1========================== 2Exception Handling in LLVM 3========================== 4 5.. contents:: 6 :local: 7 8Introduction 9============ 10 11This document is the central repository for all information pertaining to 12exception handling in LLVM. It describes the format that LLVM exception 13handling information takes, which is useful for those interested in creating 14front-ends or dealing directly with the information. Further, this document 15provides specific examples of what exception handling information is used for in 16C and C++. 17 18Itanium ABI Zero-cost Exception Handling 19---------------------------------------- 20 21Exception handling for most programming languages is designed to recover from 22conditions that rarely occur during general use of an application. To that end, 23exception handling should not interfere with the main flow of an application's 24algorithm by performing checkpointing tasks, such as saving the current pc or 25register state. 26 27The Itanium ABI Exception Handling Specification defines a methodology for 28providing outlying data in the form of exception tables without inlining 29speculative exception handling code in the flow of an application's main 30algorithm. Thus, the specification is said to add "zero-cost" to the normal 31execution of an application. 32 33A more complete description of the Itanium ABI exception handling runtime 34support of can be found at `Itanium C++ ABI: Exception Handling 35<http://mentorembedded.github.com/cxx-abi/abi-eh.html>`_. A description of the 36exception frame format can be found at `Exception Frames 37<http://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html>`_, 38with details of the DWARF 4 specification at `DWARF 4 Standard 39<http://dwarfstd.org/Dwarf4Std.php>`_. A description for the C++ exception 40table formats can be found at `Exception Handling Tables 41<http://mentorembedded.github.com/cxx-abi/exceptions.pdf>`_. 42 43Setjmp/Longjmp Exception Handling 44--------------------------------- 45 46Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics 47`llvm.eh.sjlj.setjmp`_ and `llvm.eh.sjlj.longjmp`_ to handle control flow for 48exception handling. 49 50For each function which does exception processing --- be it ``try``/``catch`` 51blocks or cleanups --- that function registers itself on a global frame 52list. When exceptions are unwinding, the runtime uses this list to identify 53which functions need processing. 54 55Landing pad selection is encoded in the call site entry of the function 56context. The runtime returns to the function via `llvm.eh.sjlj.longjmp`_, where 57a switch table transfers control to the appropriate landing pad based on the 58index stored in the function context. 59 60In contrast to DWARF exception handling, which encodes exception regions and 61frame information in out-of-line tables, SJLJ exception handling builds and 62removes the unwind frame context at runtime. This results in faster exception 63handling at the expense of slower execution when no exceptions are thrown. As 64exceptions are, by their nature, intended for uncommon code paths, DWARF 65exception handling is generally preferred to SJLJ. 66 67Windows Runtime Exception Handling 68----------------------------------- 69 70Windows runtime based exception handling uses the same basic IR structure as 71Itanium ABI based exception handling, but it relies on the personality 72functions provided by the native Windows runtime library, ``__CxxFrameHandler3`` 73for C++ exceptions: ``__C_specific_handler`` for 64-bit SEH or 74``_frame_handler3/4`` for 32-bit SEH. This results in a very different 75execution model and requires some minor modifications to the initial IR 76representation and a significant restructuring just before code generation. 77 78General information about the Windows x64 exception handling mechanism can be 79found at `MSDN Exception Handling (x64) 80<https://msdn.microsoft.com/en-us/library/1eyas8tf(v=vs.80).aspx>`_. 81 82Overview 83-------- 84 85When an exception is thrown in LLVM code, the runtime does its best to find a 86handler suited to processing the circumstance. 87 88The runtime first attempts to find an *exception frame* corresponding to the 89function where the exception was thrown. If the programming language supports 90exception handling (e.g. C++), the exception frame contains a reference to an 91exception table describing how to process the exception. If the language does 92not support exception handling (e.g. C), or if the exception needs to be 93forwarded to a prior activation, the exception frame contains information about 94how to unwind the current activation and restore the state of the prior 95activation. This process is repeated until the exception is handled. If the 96exception is not handled and no activations remain, then the application is 97terminated with an appropriate error message. 98 99Because different programming languages have different behaviors when handling 100exceptions, the exception handling ABI provides a mechanism for 101supplying *personalities*. An exception handling personality is defined by 102way of a *personality function* (e.g. ``__gxx_personality_v0`` in C++), 103which receives the context of the exception, an *exception structure* 104containing the exception object type and value, and a reference to the exception 105table for the current function. The personality function for the current 106compile unit is specified in a *common exception frame*. 107 108The organization of an exception table is language dependent. For C++, an 109exception table is organized as a series of code ranges defining what to do if 110an exception occurs in that range. Typically, the information associated with a 111range defines which types of exception objects (using C++ *type info*) that are 112handled in that range, and an associated action that should take place. Actions 113typically pass control to a *landing pad*. 114 115A landing pad corresponds roughly to the code found in the ``catch`` portion of 116a ``try``/``catch`` sequence. When execution resumes at a landing pad, it 117receives an *exception structure* and a *selector value* corresponding to the 118*type* of exception thrown. The selector is then used to determine which *catch* 119should actually process the exception. 120 121LLVM Code Generation 122==================== 123 124From a C++ developer's perspective, exceptions are defined in terms of the 125``throw`` and ``try``/``catch`` statements. In this section we will describe the 126implementation of LLVM exception handling in terms of C++ examples. 127 128Throw 129----- 130 131Languages that support exception handling typically provide a ``throw`` 132operation to initiate the exception process. Internally, a ``throw`` operation 133breaks down into two steps. 134 135#. A request is made to allocate exception space for an exception structure. 136 This structure needs to survive beyond the current activation. This structure 137 will contain the type and value of the object being thrown. 138 139#. A call is made to the runtime to raise the exception, passing the exception 140 structure as an argument. 141 142In C++, the allocation of the exception structure is done by the 143``__cxa_allocate_exception`` runtime function. The exception raising is handled 144by ``__cxa_throw``. The type of the exception is represented using a C++ RTTI 145structure. 146 147Try/Catch 148--------- 149 150A call within the scope of a *try* statement can potentially raise an 151exception. In those circumstances, the LLVM C++ front-end replaces the call with 152an ``invoke`` instruction. Unlike a call, the ``invoke`` has two potential 153continuation points: 154 155#. where to continue when the call succeeds as per normal, and 156 157#. where to continue if the call raises an exception, either by a throw or the 158 unwinding of a throw 159 160The term used to define the place where an ``invoke`` continues after an 161exception is called a *landing pad*. LLVM landing pads are conceptually 162alternative function entry points where an exception structure reference and a 163type info index are passed in as arguments. The landing pad saves the exception 164structure reference and then proceeds to select the catch block that corresponds 165to the type info of the exception object. 166 167The LLVM :ref:`i_landingpad` is used to convey information about the landing 168pad to the back end. For C++, the ``landingpad`` instruction returns a pointer 169and integer pair corresponding to the pointer to the *exception structure* and 170the *selector value* respectively. 171 172The ``landingpad`` instruction takes a reference to the personality function to 173be used for this ``try``/``catch`` sequence. The remainder of the instruction is 174a list of *cleanup*, *catch*, and *filter* clauses. The exception is tested 175against the clauses sequentially from first to last. The clauses have the 176following meanings: 177 178- ``catch <type> @ExcType`` 179 180 - This clause means that the landingpad block should be entered if the 181 exception being thrown is of type ``@ExcType`` or a subtype of 182 ``@ExcType``. For C++, ``@ExcType`` is a pointer to the ``std::type_info`` 183 object (an RTTI object) representing the C++ exception type. 184 185 - If ``@ExcType`` is ``null``, any exception matches, so the landingpad 186 should always be entered. This is used for C++ catch-all blocks ("``catch 187 (...)``"). 188 189 - When this clause is matched, the selector value will be equal to the value 190 returned by "``@llvm.eh.typeid.for(i8* @ExcType)``". This will always be a 191 positive value. 192 193- ``filter <type> [<type> @ExcType1, ..., <type> @ExcTypeN]`` 194 195 - This clause means that the landingpad should be entered if the exception 196 being thrown does *not* match any of the types in the list (which, for C++, 197 are again specified as ``std::type_info`` pointers). 198 199 - C++ front-ends use this to implement C++ exception specifications, such as 200 "``void foo() throw (ExcType1, ..., ExcTypeN) { ... }``". 201 202 - When this clause is matched, the selector value will be negative. 203 204 - The array argument to ``filter`` may be empty; for example, "``[0 x i8**] 205 undef``". This means that the landingpad should always be entered. (Note 206 that such a ``filter`` would not be equivalent to "``catch i8* null``", 207 because ``filter`` and ``catch`` produce negative and positive selector 208 values respectively.) 209 210- ``cleanup`` 211 212 - This clause means that the landingpad should always be entered. 213 214 - C++ front-ends use this for calling objects' destructors. 215 216 - When this clause is matched, the selector value will be zero. 217 218 - The runtime may treat "``cleanup``" differently from "``catch <type> 219 null``". 220 221 In C++, if an unhandled exception occurs, the language runtime will call 222 ``std::terminate()``, but it is implementation-defined whether the runtime 223 unwinds the stack and calls object destructors first. For example, the GNU 224 C++ unwinder does not call object destructors when an unhandled exception 225 occurs. The reason for this is to improve debuggability: it ensures that 226 ``std::terminate()`` is called from the context of the ``throw``, so that 227 this context is not lost by unwinding the stack. A runtime will typically 228 implement this by searching for a matching non-``cleanup`` clause, and 229 aborting if it does not find one, before entering any landingpad blocks. 230 231Once the landing pad has the type info selector, the code branches to the code 232for the first catch. The catch then checks the value of the type info selector 233against the index of type info for that catch. Since the type info index is not 234known until all the type infos have been gathered in the backend, the catch code 235must call the `llvm.eh.typeid.for`_ intrinsic to determine the index for a given 236type info. If the catch fails to match the selector then control is passed on to 237the next catch. 238 239Finally, the entry and exit of catch code is bracketed with calls to 240``__cxa_begin_catch`` and ``__cxa_end_catch``. 241 242* ``__cxa_begin_catch`` takes an exception structure reference as an argument 243 and returns the value of the exception object. 244 245* ``__cxa_end_catch`` takes no arguments. This function: 246 247 #. Locates the most recently caught exception and decrements its handler 248 count, 249 250 #. Removes the exception from the *caught* stack if the handler count goes to 251 zero, and 252 253 #. Destroys the exception if the handler count goes to zero and the exception 254 was not re-thrown by throw. 255 256 .. note:: 257 258 a rethrow from within the catch may replace this call with a 259 ``__cxa_rethrow``. 260 261Cleanups 262-------- 263 264A cleanup is extra code which needs to be run as part of unwinding a scope. C++ 265destructors are a typical example, but other languages and language extensions 266provide a variety of different kinds of cleanups. In general, a landing pad may 267need to run arbitrary amounts of cleanup code before actually entering a catch 268block. To indicate the presence of cleanups, a :ref:`i_landingpad` should have 269a *cleanup* clause. Otherwise, the unwinder will not stop at the landing pad if 270there are no catches or filters that require it to. 271 272.. note:: 273 274 Do not allow a new exception to propagate out of the execution of a 275 cleanup. This can corrupt the internal state of the unwinder. Different 276 languages describe different high-level semantics for these situations: for 277 example, C++ requires that the process be terminated, whereas Ada cancels both 278 exceptions and throws a third. 279 280When all cleanups are finished, if the exception is not handled by the current 281function, resume unwinding by calling the :ref:`resume instruction <i_resume>`, 282passing in the result of the ``landingpad`` instruction for the original 283landing pad. 284 285Throw Filters 286------------- 287 288C++ allows the specification of which exception types may be thrown from a 289function. To represent this, a top level landing pad may exist to filter out 290invalid types. To express this in LLVM code the :ref:`i_landingpad` will have a 291filter clause. The clause consists of an array of type infos. 292``landingpad`` will return a negative value 293if the exception does not match any of the type infos. If no match is found then 294a call to ``__cxa_call_unexpected`` should be made, otherwise 295``_Unwind_Resume``. Each of these functions requires a reference to the 296exception structure. Note that the most general form of a ``landingpad`` 297instruction can have any number of catch, cleanup, and filter clauses (though 298having more than one cleanup is pointless). The LLVM C++ front-end can generate 299such ``landingpad`` instructions due to inlining creating nested exception 300handling scopes. 301 302.. _undefined: 303 304Restrictions 305------------ 306 307The unwinder delegates the decision of whether to stop in a call frame to that 308call frame's language-specific personality function. Not all unwinders guarantee 309that they will stop to perform cleanups. For example, the GNU C++ unwinder 310doesn't do so unless the exception is actually caught somewhere further up the 311stack. 312 313In order for inlining to behave correctly, landing pads must be prepared to 314handle selector results that they did not originally advertise. Suppose that a 315function catches exceptions of type ``A``, and it's inlined into a function that 316catches exceptions of type ``B``. The inliner will update the ``landingpad`` 317instruction for the inlined landing pad to include the fact that ``B`` is also 318caught. If that landing pad assumes that it will only be entered to catch an 319``A``, it's in for a rude awakening. Consequently, landing pads must test for 320the selector results they understand and then resume exception propagation with 321the `resume instruction <LangRef.html#i_resume>`_ if none of the conditions 322match. 323 324C++ Exception Handling using the Windows Runtime 325================================================= 326 327(Note: Windows C++ exception handling support is a work in progress and is 328 not yet fully implemented. The text below describes how it will work 329 when completed.) 330 331The Windows runtime function for C++ exception handling uses a multi-phase 332approach. When an exception occurs it searches the current callstack for a 333frame that has a handler for the exception. If a handler is found, it then 334calls the cleanup handler for each frame above the handler which has a 335cleanup handler before calling the catch handler. These calls are all made 336from a stack context different from the original frame in which the handler 337is defined. Therefore, it is necessary to outline these handlers from their 338original context before code generation. 339 340Catch handlers are called with a pointer to the handler itself as the first 341argument and a pointer to the parent function's stack frame as the second 342argument. The catch handler uses the `llvm.recoverframe 343<LangRef.html#llvm-frameallocate-and-llvm-framerecover-intrinsics>`_ to get a 344pointer to a frame allocation block that is created in the parent frame using 345the `llvm.allocateframe 346<LangRef.html#llvm-frameallocate-and-llvm-framerecover-intrinsics>`_ intrinsic. 347The ``WinEHPrepare`` pass will have created a structure definition for the 348contents of this block. The first two members of the structure will always be 349(1) a 32-bit integer that the runtime uses to track the exception state of the 350parent frame for the purposes of handling chained exceptions and (2) a pointer 351to the object associated with the exception (roughly, the parameter of the 352catch clause). These two members will be followed by any frame variables from 353the parent function which must be accessed in any of the functions unwind or 354catch handlers. The catch handler returns the address at which execution 355should continue. 356 357Cleanup handlers perform any cleanup necessary as the frame goes out of scope, 358such as calling object destructors. The runtime handles the actual unwinding 359of the stack. If an exception occurs in a cleanup handler the runtime manages 360termination of the process. Cleanup handlers are called with the same arguments 361as catch handlers (a pointer to the handler and a pointer to the parent stack 362frame) and use the same mechanism described above to access frame variables 363in the parent function. Cleanup handlers do not return a value. 364 365The IR generated for Windows runtime based C++ exception handling is initially 366very similar to the ``landingpad`` mechanism described above. Calls to 367libc++abi functions (such as ``__cxa_begin_catch``/``__cxa_end_catch`` and 368``__cxa_throw_exception`` are replaced with calls to intrinsics or Windows 369runtime functions (such as ``llvm.eh.begincatch``/``llvm.eh.endcatch`` and 370``__CxxThrowException``). 371 372During the WinEHPrepare pass, the handler functions are outlined into handler 373functions and the original landing pad code is replaced with a call to the 374``llvm.eh.actions`` intrinsic that describes the order in which handlers will 375be processed from the logical location of the landing pad and an indirect 376branch to the return value of the ``llvm.eh.actions`` intrinsic. The 377``llvm.eh.actions`` intrinsic is defined as returning the address at which 378execution will continue. This is a temporary construct which will be removed 379before code generation, but it allows for the accurate tracking of control 380flow until then. 381 382A typical landing pad will look like this after outlining: 383 384.. code-block:: llvm 385 386 lpad: 387 %vals = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)* @__CxxFrameHandler3 to i8*) 388 cleanup 389 catch i8* bitcast (i8** @_ZTIi to i8*) 390 catch i8* bitcast (i8** @_ZTIf to i8*) 391 %recover = call i8* (...)* @llvm.eh.actions( 392 i32 3, i8* bitcast (i8** @_ZTIi to i8*), i8* (i8*, i8*)* @_Z4testb.catch.1) 393 i32 2, i8* null, void (i8*, i8*)* @_Z4testb.cleanup.1) 394 i32 1, i8* bitcast (i8** @_ZTIf to i8*), i8* (i8*, i8*)* @_Z4testb.catch.0) 395 i32 0, i8* null, void (i8*, i8*)* @_Z4testb.cleanup.0) 396 indirectbr i8* %recover, [label %try.cont1, label %try.cont2] 397 398In this example, the landing pad represents an exception handling context with 399two catch handlers and a cleanup handler that have been outlined. If an 400exception is thrown with a type that matches ``_ZTIi``, the ``_Z4testb.catch.1`` 401handler will be called an no clean-up is needed. If an exception is thrown 402with a type that matches ``_ZTIf``, first the ``_Z4testb.cleanup.1`` handler 403will be called to perform unwind-related cleanup, then the ``_Z4testb.catch.1`` 404handler will be called. If an exception is throw which does not match either 405of these types and the exception is handled by another frame further up the 406call stack, first the ``_Z4testb.cleanup.1`` handler will be called, then the 407``_Z4testb.cleanup.0`` handler (which corresponds to a different scope) will be 408called, and exception handling will continue at the next frame in the call 409stack will be called. One of the catch handlers will return the address of 410``%try.cont1`` in the parent function and the other will return the address of 411``%try.cont2``, meaning that execution continues at one of those blocks after 412an exception is caught. 413 414 415Exception Handling Intrinsics 416============================= 417 418In addition to the ``landingpad`` and ``resume`` instructions, LLVM uses several 419intrinsic functions (name prefixed with ``llvm.eh``) to provide exception 420handling information at various points in generated code. 421 422.. _llvm.eh.typeid.for: 423 424``llvm.eh.typeid.for`` 425---------------------- 426 427.. code-block:: llvm 428 429 i32 @llvm.eh.typeid.for(i8* %type_info) 430 431 432This intrinsic returns the type info index in the exception table of the current 433function. This value can be used to compare against the result of 434``landingpad`` instruction. The single argument is a reference to a type info. 435 436Uses of this intrinsic are generated by the C++ front-end. 437 438.. _llvm.eh.begincatch: 439 440``llvm.eh.begincatch`` 441---------------------- 442 443.. code-block:: llvm 444 445 void @llvm.eh.begincatch(i8* %ehptr, i8* %ehobj) 446 447 448This intrinsic marks the beginning of catch handling code within the blocks 449following a ``landingpad`` instruction. The exact behavior of this function 450depends on the compilation target and the personality function associated 451with the ``landingpad`` instruction. 452 453The first argument to this intrinsic is a pointer that was previously extracted 454from the aggregate return value of the ``landingpad`` instruction. The second 455argument to the intrinsic is a pointer to stack space where the exception object 456should be stored. The runtime handles the details of copying the exception 457object into the slot. If the second parameter is null, no copy occurs. 458 459Uses of this intrinsic are generated by the C++ front-end. Many targets will 460use implementation-specific functions (such as ``__cxa_begin_catch``) instead 461of this intrinsic. The intrinsic is provided for targets that require a more 462abstract interface. 463 464When used in the native Windows C++ exception handling implementation, this 465intrinsic serves as a placeholder to delimit code before a catch handler is 466outlined. When the handler is is outlined, this intrinsic will be replaced 467by instructions that retrieve the exception object pointer from the frame 468allocation block. 469 470 471.. _llvm.eh.endcatch: 472 473``llvm.eh.endcatch`` 474---------------------- 475 476.. code-block:: llvm 477 478 void @llvm.eh.endcatch() 479 480 481This intrinsic marks the end of catch handling code within the current block, 482which will be a successor of a block which called ``llvm.eh.begincatch''. 483The exact behavior of this function depends on the compilation target and the 484personality function associated with the corresponding ``landingpad`` 485instruction. 486 487There may be more than one call to ``llvm.eh.endcatch`` for any given call to 488``llvm.eh.begincatch`` with each ``llvm.eh.endcatch`` call corresponding to the 489end of a different control path. All control paths following a call to 490``llvm.eh.begincatch`` must reach a call to ``llvm.eh.endcatch``. 491 492Uses of this intrinsic are generated by the C++ front-end. Many targets will 493use implementation-specific functions (such as ``__cxa_begin_catch``) instead 494of this intrinsic. The intrinsic is provided for targets that require a more 495abstract interface. 496 497When used in the native Windows C++ exception handling implementation, this 498intrinsic serves as a placeholder to delimit code before a catch handler is 499outlined. After the handler is outlined, this intrinsic is simply removed. 500 501.. _llvm.eh.actions: 502 503``llvm.eh.actions`` 504---------------------- 505 506.. code-block:: llvm 507 508 void @llvm.eh.actions() 509 510This intrinsic represents the list of actions to take when an exception is 511thrown. It is typically used by Windows exception handling schemes where cleanup 512outlining is required by the runtime. The arguments are a sequence of ``i32`` 513sentinels indicating the action type followed by some pre-determined number of 514arguments required to implement that action. 515 516A code of ``i32 0`` indicates a cleanup action, which expects one additional 517argument. The argument is a pointer to a function that implements the cleanup 518action. 519 520A code of ``i32 1`` indicates a catch action, which expects three additional 521arguments. Different EH schemes give different meanings to the three arguments, 522but the first argument indicates whether the catch should fire, the second is 523the frameescape index of the exception object, and the third is the code to run 524to catch the exception. 525 526For Windows C++ exception handling, the first argument for a catch handler is a 527pointer to the RTTI type descriptor for the object to catch. The second 528argument is an index into the argument list of the ``llvm.frameescape`` call in 529the main function. The exception object will be copied into the provided stack 530object. If the exception object is not required, this argument should be -1. 531The third argument is a pointer to a function implementing the catch. This 532function returns the address of the basic block where execution should resume 533after handling the exception. 534 535For Windows SEH, the first argument is a pointer to the filter function, which 536indicates if the exception should be caught or not. The second argument is 537typically negative one. The third argument is the address of a basic block 538where the exception will be handled. In other words, catch handlers are not 539outlined in SEH. After running cleanups, execution immediately resumes at this 540PC. 541 542In order to preserve the structure of the CFG, a call to '``llvm.eh.actions``' 543must be followed by an ':ref:`indirectbr <i_indirectbr>`' instruction that 544jumps to the result of the intrinsic call. 545 546 547SJLJ Intrinsics 548--------------- 549 550The ``llvm.eh.sjlj`` intrinsics are used internally within LLVM's 551backend. Uses of them are generated by the backend's 552``SjLjEHPrepare`` pass. 553 554.. _llvm.eh.sjlj.setjmp: 555 556``llvm.eh.sjlj.setjmp`` 557~~~~~~~~~~~~~~~~~~~~~~~ 558 559.. code-block:: llvm 560 561 i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf) 562 563For SJLJ based exception handling, this intrinsic forces register saving for the 564current function and stores the address of the following instruction for use as 565a destination address by `llvm.eh.sjlj.longjmp`_. The buffer format and the 566overall functioning of this intrinsic is compatible with the GCC 567``__builtin_setjmp`` implementation allowing code built with the clang and GCC 568to interoperate. 569 570The single parameter is a pointer to a five word buffer in which the calling 571context is saved. The front end places the frame pointer in the first word, and 572the target implementation of this intrinsic should place the destination address 573for a `llvm.eh.sjlj.longjmp`_ in the second word. The following three words are 574available for use in a target-specific manner. 575 576.. _llvm.eh.sjlj.longjmp: 577 578``llvm.eh.sjlj.longjmp`` 579~~~~~~~~~~~~~~~~~~~~~~~~ 580 581.. code-block:: llvm 582 583 void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf) 584 585For SJLJ based exception handling, the ``llvm.eh.sjlj.longjmp`` intrinsic is 586used to implement ``__builtin_longjmp()``. The single parameter is a pointer to 587a buffer populated by `llvm.eh.sjlj.setjmp`_. The frame pointer and stack 588pointer are restored from the buffer, then control is transferred to the 589destination address. 590 591``llvm.eh.sjlj.lsda`` 592~~~~~~~~~~~~~~~~~~~~~ 593 594.. code-block:: llvm 595 596 i8* @llvm.eh.sjlj.lsda() 597 598For SJLJ based exception handling, the ``llvm.eh.sjlj.lsda`` intrinsic returns 599the address of the Language Specific Data Area (LSDA) for the current 600function. The SJLJ front-end code stores this address in the exception handling 601function context for use by the runtime. 602 603``llvm.eh.sjlj.callsite`` 604~~~~~~~~~~~~~~~~~~~~~~~~~ 605 606.. code-block:: llvm 607 608 void @llvm.eh.sjlj.callsite(i32 %call_site_num) 609 610For SJLJ based exception handling, the ``llvm.eh.sjlj.callsite`` intrinsic 611identifies the callsite value associated with the following ``invoke`` 612instruction. This is used to ensure that landing pad entries in the LSDA are 613generated in matching order. 614 615Asm Table Formats 616================= 617 618There are two tables that are used by the exception handling runtime to 619determine which actions should be taken when an exception is thrown. 620 621Exception Handling Frame 622------------------------ 623 624An exception handling frame ``eh_frame`` is very similar to the unwind frame 625used by DWARF debug info. The frame contains all the information necessary to 626tear down the current frame and restore the state of the prior frame. There is 627an exception handling frame for each function in a compile unit, plus a common 628exception handling frame that defines information common to all functions in the 629unit. 630 631Exception Tables 632---------------- 633 634An exception table contains information about what actions to take when an 635exception is thrown in a particular part of a function's code. There is one 636exception table per function, except leaf functions and functions that have 637calls only to non-throwing functions. They do not need an exception table. 638