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    5. 

  5.   <title>Exception Handling in LLVM</title>

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    7. 

  7.   <meta name="description"

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  8.         content="Exception Handling in LLVM.">

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  9.   <link rel="stylesheet" href="llvm.css" type="text/css">

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  10. </head>

    11. 

  11. 

    12. 

  12. <body>

    13. 

  13. 

    14. 

  14. <h1>Exception Handling in LLVM</h1>

    15. 

  15. 

    16. 

  16. <table class="layout" style="width:100%">

    17. 

  17.   <tr class="layout">

    18. 

  18.     <td class="left">

    19. 

  19. <ul>

    20. 

  20.   <li><a href="#introduction">Introduction</a>

    21. 

  21.   <ol>

    22. 

  22.     <li><a href="#itanium">Itanium ABI Zero-cost Exception Handling</a></li>

    23. 

  23.     <li><a href="#sjlj">Setjmp/Longjmp Exception Handling</a></li>

    24. 

  24.     <li><a href="#overview">Overview</a></li>

    25. 

  25.   </ol></li>

    26. 

  26.   <li><a href="#codegen">LLVM Code Generation</a>

    27. 

  27.   <ol>

    28. 

  28.     <li><a href="#throw">Throw</a></li>

    29. 

  29.     <li><a href="#try_catch">Try/Catch</a></li>

    30. 

  30.     <li><a href="#cleanups">Cleanups</a></li>

    31. 

  31.     <li><a href="#throw_filters">Throw Filters</a></li>

    32. 

  32.     <li><a href="#restrictions">Restrictions</a></li>

    33. 

  33.   </ol></li>

    34. 

  34.   <li><a href="#format_common_intrinsics">Exception Handling Intrinsics</a>

    35. 

  35.   <ol>

    36. 

  36.   	<li><a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a></li>

    37. 

  37.   	<li><a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a></li>

    38. 

  38.   	<li><a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a></li>

    39. 

  39.   	<li><a href="#llvm_eh_sjlj_lsda"><tt>llvm.eh.sjlj.lsda</tt></a></li>

    40. 

  40.   	<li><a href="#llvm_eh_sjlj_callsite"><tt>llvm.eh.sjlj.callsite</tt></a></li>

    41. 

  41.   	<li><a href="#llvm_eh_sjlj_dispatchsetup"><tt>llvm.eh.sjlj.dispatchsetup</tt></a></li>

    42. 

  42.   </ol></li>

    43. 

  43.   <li><a href="#asm">Asm Table Formats</a>

    44. 

  44.   <ol>

    45. 

  45.     <li><a href="#unwind_tables">Exception Handling Frame</a></li>

    46. 

  46.     <li><a href="#exception_tables">Exception Tables</a></li>

    47. 

  47.   </ol></li>

    48. 

  48. </ul>

    49. 

  49. </td>

    50. 

  50. </tr></table>

    51. 

  51. 

    52. 

  52. <div class="doc_author">

    53. 

  53.   <p>Written by <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>

    54. 

  54. </div>

    55. 

  55. 

    56. 

  56. 

    57. 

  57. <!-- *********************************************************************** -->

    58. 

  58. <h2><a name="introduction">Introduction</a></h2>

    59. 

  59. <!-- *********************************************************************** -->

    60. 

  60. 

    61. 

  61. <div>

    62. 

  62. 

    63. 

  63. <p>This document is the central repository for all information pertaining to

    64. 

  64.    exception handling in LLVM.  It describes the format that LLVM exception

    65. 

  65.    handling information takes, which is useful for those interested in creating

    66. 

  66.    front-ends or dealing directly with the information.  Further, this document

    67. 

  67.    provides specific examples of what exception handling information is used for

    68. 

  68.    in C and C++.</p>

    69. 

  69. 

    70. 

  70. <!-- ======================================================================= -->

    71. 

  71. <h3>

    72. 

  72.   <a name="itanium">Itanium ABI Zero-cost Exception Handling</a>

    73. 

  73. </h3>

    74. 

  74. 

    75. 

  75. <div>

    76. 

  76. 

    77. 

  77. <p>Exception handling for most programming languages is designed to recover from

    78. 

  78.    conditions that rarely occur during general use of an application.  To that

    79. 

  79.    end, exception handling should not interfere with the main flow of an

    80. 

  80.    application's algorithm by performing checkpointing tasks, such as saving the

    81. 

  81.    current pc or register state.</p>

    82. 

  82. 

    83. 

  83. <p>The Itanium ABI Exception Handling Specification defines a methodology for

    84. 

  84.    providing outlying data in the form of exception tables without inlining

    85. 

  85.    speculative exception handling code in the flow of an application's main

    86. 

  86.    algorithm.  Thus, the specification is said to add "zero-cost" to the normal

    87. 

  87.    execution of an application.</p>

    88. 

  88. 

    89. 

  89. <p>A more complete description of the Itanium ABI exception handling runtime

    90. 

  90.    support of can be found at

    91. 

  91.    <a href="http://www.codesourcery.com/cxx-abi/abi-eh.html">Itanium C++ ABI:

    92. 

  92.    Exception Handling</a>. A description of the exception frame format can be

    93. 

  93.    found at

    94. 

  94.    <a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html">Exception

    95. 

  95.    Frames</a>, with details of the DWARF 4 specification at

    96. 

  96.    <a href="http://dwarfstd.org/Dwarf4Std.php">DWARF 4 Standard</a>.

    97. 

  97.    A description for the C++ exception table formats can be found at

    98. 

  98.    <a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling

    99. 

  99.    Tables</a>.</p>

    100. 

  100. 

    101. 

  101. </div>

    102. 

  102. 

    103. 

  103. <!-- ======================================================================= -->

    104. 

  104. <h3>

    105. 

  105.   <a name="sjlj">Setjmp/Longjmp Exception Handling</a>

    106. 

  106. </h3>

    107. 

  107. 

    108. 

  108. <div>

    109. 

  109. 

    110. 

  110. <p>Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics

    111. 

  111.    <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a> and

    112. 

  112.    <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to

    113. 

  113.    handle control flow for exception handling.</p>

    114. 

  114. 

    115. 

  115. <p>For each function which does exception processing &mdash; be

    116. 

  116.    it <tt>try</tt>/<tt>catch</tt> blocks or cleanups &mdash; that function

    117. 

  117.    registers itself on a global frame list. When exceptions are unwinding, the

    118. 

  118.    runtime uses this list to identify which functions need processing.<p>

    119. 

  119. 

    120. 

  120. <p>Landing pad selection is encoded in the call site entry of the function

    121. 

  121.    context. The runtime returns to the function via

    122. 

  122.    <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>, where

    123. 

  123.    a switch table transfers control to the appropriate landing pad based on

    124. 

  124.    the index stored in the function context.</p>

    125. 

  125. 

    126. 

  126. <p>In contrast to DWARF exception handling, which encodes exception regions

    127. 

  127.    and frame information in out-of-line tables, SJLJ exception handling

    128. 

  128.    builds and removes the unwind frame context at runtime. This results in

    129. 

  129.    faster exception handling at the expense of slower execution when no

    130. 

  130.    exceptions are thrown. As exceptions are, by their nature, intended for

    131. 

  131.    uncommon code paths, DWARF exception handling is generally preferred to

    132. 

  132.    SJLJ.</p>

    133. 

  133. 

    134. 

  134. </div>

    135. 

  135. 

    136. 

  136. <!-- ======================================================================= -->

    137. 

  137. <h3>

    138. 

  138.   <a name="overview">Overview</a>

    139. 

  139. </h3>

    140. 

  140. 

    141. 

  141. <div>

    142. 

  142. 

    143. 

  143. <p>When an exception is thrown in LLVM code, the runtime does its best to find a

    144. 

  144.    handler suited to processing the circumstance.</p>

    145. 

  145. 

    146. 

  146. <p>The runtime first attempts to find an <i>exception frame</i> corresponding to

    147. 

  147.    the function where the exception was thrown.  If the programming language

    148. 

  148.    supports exception handling (e.g. C++), the exception frame contains a

    149. 

  149.    reference to an exception table describing how to process the exception.  If

    150. 

  150.    the language does not support exception handling (e.g. C), or if the

    151. 

  151.    exception needs to be forwarded to a prior activation, the exception frame

    152. 

  152.    contains information about how to unwind the current activation and restore

    153. 

  153.    the state of the prior activation.  This process is repeated until the

    154. 

  154.    exception is handled. If the exception is not handled and no activations

    155. 

  155.    remain, then the application is terminated with an appropriate error

    156. 

  156.    message.</p>

    157. 

  157. 

    158. 

  158. <p>Because different programming languages have different behaviors when

    159. 

  159.    handling exceptions, the exception handling ABI provides a mechanism for

    160. 

  160.    supplying <i>personalities</i>. An exception handling personality is defined

    161. 

  161.    by way of a <i>personality function</i> (e.g. <tt>__gxx_personality_v0</tt>

    162. 

  162.    in C++), which receives the context of the exception, an <i>exception

    163. 

  163.    structure</i> containing the exception object type and value, and a reference

    164. 

  164.    to the exception table for the current function.  The personality function

    165. 

  165.    for the current compile unit is specified in a <i>common exception

    166. 

  166.    frame</i>.</p>

    167. 

  167. 

    168. 

  168. <p>The organization of an exception table is language dependent. For C++, an

    169. 

  169.    exception table is organized as a series of code ranges defining what to do

    170. 

  170.    if an exception occurs in that range. Typically, the information associated

    171. 

  171.    with a range defines which types of exception objects (using C++ <i>type

    172. 

  172.    info</i>) that are handled in that range, and an associated action that

    173. 

  173.    should take place. Actions typically pass control to a <i>landing

    174. 

  174.    pad</i>.</p>

    175. 

  175. 

    176. 

  176. <p>A landing pad corresponds roughly to the code found in the <tt>catch</tt>

    177. 

  177.    portion of a <tt>try</tt>/<tt>catch</tt> sequence. When execution resumes at

    178. 

  178.    a landing pad, it receives an <i>exception structure</i> and a

    179. 

  179.    <i>selector value</i> corresponding to the <i>type</i> of exception

    180. 

  180.    thrown. The selector is then used to determine which <i>catch</i> should

    181. 

  181.    actually process the exception.</p>

    182. 

  182. 

    183. 

  183. </div>

    184. 

  184. 

    185. 

  185. </div>

    186. 

  186. 

    187. 

  187. <!-- ======================================================================= -->

    188. 

  188. <h2>

    189. 

  189.   <a name="codegen">LLVM Code Generation</a>

    190. 

  190. </h2>

    191. 

  191. 

    192. 

  192. <div>

    193. 

  193. 

    194. 

  194. <p>From a C++ developer's perspective, exceptions are defined in terms of the

    195. 

  195.    <tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements. In this section

    196. 

  196.    we will describe the implementation of LLVM exception handling in terms of

    197. 

  197.    C++ examples.</p>

    198. 

  198. 

    199. 

  199. <!-- ======================================================================= -->

    200. 

  200. <h3>

    201. 

  201.   <a name="throw">Throw</a>

    202. 

  202. </h3>

    203. 

  203. 

    204. 

  204. <div>

    205. 

  205. 

    206. 

  206. <p>Languages that support exception handling typically provide a <tt>throw</tt>

    207. 

  207.    operation to initiate the exception process. Internally, a <tt>throw</tt>

    208. 

  208.    operation breaks down into two steps.</p>

    209. 

  209. 

    210. 

  210. <ol>

    211. 

  211.   <li>A request is made to allocate exception space for an exception structure.

    212. 

  212.       This structure needs to survive beyond the current activation. This

    213. 

  213.       structure will contain the type and value of the object being thrown.</li>

    214. 

  214. 

    215. 

  215.   <li>A call is made to the runtime to raise the exception, passing the

    216. 

  216.       exception structure as an argument.</li>

    217. 

  217. </ol>

    218. 

  218. 

    219. 

  219. <p>In C++, the allocation of the exception structure is done by the

    220. 

  220.    <tt>__cxa_allocate_exception</tt> runtime function. The exception raising is

    221. 

  221.    handled by <tt>__cxa_throw</tt>. The type of the exception is represented

    222. 

  222.    using a C++ RTTI structure.</p>

    223. 

  223. 

    224. 

  224. </div>

    225. 

  225. 

    226. 

  226. <!-- ======================================================================= -->

    227. 

  227. <h3>

    228. 

  228.   <a name="try_catch">Try/Catch</a>

    229. 

  229. </h3>

    230. 

  230. 

    231. 

  231. <div>

    232. 

  232. 

    233. 

  233. <p>A call within the scope of a <i>try</i> statement can potentially raise an

    234. 

  234.    exception. In those circumstances, the LLVM C++ front-end replaces the call

    235. 

  235.    with an <tt>invoke</tt> instruction. Unlike a call, the <tt>invoke</tt> has

    236. 

  236.    two potential continuation points:</p>

    237. 

  237. 

    238. 

  238. <ol>

    239. 

  239.   <li>where to continue when the call succeeds as per normal, and</li>

    240. 

  240. 

    241. 

  241.   <li>where to continue if the call raises an exception, either by a throw or

    242. 

  242.       the unwinding of a throw</li>

    243. 

  243. </ol>

    244. 

  244. 

    245. 

  245. <p>The term used to define a the place where an <tt>invoke</tt> continues after

    246. 

  246.    an exception is called a <i>landing pad</i>. LLVM landing pads are

    247. 

  247.    conceptually alternative function entry points where an exception structure

    248. 

  248.    reference and a type info index are passed in as arguments. The landing pad

    249. 

  249.    saves the exception structure reference and then proceeds to select the catch

    250. 

  250.    block that corresponds to the type info of the exception object.</p>

    251. 

  251. 

    252. 

  252. <p>The LLVM <a href="LangRef.html#i_landingpad"><tt>landingpad</tt>

    253. 

  253.    instruction</a> is used to convey information about the landing pad to the

    254. 

  254.    back end. For C++, the <tt>landingpad</tt> instruction returns a pointer and

    255. 

  255.    integer pair corresponding to the pointer to the <i>exception structure</i>

    256. 

  256.    and the <i>selector value</i> respectively.</p>

    257. 

  257. 

    258. 

  258. <p>The <tt>landingpad</tt> instruction takes a reference to the personality

    259. 

  259.    function to be used for this <tt>try</tt>/<tt>catch</tt> sequence. The

    260. 

  260.    remainder of the instruction is a list of <i>cleanup</i>, <i>catch</i>,

    261. 

  261.    and <i>filter</i> clauses. The exception is tested against the clauses

    262. 

  262.    sequentially from first to last. The selector value is a positive number if

    263. 

  263.    the exception matched a type info, a negative number if it matched a filter,

    264. 

  264.    and zero if it matched a cleanup. If nothing is matched, the behavior of

    265. 

  265.    the program is <a href="#restrictions">undefined</a>. If a type info matched,

    266. 

  266.    then the selector value is the index of the type info in the exception table,

    267. 

  267.    which can be obtained using the

    268. 

  268.    <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>

    269. 

  269. 

    270. 

  270. <p>Once the landing pad has the type info selector, the code branches to the

    271. 

  271.    code for the first catch. The catch then checks the value of the type info

    272. 

  272.    selector against the index of type info for that catch.  Since the type info

    273. 

  273.    index is not known until all the type infos have been gathered in the

    274. 

  274.    backend, the catch code must call the

    275. 

  275.    <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic to

    276. 

  276.    determine the index for a given type info. If the catch fails to match the

    277. 

  277.    selector then control is passed on to the next catch.</p>

    278. 

  278. 

    279. 

  279. <p>Finally, the entry and exit of catch code is bracketed with calls to

    280. 

  280.    <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p>

    281. 

  281. 

    282. 

  282. <ul>

    283. 

  283.   <li><tt>__cxa_begin_catch</tt> takes an exception structure reference as an

    284. 

  284.       argument and returns the value of the exception object.</li>

    285. 

  285. 

    286. 

  286.   <li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br>

    287. 

  287.     <ol>

    288. 

  288.       <li>Locates the most recently caught exception and decrements its handler

    289. 

  289.           count,</li>

    290. 

  290.       <li>Removes the exception from the <i>caught</i> stack if the handler

    291. 

  291.           count goes to zero, and</li>

    292. 

  292.       <li>Destroys the exception if the handler count goes to zero and the

    293. 

  293.           exception was not re-thrown by throw.</li>

    294. 

  294.     </ol>

    295. 

  295.     <p><b>Note:</b> a rethrow from within the catch may replace this call with

    296. 

  296.        a <tt>__cxa_rethrow</tt>.</p></li>

    297. 

  297. </ul>

    298. 

  298. 

    299. 

  299. </div>

    300. 

  300. 

    301. 

  301. <!-- ======================================================================= -->

    302. 

  302. <h3>

    303. 

  303.   <a name="cleanups">Cleanups</a>

    304. 

  304. </h3>

    305. 

  305. 

    306. 

  306. <div>

    307. 

  307. 

    308. 

  308. <p>A cleanup is extra code which needs to be run as part of unwinding a scope.

    309. 

  309.    C++ destructors are a typical example, but other languages and language

    310. 

  310.    extensions provide a variety of different kinds of cleanups. In general, a

    311. 

  311.    landing pad may need to run arbitrary amounts of cleanup code before actually

    312. 

  312.    entering a catch block. To indicate the presence of cleanups, a

    313. 

  313.    <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>

    314. 

  314.    should have a <i>cleanup</i> clause. Otherwise, the unwinder will not stop at

    315. 

  315.    the landing pad if there are no catches or filters that require it to.</p>

    316. 

  316. 

    317. 

  317. <p><b>Note:</b> Do not allow a new exception to propagate out of the execution

    318. 

  318.    of a cleanup. This can corrupt the internal state of the unwinder.

    319. 

  319.    Different languages describe different high-level semantics for these

    320. 

  320.    situations: for example, C++ requires that the process be terminated, whereas

    321. 

  321.    Ada cancels both exceptions and throws a third.</p>

    322. 

  322. 

    323. 

  323. <p>When all cleanups are finished, if the exception is not handled by the

    324. 

  324.    current function, resume unwinding by calling the

    325. 

  325.    <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a>, passing in

    326. 

  326.    the result of the <tt>landingpad</tt> instruction for the original landing

    327. 

  327.    pad.</p>

    328. 

  328. 

    329. 

  329. </div>

    330. 

  330. 

    331. 

  331. <!-- ======================================================================= -->

    332. 

  332. <h3>

    333. 

  333.   <a name="throw_filters">Throw Filters</a>

    334. 

  334. </h3>

    335. 

  335. 

    336. 

  336. <div>

    337. 

  337. 

    338. 

  338. <p>C++ allows the specification of which exception types may be thrown from a

    339. 

  339.    function. To represent this, a top level landing pad may exist to filter out

    340. 

  340.    invalid types. To express this in LLVM code the

    341. 

  341.    <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> will

    342. 

  342.    have a filter clause. The clause consists of an array of type infos.

    343. 

  343.    <tt>landingpad</tt> will return a negative value if the exception does not

    344. 

  344.    match any of the type infos. If no match is found then a call

    345. 

  345.    to <tt>__cxa_call_unexpected</tt> should be made, otherwise

    346. 

  346.    <tt>_Unwind_Resume</tt>.  Each of these functions requires a reference to the

    347. 

  347.    exception structure.  Note that the most general form of a

    348. 

  348.    <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> can

    349. 

  349.    have any number of catch, cleanup, and filter clauses (though having more

    350. 

  350.    than one cleanup is pointless). The LLVM C++ front-end can generate such

    351. 

  351.    <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instructions</a> due

    352. 

  352.    to inlining creating nested exception handling scopes.</p>

    353. 

  353. 

    354. 

  354. </div>

    355. 

  355. 

    356. 

  356. <!-- ======================================================================= -->

    357. 

  357. <h3>

    358. 

  358.   <a name="restrictions">Restrictions</a>

    359. 

  359. </h3>

    360. 

  360. 

    361. 

  361. <div>

    362. 

  362. 

    363. 

  363. <p>The unwinder delegates the decision of whether to stop in a call frame to

    364. 

  364.    that call frame's language-specific personality function. Not all unwinders

    365. 

  365.    guarantee that they will stop to perform cleanups. For example, the GNU C++

    366. 

  366.    unwinder doesn't do so unless the exception is actually caught somewhere

    367. 

  367.    further up the stack.</p>

    368. 

  368. 

    369. 

  369. <p>In order for inlining to behave correctly, landing pads must be prepared to

    370. 

  370.    handle selector results that they did not originally advertise. Suppose that

    371. 

  371.    a function catches exceptions of type <tt>A</tt>, and it's inlined into a

    372. 

  372.    function that catches exceptions of type <tt>B</tt>. The inliner will update

    373. 

  373.    the <tt>landingpad</tt> instruction for the inlined landing pad to include

    374. 

  374.    the fact that <tt>B</tt> is also caught. If that landing pad assumes that it

    375. 

  375.    will only be entered to catch an <tt>A</tt>, it's in for a rude awakening.

    376. 

  376.    Consequently, landing pads must test for the selector results they understand

    377. 

  377.    and then resume exception propagation with the

    378. 

  378.    <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a> if none of

    379. 

  379.    the conditions match.</p>

    380. 

  380. 

    381. 

  381. </div>

    382. 

  382. 

    383. 

  383. </div>

    384. 

  384. 

    385. 

  385. <!-- ======================================================================= -->

    386. 

  386. <h2>

    387. 

  387.   <a name="format_common_intrinsics">Exception Handling Intrinsics</a>

    388. 

  388. </h2>

    389. 

  389. 

    390. 

  390. <div>

    391. 

  391. 

    392. 

  392. <p>In addition to the

    393. 

  393.    <a href="LangRef.html#i_landingpad"><tt>landingpad</tt></a> and

    394. 

  394.    <a href="LangRef.html#i_resume"><tt>resume</tt></a> instructions, LLVM uses

    395. 

  395.    several intrinsic functions (name prefixed with <i><tt>llvm.eh</tt></i>) to

    396. 

  396.    provide exception handling information at various points in generated

    397. 

  397.    code.</p>

    398. 

  398. 

    399. 

  399. <!-- ======================================================================= -->

    400. 

  400. <h4>

    401. 

  401.   <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>

    402. 

  402. </h4>

    403. 

  403. 

    404. 

  404. <div>

    405. 

  405. 

    406. 

  406. <pre>

    407. 

  407.   i32 @llvm.eh.typeid.for(i8* %type_info)

    408. 

  408. </pre>

    409. 

  409. 

    410. 

  410. <p>This intrinsic returns the type info index in the exception table of the

    411. 

  411.    current function.  This value can be used to compare against the result

    412. 

  412.    of <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>.

    413. 

  413.    The single argument is a reference to a type info.</p>

    414. 

  414. 

    415. 

  415. </div>

    416. 

  416. 

    417. 

  417. <!-- ======================================================================= -->

    418. 

  418. <h4>

    419. 

  419.   <a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>

    420. 

  420. </h4>

    421. 

  421. 

    422. 

  422. <div>

    423. 

  423. 

    424. 

  424. <pre>

    425. 

  425.   i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf)

    426. 

  426. </pre>

    427. 

  427. 

    428. 

  428. <p>For SJLJ based exception handling, this intrinsic forces register saving for

    429. 

  429.    the current function and stores the address of the following instruction for

    430. 

  430.    use as a destination address

    431. 

  431.    by <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>. The

    432. 

  432.    buffer format and the overall functioning of this intrinsic is compatible

    433. 

  433.    with the GCC <tt>__builtin_setjmp</tt> implementation allowing code built

    434. 

  434.    with the clang and GCC to interoperate.</p>

    435. 

  435. 

    436. 

  436. <p>The single parameter is a pointer to a five word buffer in which the calling

    437. 

  437.    context is saved. The front end places the frame pointer in the first word,

    438. 

  438.    and the target implementation of this intrinsic should place the destination

    439. 

  439.    address for a

    440. 

  440.    <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> in the

    441. 

  441.    second word. The following three words are available for use in a

    442. 

  442.    target-specific manner.</p>

    443. 

  443. 

    444. 

  444. </div>

    445. 

  445. 

    446. 

  446. <!-- ======================================================================= -->

    447. 

  447. <h4>

    448. 

  448.   <a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a>

    449. 

  449. </h4>

    450. 

  450. 

    451. 

  451. <div>

    452. 

  452. 

    453. 

  453. <pre>

    454. 

  454.   void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf)

    455. 

  455. </pre>

    456. 

  456. 

    457. 

  457. <p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.longjmp</tt>

    458. 

  458.    intrinsic is used to implement <tt>__builtin_longjmp()</tt>. The single

    459. 

  459.    parameter is a pointer to a buffer populated

    460. 

  460.    by <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a>. The frame

    461. 

  461.    pointer and stack pointer are restored from the buffer, then control is

    462. 

  462.    transferred to the destination address.</p>

    463. 

  463. 

    464. 

  464. </div>

    465. 

  465. <!-- ======================================================================= -->

    466. 

  466. <h4>

    467. 

  467.   <a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>

    468. 

  468. </h4>

    469. 

  469. 

    470. 

  470. <div>

    471. 

  471. 

    472. 

  472. <pre>

    473. 

  473.   i8* @llvm.eh.sjlj.lsda()

    474. 

  474. </pre>

    475. 

  475. 

    476. 

  476. <p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.lsda</tt> intrinsic

    477. 

  477.    returns the address of the Language Specific Data Area (LSDA) for the current

    478. 

  478.    function. The SJLJ front-end code stores this address in the exception

    479. 

  479.    handling function context for use by the runtime.</p>

    480. 

  480. 

    481. 

  481. </div>

    482. 

  482. 

    483. 

  483. <!-- ======================================================================= -->

    484. 

  484. <h4>

    485. 

  485.   <a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>

    486. 

  486. </h4>

    487. 

  487. 

    488. 

  488. <div>

    489. 

  489. 

    490. 

  490. <pre>

    491. 

  491.   void @llvm.eh.sjlj.callsite(i32 %call_site_num)

    492. 

  492. </pre>

    493. 

  493. 

    494. 

  494. <p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.callsite</tt>

    495. 

  495.    intrinsic identifies the callsite value associated with the

    496. 

  496.    following <tt>invoke</tt> instruction. This is used to ensure that landing

    497. 

  497.    pad entries in the LSDA are generated in matching order.</p>

    498. 

  498. 

    499. 

  499. </div>

    500. 

  500. 

    501. 

  501. <!-- ======================================================================= -->

    502. 

  502. <h4>

    503. 

  503.   <a name="llvm_eh_sjlj_dispatchsetup">llvm.eh.sjlj.dispatchsetup</a>

    504. 

  504. </h4>

    505. 

  505. 

    506. 

  506. <div>

    507. 

  507. 

    508. 

  508. <pre>

    509. 

  509.   void @llvm.eh.sjlj.dispatchsetup(i32 %dispatch_value)

    510. 

  510. </pre>

    511. 

  511. 

    512. 

  512. <p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.dispatchsetup</tt>

    513. 

  513.    intrinsic is used by targets to do any unwind edge setup they need. By

    514. 

  514.    default, no action is taken.</p>

    515. 

  515. 

    516. 

  516. </div>

    517. 

  517. 

    518. 

  518. </div>

    519. 

  519. 

    520. 

  520. <!-- ======================================================================= -->

    521. 

  521. <h2>

    522. 

  522.   <a name="asm">Asm Table Formats</a>

    523. 

  523. </h2>

    524. 

  524. 

    525. 

  525. <div>

    526. 

  526. 

    527. 

  527. <p>There are two tables that are used by the exception handling runtime to

    528. 

  528.    determine which actions should be taken when an exception is thrown.</p>

    529. 

  529. 

    530. 

  530. <!-- ======================================================================= -->

    531. 

  531. <h3>

    532. 

  532.   <a name="unwind_tables">Exception Handling Frame</a>

    533. 

  533. </h3>

    534. 

  534. 

    535. 

  535. <div>

    536. 

  536. 

    537. 

  537. <p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind

    538. 

  538.    frame used by DWARF debug info. The frame contains all the information

    539. 

  539.    necessary to tear down the current frame and restore the state of the prior

    540. 

  540.    frame. There is an exception handling frame for each function in a compile

    541. 

  541.    unit, plus a common exception handling frame that defines information common

    542. 

  542.    to all functions in the unit.</p>

    543. 

  543. 

    544. 

  544. <!-- Todo - Table details here. -->

    545. 

  545. 

    546. 

  546. </div>

    547. 

  547. 

    548. 

  548. <!-- ======================================================================= -->

    549. 

  549. <h3>

    550. 

  550.   <a name="exception_tables">Exception Tables</a>

    551. 

  551. </h3>

    552. 

  552. 

    553. 

  553. <div>

    554. 

  554. 

    555. 

  555. <p>An exception table contains information about what actions to take when an

    556. 

  556.    exception is thrown in a particular part of a function's code. There is one

    557. 

  557.    exception table per function, except leaf functions and functions that have

    558. 

  558.    calls only to non-throwing functions. They do not need an exception

    559. 

  559.    table.</p>

    560. 

  560. 

    561. 

  561. <!-- Todo - Table details here. -->

    562. 

  562. 

    563. 

  563. </div>

    564. 

  564. 

    565. 

  565. </div>

    566. 

  566. 

    567. 

  567. <!-- *********************************************************************** -->

    568. 

  568. 

    569. 

  569. <hr>

    570. 

  570. <address>

    571. 

  571.   <a href="http://jigsaw.w3.org/css-validator/check/referer"><img

    572. 

  572.   src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>

    573. 

  573.   <a href="http://validator.w3.org/check/referer"><img

    574. 

  574.   src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>

    575. 

  575. 

    576. 

  576.   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>

    577. 

  577.   <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>

    578. 

  578.   Last modified: $Date: 2011-09-28 04:16:57 +0800 (Wed, 28 Sep 2011) $

    579. 

  579. </address>

    580. 

  580. 

    581. 

  581. </body>

    582. 

  582. </html>
    583.