1 ========================== 2 Exception Handling in LLVM 3 ========================== 4 5 .. contents:: 6 :local: 7 8 Introduction 9 ============ 10 11 This document is the central repository for all information pertaining to 12 exception handling in LLVM. It describes the format that LLVM exception 13 handling information takes, which is useful for those interested in creating 14 front-ends or dealing directly with the information. Further, this document 15 provides specific examples of what exception handling information is used for in 16 C and C++. 17 18 Itanium ABI Zero-cost Exception Handling 19 ---------------------------------------- 20 21 Exception handling for most programming languages is designed to recover from 22 conditions that rarely occur during general use of an application. To that end, 23 exception handling should not interfere with the main flow of an application's 24 algorithm by performing checkpointing tasks, such as saving the current pc or 25 register state. 26 27 The Itanium ABI Exception Handling Specification defines a methodology for 28 providing outlying data in the form of exception tables without inlining 29 speculative exception handling code in the flow of an application's main 30 algorithm. Thus, the specification is said to add "zero-cost" to the normal 31 execution of an application. 32 33 A more complete description of the Itanium ABI exception handling runtime 34 support of can be found at `Itanium C++ ABI: Exception Handling 35 <http://mentorembedded.github.com/cxx-abi/abi-eh.html>`_. A description of the 36 exception 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>`_, 38 with details of the DWARF 4 specification at `DWARF 4 Standard 39 <http://dwarfstd.org/Dwarf4Std.php>`_. A description for the C++ exception 40 table formats can be found at `Exception Handling Tables 41 <http://mentorembedded.github.com/cxx-abi/exceptions.pdf>`_. 42 43 Setjmp/Longjmp Exception Handling 44 --------------------------------- 45 46 Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics 47 `llvm.eh.sjlj.setjmp`_ and `llvm.eh.sjlj.longjmp`_ to handle control flow for 48 exception handling. 49 50 For each function which does exception processing --- be it ``try``/``catch`` 51 blocks or cleanups --- that function registers itself on a global frame 52 list. When exceptions are unwinding, the runtime uses this list to identify 53 which functions need processing. 54 55 Landing pad selection is encoded in the call site entry of the function 56 context. The runtime returns to the function via `llvm.eh.sjlj.longjmp`_, where 57 a switch table transfers control to the appropriate landing pad based on the 58 index stored in the function context. 59 60 In contrast to DWARF exception handling, which encodes exception regions and 61 frame information in out-of-line tables, SJLJ exception handling builds and 62 removes the unwind frame context at runtime. This results in faster exception 63 handling at the expense of slower execution when no exceptions are thrown. As 64 exceptions are, by their nature, intended for uncommon code paths, DWARF 65 exception handling is generally preferred to SJLJ. 66 67 Overview 68 -------- 69 70 When an exception is thrown in LLVM code, the runtime does its best to find a 71 handler suited to processing the circumstance. 72 73 The runtime first attempts to find an *exception frame* corresponding to the 74 function where the exception was thrown. If the programming language supports 75 exception handling (e.g. C++), the exception frame contains a reference to an 76 exception table describing how to process the exception. If the language does 77 not support exception handling (e.g. C), or if the exception needs to be 78 forwarded to a prior activation, the exception frame contains information about 79 how to unwind the current activation and restore the state of the prior 80 activation. This process is repeated until the exception is handled. If the 81 exception is not handled and no activations remain, then the application is 82 terminated with an appropriate error message. 83 84 Because different programming languages have different behaviors when handling 85 exceptions, the exception handling ABI provides a mechanism for 86 supplying *personalities*. An exception handling personality is defined by 87 way of a *personality function* (e.g. ``__gxx_personality_v0`` in C++), 88 which receives the context of the exception, an *exception structure* 89 containing the exception object type and value, and a reference to the exception 90 table for the current function. The personality function for the current 91 compile unit is specified in a *common exception frame*. 92 93 The organization of an exception table is language dependent. For C++, an 94 exception table is organized as a series of code ranges defining what to do if 95 an exception occurs in that range. Typically, the information associated with a 96 range defines which types of exception objects (using C++ *type info*) that are 97 handled in that range, and an associated action that should take place. Actions 98 typically pass control to a *landing pad*. 99 100 A landing pad corresponds roughly to the code found in the ``catch`` portion of 101 a ``try``/``catch`` sequence. When execution resumes at a landing pad, it 102 receives an *exception structure* and a *selector value* corresponding to the 103 *type* of exception thrown. The selector is then used to determine which *catch* 104 should actually process the exception. 105 106 LLVM Code Generation 107 ==================== 108 109 From a C++ developer's perspective, exceptions are defined in terms of the 110 ``throw`` and ``try``/``catch`` statements. In this section we will describe the 111 implementation of LLVM exception handling in terms of C++ examples. 112 113 Throw 114 ----- 115 116 Languages that support exception handling typically provide a ``throw`` 117 operation to initiate the exception process. Internally, a ``throw`` operation 118 breaks down into two steps. 119 120 #. A request is made to allocate exception space for an exception structure. 121 This structure needs to survive beyond the current activation. This structure 122 will contain the type and value of the object being thrown. 123 124 #. A call is made to the runtime to raise the exception, passing the exception 125 structure as an argument. 126 127 In C++, the allocation of the exception structure is done by the 128 ``__cxa_allocate_exception`` runtime function. The exception raising is handled 129 by ``__cxa_throw``. The type of the exception is represented using a C++ RTTI 130 structure. 131 132 Try/Catch 133 --------- 134 135 A call within the scope of a *try* statement can potentially raise an 136 exception. In those circumstances, the LLVM C++ front-end replaces the call with 137 an ``invoke`` instruction. Unlike a call, the ``invoke`` has two potential 138 continuation points: 139 140 #. where to continue when the call succeeds as per normal, and 141 142 #. where to continue if the call raises an exception, either by a throw or the 143 unwinding of a throw 144 145 The term used to define a the place where an ``invoke`` continues after an 146 exception is called a *landing pad*. LLVM landing pads are conceptually 147 alternative function entry points where an exception structure reference and a 148 type info index are passed in as arguments. The landing pad saves the exception 149 structure reference and then proceeds to select the catch block that corresponds 150 to the type info of the exception object. 151 152 The LLVM :ref:`i_landingpad` is used to convey information about the landing 153 pad to the back end. For C++, the ``landingpad`` instruction returns a pointer 154 and integer pair corresponding to the pointer to the *exception structure* and 155 the *selector value* respectively. 156 157 The ``landingpad`` instruction takes a reference to the personality function to 158 be used for this ``try``/``catch`` sequence. The remainder of the instruction is 159 a list of *cleanup*, *catch*, and *filter* clauses. The exception is tested 160 against the clauses sequentially from first to last. The selector value is a 161 positive number if the exception matched a type info, a negative number if it 162 matched a filter, and zero if it matched a cleanup. If nothing is matched, the 163 behavior of the program is `undefined`_. If a type info matched, then the 164 selector value is the index of the type info in the exception table, which can 165 be obtained using the `llvm.eh.typeid.for`_ intrinsic. 166 167 Once the landing pad has the type info selector, the code branches to the code 168 for the first catch. The catch then checks the value of the type info selector 169 against the index of type info for that catch. Since the type info index is not 170 known until all the type infos have been gathered in the backend, the catch code 171 must call the `llvm.eh.typeid.for`_ intrinsic to determine the index for a given 172 type info. If the catch fails to match the selector then control is passed on to 173 the next catch. 174 175 Finally, the entry and exit of catch code is bracketed with calls to 176 ``__cxa_begin_catch`` and ``__cxa_end_catch``. 177 178 * ``__cxa_begin_catch`` takes an exception structure reference as an argument 179 and returns the value of the exception object. 180 181 * ``__cxa_end_catch`` takes no arguments. This function: 182 183 #. Locates the most recently caught exception and decrements its handler 184 count, 185 186 #. Removes the exception from the *caught* stack if the handler count goes to 187 zero, and 188 189 #. Destroys the exception if the handler count goes to zero and the exception 190 was not re-thrown by throw. 191 192 .. note:: 193 194 a rethrow from within the catch may replace this call with a 195 ``__cxa_rethrow``. 196 197 Cleanups 198 -------- 199 200 A cleanup is extra code which needs to be run as part of unwinding a scope. C++ 201 destructors are a typical example, but other languages and language extensions 202 provide a variety of different kinds of cleanups. In general, a landing pad may 203 need to run arbitrary amounts of cleanup code before actually entering a catch 204 block. To indicate the presence of cleanups, a :ref:`i_landingpad` should have 205 a *cleanup* clause. Otherwise, the unwinder will not stop at the landing pad if 206 there are no catches or filters that require it to. 207 208 .. note:: 209 210 Do not allow a new exception to propagate out of the execution of a 211 cleanup. This can corrupt the internal state of the unwinder. Different 212 languages describe different high-level semantics for these situations: for 213 example, C++ requires that the process be terminated, whereas Ada cancels both 214 exceptions and throws a third. 215 216 When all cleanups are finished, if the exception is not handled by the current 217 function, resume unwinding by calling the `resume 218 instruction <LangRef.html#i_resume>`_, passing in the result of the 219 ``landingpad`` instruction for the original landing pad. 220 221 Throw Filters 222 ------------- 223 224 C++ allows the specification of which exception types may be thrown from a 225 function. To represent this, a top level landing pad may exist to filter out 226 invalid types. To express this in LLVM code the :ref:`i_landingpad` will have a 227 filter clause. The clause consists of an array of type infos. 228 ``landingpad`` will return a negative value 229 if the exception does not match any of the type infos. If no match is found then 230 a call to ``__cxa_call_unexpected`` should be made, otherwise 231 ``_Unwind_Resume``. Each of these functions requires a reference to the 232 exception structure. Note that the most general form of a ``landingpad`` 233 instruction can have any number of catch, cleanup, and filter clauses (though 234 having more than one cleanup is pointless). The LLVM C++ front-end can generate 235 such ``landingpad`` instructions due to inlining creating nested exception 236 handling scopes. 237 238 .. _undefined: 239 240 Restrictions 241 ------------ 242 243 The unwinder delegates the decision of whether to stop in a call frame to that 244 call frame's language-specific personality function. Not all unwinders guarantee 245 that they will stop to perform cleanups. For example, the GNU C++ unwinder 246 doesn't do so unless the exception is actually caught somewhere further up the 247 stack. 248 249 In order for inlining to behave correctly, landing pads must be prepared to 250 handle selector results that they did not originally advertise. Suppose that a 251 function catches exceptions of type ``A``, and it's inlined into a function that 252 catches exceptions of type ``B``. The inliner will update the ``landingpad`` 253 instruction for the inlined landing pad to include the fact that ``B`` is also 254 caught. If that landing pad assumes that it will only be entered to catch an 255 ``A``, it's in for a rude awakening. Consequently, landing pads must test for 256 the selector results they understand and then resume exception propagation with 257 the `resume instruction <LangRef.html#i_resume>`_ if none of the conditions 258 match. 259 260 Exception Handling Intrinsics 261 ============================= 262 263 In addition to the ``landingpad`` and ``resume`` instructions, LLVM uses several 264 intrinsic functions (name prefixed with ``llvm.eh``) to provide exception 265 handling information at various points in generated code. 266 267 .. _llvm.eh.typeid.for: 268 269 ``llvm.eh.typeid.for`` 270 ---------------------- 271 272 .. code-block:: llvm 273 274 i32 @llvm.eh.typeid.for(i8* %type_info) 275 276 277 This intrinsic returns the type info index in the exception table of the current 278 function. This value can be used to compare against the result of 279 ``landingpad`` instruction. The single argument is a reference to a type info. 280 281 .. _llvm.eh.sjlj.setjmp: 282 283 ``llvm.eh.sjlj.setjmp`` 284 ----------------------- 285 286 .. code-block:: llvm 287 288 i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf) 289 290 For SJLJ based exception handling, this intrinsic forces register saving for the 291 current function and stores the address of the following instruction for use as 292 a destination address by `llvm.eh.sjlj.longjmp`_. The buffer format and the 293 overall functioning of this intrinsic is compatible with the GCC 294 ``__builtin_setjmp`` implementation allowing code built with the clang and GCC 295 to interoperate. 296 297 The single parameter is a pointer to a five word buffer in which the calling 298 context is saved. The front end places the frame pointer in the first word, and 299 the target implementation of this intrinsic should place the destination address 300 for a `llvm.eh.sjlj.longjmp`_ in the second word. The following three words are 301 available for use in a target-specific manner. 302 303 .. _llvm.eh.sjlj.longjmp: 304 305 ``llvm.eh.sjlj.longjmp`` 306 ------------------------ 307 308 .. code-block:: llvm 309 310 void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf) 311 312 For SJLJ based exception handling, the ``llvm.eh.sjlj.longjmp`` intrinsic is 313 used to implement ``__builtin_longjmp()``. The single parameter is a pointer to 314 a buffer populated by `llvm.eh.sjlj.setjmp`_. The frame pointer and stack 315 pointer are restored from the buffer, then control is transferred to the 316 destination address. 317 318 ``llvm.eh.sjlj.lsda`` 319 --------------------- 320 321 .. code-block:: llvm 322 323 i8* @llvm.eh.sjlj.lsda() 324 325 For SJLJ based exception handling, the ``llvm.eh.sjlj.lsda`` intrinsic returns 326 the address of the Language Specific Data Area (LSDA) for the current 327 function. The SJLJ front-end code stores this address in the exception handling 328 function context for use by the runtime. 329 330 ``llvm.eh.sjlj.callsite`` 331 ------------------------- 332 333 .. code-block:: llvm 334 335 void @llvm.eh.sjlj.callsite(i32 %call_site_num) 336 337 For SJLJ based exception handling, the ``llvm.eh.sjlj.callsite`` intrinsic 338 identifies the callsite value associated with the following ``invoke`` 339 instruction. This is used to ensure that landing pad entries in the LSDA are 340 generated in matching order. 341 342 Asm Table Formats 343 ================= 344 345 There are two tables that are used by the exception handling runtime to 346 determine which actions should be taken when an exception is thrown. 347 348 Exception Handling Frame 349 ------------------------ 350 351 An exception handling frame ``eh_frame`` is very similar to the unwind frame 352 used by DWARF debug info. The frame contains all the information necessary to 353 tear down the current frame and restore the state of the prior frame. There is 354 an exception handling frame for each function in a compile unit, plus a common 355 exception handling frame that defines information common to all functions in the 356 unit. 357 358 Exception Tables 359 ---------------- 360 361 An exception table contains information about what actions to take when an 362 exception is thrown in a particular part of a function's code. There is one 363 exception table per function, except leaf functions and functions that have 364 calls only to non-throwing functions. They do not need an exception table. 365
