 527
 528 <pre>
 529   i32 %<a href="#llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>(i8*)
 530 </pre>
 531
 532 <p>The SJLJ exception handling uses this intrinsic to force register saving for
 533    the current function and to store the address of the following instruction
 534    for use as a destination address by <a href="#llvm_eh_sjlj_longjmp">
 535    <tt>llvm.eh.sjlj.longjmp</tt></a>. The buffer format and the overall
 536    functioning of this intrinsic is compatible with the GCC
 537    <tt>__builtin_setjmp</tt> implementation, allowing code built with the
 538    two compilers to interoperate.</p>
 539
 540 <p>The single parameter is a pointer to a five word buffer in which the calling
 541    context is saved. The front end places the frame pointer in the first word,
 542    and the target implementation of this intrinsic should place the destination
 543    address for a
 544    <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> in the
 545    second word. The following three words are available for use in a
 546    target-specific manner.</p>
 547
 548 </div>
 549
 550 <!-- ======================================================================= -->
 551 <h4>
 552   <a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a>
 553 </h4>
 554
 555 <div>
 556
 557 <pre>
 558   void %<a href="#llvm_eh_sjlj_longjmp">llvm.eh.sjlj.setjmp</a>(i8*)
 559 </pre>
 560
 561 <p>The <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>
 562    intrinsic is used to implement <tt>__builtin_longjmp()</tt> for SJLJ
 563    style exception handling. The single parameter is a pointer to a
 564    buffer populated by <a href="#llvm_eh_sjlj_setjmp">
 565      <tt>llvm.eh.sjlj.setjmp</tt></a>. The frame pointer and stack pointer
 566    are restored from the buffer, then control is transferred to the
 567    destination address.</p>
 568
 569 </div>
 570 <!-- ======================================================================= -->
 571 <h4>
 572   <a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>
 573 </h4>
 574
 575 <div>
 576
 577 <pre>
 578   i8* %<a href="#llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>()
 579 </pre>
 580
 581 <p>Used for SJLJ based exception handling, the <a href="#llvm_eh_sjlj_lsda">
 582    <tt>llvm.eh.sjlj.lsda</tt></a> intrinsic returns the address of the Language
 583    Specific Data Area (LSDA) for the current function. The SJLJ front-end code
 584    stores this address in the exception handling function context for use by the
 585    runtime.</p>
 586
 587 </div>
 588
 589 <!-- ======================================================================= -->
 590 <h4>
 591   <a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>
 592 </h4>
 593
 594 <div>
 595
 596 <pre>
 597   void %<a href="#llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>(i32)
 598 </pre>
 599
 600 <p>For SJLJ based exception handling, the <a href="#llvm_eh_sjlj_callsite">
 601   <tt>llvm.eh.sjlj.callsite</tt></a> intrinsic identifies the callsite value
 602   associated with the following invoke instruction. This is used to ensure
 603   that landing pad entries in the LSDA are generated in the matching order.</p>
 604
 605 </div>
 606
 607 <!-- ======================================================================= -->
 608 <h4>
 609   <a name="llvm_eh_sjlj_dispatchsetup">llvm.eh.sjlj.dispatchsetup</a>
 610 </h4>
 611
 612 <div>
 613
 614 <pre>
 615   void %<a href="#llvm_eh_sjlj_dispatchsetup">llvm.eh.sjlj.dispatchsetup</a>(i32)
 616 </pre>
 617
 618 <p>For SJLJ based exception handling, the <a href="#llvm_eh_sjlj_dispatchsetup">
 619   <tt>llvm.eh.sjlj.dispatchsetup</tt></a> intrinsic is used by targets to do
 620   any unwind-edge setup they need. By default, no action is taken.  </p>
 621
 622 </div>
 623
 624 </div>
 625
 626 <!-- ======================================================================= -->
 627 <h2>
 628   <a name="asm">Asm Table Formats</a>
 629 </h2>
 630
 631 <div>
 632
 633 <p>There are two tables that are used by the exception handling runtime to
 634    determine which actions should take place when an exception is thrown.</p>
 635
 636 <!-- ======================================================================= -->
 637 <h3>
 638   <a name="unwind_tables">Exception Handling Frame</a>
 639 </h3>
 640
 641 <div>
 642
 643 <p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind
 644    frame used by dwarf debug info.  The frame contains all the information
 645    necessary to tear down the current frame and restore the state of the prior
 646    frame.  There is an exception handling frame for each function in a compile
 647    unit, plus a common exception handling frame that defines information common
 648    to all functions in the unit.</p>
 649
 650 <p>Todo - Table details here.</p>
 651
 652 </div>
 653
 654 <!-- ======================================================================= -->
 655 <h3>
 656   <a name="exception_tables">Exception Tables</a>
 657 </h3>
 658
 659 <div>
 660
 661 <p>An exception table contains information about what actions to take when an
 662    exception is thrown in a particular part of a function's code.  There is one
 663    exception table per function except leaf routines and functions that have
 664    only calls to non-throwing functions will not need an exception table.</p>
 665
 666 <p>Todo - Table details here.</p>
 667
 668 </div>
 669
 670 </div>
 671
 672 <!-- ======================================================================= -->
 673 <h2>
 674   <a name="todo">ToDo</a>
 675 </h2>
 676
 677 <div>
 678
 679 <ol>
 680
 681   <li>Testing/Testing/Testing.</li>
 682
 683 </ol>
 684
 685 </div>
 686
 687 <!-- *********************************************************************** -->
 688
 689 <hr>
 690 <address>
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 695
 696   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
 697   <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
 698   Last modified: $Date$
 699 </address>
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