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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   </ol></li>
  44   <li><a href="#asm">Asm Table Formats</a>
  45   <ol>
  46     <li><a href="#unwind_tables">Exception Handling Frame</a></li>
  47     <li><a href="#exception_tables">Exception Tables</a></li>
  48   </ol></li>
  49   <li><a href="#todo">ToDo</a></li>
  50 </ul>
  51 </td>
  52 </tr></table>
  53
  54 <div class="doc_author">
  55   <p>Written by <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
  56 </div>
  57
  58
  59 <!-- *********************************************************************** -->
  60 <div class="doc_section"><a name="introduction">Introduction</a></div>
  61 <!-- *********************************************************************** -->
  62
  63 <div class="doc_text">
  64
  65 <p>This document is the central repository for all information pertaining to
  66    exception handling in LLVM.  It describes the format that LLVM exception
  67    handling information takes, which is useful for those interested in creating
  68    front-ends or dealing directly with the information.  Further, this document
  69    provides specific examples of what exception handling information is used for
  70    in C/C++.</p>
  71
  72 </div>
  73
  74 <!-- ======================================================================= -->
  75 <div class="doc_subsection">
  76   <a name="itanium">Itanium ABI Zero-cost Exception Handling</a>
  77 </div>
  78
  79 <div class="doc_text">
  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 <div class="doc_subsection">
 109   <a name="sjlj">Setjmp/Longjmp Exception Handling</a>
 110 </div>
 111
 112 <div class="doc_text">
 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 <div class="doc_subsection">
 141   <a name="overview">Overview</a>
 142 </div>
 143
 144 <div class="doc_text">
 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 <!-- ======================================================================= -->
 188 <div class="doc_section">
 189   <a name="codegen">LLVM Code Generation</a>
 190 </div>
 191
 192 <div class="doc_text">
 193
 194 <p>At the time of this writing, only C++ exception handling support is available
 195    in LLVM.  So the remainder of this document will be somewhat C++-centric.</p>
 196
 197 <p>From the C++ developers perspective, exceptions are defined in terms of the
 198    <tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements.  In this section
 199    we will describe the implementation of LLVM exception handling in terms of
 200    C++ examples.</p>
 201
 202 </div>
 203
 204 <!-- ======================================================================= -->
 205 <div class="doc_subsection">
 206   <a name="throw">Throw</a>
 207 </div>
 208
 209 <div class="doc_text">
 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 <div class="doc_subsection">
 228   <a name="try_catch">Try/Catch</a>
 229 </div>
 230
 231 <div class="doc_text">
 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 <div class="doc_subsection">
 316   <a name="cleanups">Cleanups</a>
 317 </div>
 318
 319 <div class="doc_text">
 320
 321 <p>To handle destructors and cleanups in <tt>try</tt> code, control may not run
 322    directly from a landing pad to the first catch.  Control may actually flow
 323    from the landing pad to clean up code and then to the first catch.  Since the
 324    required clean up for each <tt>invoke</tt> in a <tt>try</tt> may be different
 325    (e.g. intervening constructor), there may be several landing pads for a given
 326    try.  If cleanups need to be run, an <tt>i32 0</tt> should be passed as the
 327    last <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument.
 328    However, when using DWARF exception handling with C++, a <tt>i8* null</tt>
 329    <a href="#restrictions">must</a> be passed instead.</p>
 330
 331 </div>
 332
 333 <!-- ======================================================================= -->
 334 <div class="doc_subsection">
 335   <a name="throw_filters">Throw Filters</a>
 336 </div>
 337
 338 <div class="doc_text">
 339
 340 <p>C++ allows the specification of which exception types can be thrown from a
 341    function.  To represent this a top level landing pad may exist to filter out
 342    invalid types.  To express this in LLVM code the landing pad will
 343    call <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>.  The
 344    arguments are a reference to the exception structure, a reference to the
 345    personality function, the length of the filter expression (the number of type
 346    infos plus one), followed by the type infos themselves.
 347    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> will return a
 348    negative value if the exception does not match any of the type infos.  If no
 349    match is found then a call to <tt>__cxa_call_unexpected</tt> should be made,
 350    otherwise <tt>_Unwind_Resume</tt>.  Each of these functions requires a
 351    reference to the exception structure.  Note that the most general form of an
 352    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> call can contain
 353    any number of type infos, filter expressions and cleanups (though having more
 354    than one cleanup is pointless).  The LLVM C++ front-end can generate such
 355    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls due to
 356    inlining creating nested exception handling scopes.</p>
 357
 358 </div>
 359
 360 <!-- ======================================================================= -->
 361 <div class="doc_subsection">
 362   <a name="restrictions">Restrictions</a>
 363 </div>
 364
 365 <div class="doc_text">
 366
 367 <p>The semantics of the invoke instruction require that any exception that
 368    unwinds through an invoke call should result in a branch to the invoke's
 369    unwind label.  However such a branch will only happen if the
 370    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> matches. Thus in
 371    order to ensure correct operation, the front-end must only generate
 372    <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls that are
 373    guaranteed to always match whatever exception unwinds through the invoke.
 374    For most languages it is enough to pass zero, indicating the presence of
 375    a <a href="#cleanups">cleanup</a>, as the
 376    last <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument.
 377    However for C++ this is not sufficient, because the C++ personality function
 378    will terminate the program if it detects that unwinding the exception only
 379    results in matches with cleanups.  For C++ a <tt>null i8*</tt> should be
 380    passed as the last <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>
 381    argument instead.  This is interpreted as a catch-all by the C++ personality
 382    function, and will always match.</p>
 383
 384 </div>
 385
 386 <!-- ======================================================================= -->
 387 <div class="doc_section">
 388   <a name="format_common_intrinsics">Exception Handling Intrinsics</a>
 389 </div>
 390
 391 <div class="doc_text">
 392
 393 <p>LLVM uses several intrinsic functions (name prefixed with "llvm.eh") to
 394    provide exception handling information at various points in generated
 395    code.</p>
 396
 397 </div>
 398
 399 <!-- ======================================================================= -->
 400 <div class="doc_subsubsection">
 401   <a name="llvm_eh_exception">llvm.eh.exception</a>
 402 </div>
 403
 404 <div class="doc_text">
 405
 406 <pre>
 407   i8* %<a href="#llvm_eh_exception">llvm.eh.exception</a>( )
 408 </pre>
 409
 410 <p>This intrinsic returns a pointer to the exception structure.</p>
 411
 412 </div>
 413
 414 <!-- ======================================================================= -->
 415 <div class="doc_subsubsection">
 416   <a name="llvm_eh_selector">llvm.eh.selector</a>
 417 </div>
 418
 419 <div class="doc_text">
 420
 421 <pre>
 422   i32 %<a href="#llvm_eh_selector">llvm.eh.selector</a>(i8*, i8*, i8*, ...)
 423 </pre>
 424
 425 <p>This intrinsic is used to compare the exception with the given type infos,
 426    filters and cleanups.</p>
 427
 428 <p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum of
 429    three arguments.  The first argument is the reference to the exception
 430    structure. The second argument is a reference to the personality function to
 431    be used for this try catch sequence. Each of the remaining arguments is
 432    either a reference to the type info for a catch statement,
 433    a <a href="#throw_filters">filter</a> expression, or the number zero
 434    representing a <a href="#cleanups">cleanup</a>.  The exception is tested
 435    against the arguments sequentially from first to last.  The result of
 436    the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a positive
 437    number if the exception matched a type info, a negative number if it matched
 438    a filter, and zero if it matched a cleanup.  If nothing is matched, the
 439    behaviour of the program is <a href="#restrictions">undefined</a>.  If a type
 440    info matched then the selector value is the index of the type info in the
 441    exception table, which can be obtained using the
 442    <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
 443
 444 </div>
 445
 446 <!-- ======================================================================= -->
 447 <div class="doc_subsubsection">
 448   <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
 449 </div>
 450
 451 <div class="doc_text">
 452
 453 <pre>
 454   i32 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for</a>(i8*)
 455 </pre>
 456
 457 <p>This intrinsic returns the type info index in the exception table of the
 458    current function.  This value can be used to compare against the result
 459    of <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>.  The single
 460    argument is a reference to a type info.</p>
 461
 462 </div>
 463
 464 <!-- ======================================================================= -->
 465 <div class="doc_subsubsection">
 466   <a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>
 467 </div>
 468
 469 <div class="doc_text">
 470
 471 <pre>
 472   i32 %<a href="#llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>(i8*)
 473 </pre>
 474
 475 <p>The SJLJ exception handling uses this intrinsic to force register saving for
 476    the current function and to store the address of the following instruction
 477    for use as a destination address by <a href="#llvm_eh_sjlj_longjmp">
 478    <tt>llvm.eh.sjlj.longjmp</tt></a>. The buffer format and the overall
 479    functioning of this intrinsic is compatible with the GCC
 480    <tt>__builtin_setjmp</tt> implementation, allowing code built with the
 481    two compilers to interoperate.</p>
 482
 483 <p>The single parameter is a pointer to a five word buffer in which the calling
 484    context is saved. The front end places the frame pointer in the first word,
 485    and the target implementation of this intrinsic should place the destination
 486    address for a
 487    <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> in the
 488    second word. The following three words are available for use in a
 489    target-specific manner.</p>
 490
 491 </div>
 492
 493 <!-- ======================================================================= -->
 494 <div class="doc_subsubsection">
 495   <a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>
 496 </div>
 497
 498 <div class="doc_text">
 499
 500 <pre>
 501   i8* %<a href="#llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>( )
 502 </pre>
 503
 504 <p>Used for SJLJ based exception handling, the <a href="#llvm_eh_sjlj_lsda">
 505    <tt>llvm.eh.sjlj.lsda</tt></a> intrinsic returns the address of the Language
 506    Specific Data Area (LSDA) for the current function. The SJLJ front-end code
 507    stores this address in the exception handling function context for use by the
 508    runtime.</p>
 509
 510 </div>
 511
 512 <!-- ======================================================================= -->
 513 <div class="doc_subsubsection">
 514   <a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>
 515 </div>
 516
 517 <div class="doc_text">
 518
 519 <pre>
 520   void %<a href="#llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>(i32)
 521 </pre>
 522
 523 <p>For SJLJ based exception handling, the <a href="#llvm_eh_sjlj_callsite">
 524   <tt>llvm.eh.sjlj.callsite</tt></a> intrinsic identifies the callsite value
 525   associated with the following invoke instruction. This is used to ensure
 526   that landing pad entries in the LSDA are generated in the matching order.</p>
 527
 528 </div>
 529
 530 <!-- ======================================================================= -->
 531 <div class="doc_section">
 532   <a name="asm">Asm Table Formats</a>
 533 </div>
 534
 535 <div class="doc_text">
 536
 537 <p>There are two tables that are used by the exception handling runtime to
 538    determine which actions should take place when an exception is thrown.</p>
 539
 540 </div>
 541
 542 <!-- ======================================================================= -->
 543 <div class="doc_subsection">
 544   <a name="unwind_tables">Exception Handling Frame</a>
 545 </div>
 546
 547 <div class="doc_text">
 548
 549 <p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind
 550    frame used by dwarf debug info.  The frame contains all the information
 551    necessary to tear down the current frame and restore the state of the prior
 552    frame.  There is an exception handling frame for each function in a compile
 553    unit, plus a common exception handling frame that defines information common
 554    to all functions in the unit.</p>
 555
 556 <p>Todo - Table details here.</p>
 557
 558 </div>
 559
 560 <!-- ======================================================================= -->
 561 <div class="doc_subsection">
 562   <a name="exception_tables">Exception Tables</a>
 563 </div>
 564
 565 <div class="doc_text">
 566
 567 <p>An exception table contains information about what actions to take when an
 568    exception is thrown in a particular part of a function's code.  There is one
 569    exception table per function except leaf routines and functions that have
 570    only calls to non-throwing functions will not need an exception table.</p>
 571
 572 <p>Todo - Table details here.</p>
 573
 574 </div>
 575
 576 <!-- ======================================================================= -->
 577 <div class="doc_section">
 578   <a name="todo">ToDo</a>
 579 </div>
 580
 581 <div class="doc_text">
 582
 583 <ol>
 584
 585   <li>Testing/Testing/Testing.</li>
 586
 587 </ol>
 588
 589 </div>
 590
 591 <!-- *********************************************************************** -->
 592
 593 <hr>
 594 <address>
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 599
 600   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
 601   <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
 602   Last modified: $Date$
 603 </address>
 604
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