Exception Handling in LLVM

Exception handling for most programming languages is designed to recover from conditions that rarely occur during general use of an application. To that end, exception handling should not interfere with the main flow of an application's algorithm by performing checkpointing tasks, such as saving the current pc or register state.

The Itanium ABI Exception Handling Specification defines a methodology for providing outlying data in the form of exception tables without inlining speculative exception handling code in the flow of an application's main algorithm. Thus, the specification is said to add "zero-cost" to the normal execution of an application.

A more complete description of the Itanium ABI exception handling runtime support of can be found at Itanium C++ ABI: Exception Handling. A description of the exception frame format can be found at Exception Frames, with details of the DWARF 4 specification at DWARF 4 Standard. A description for the C++ exception table formats can be found at Exception Handling Tables.

Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics llvm.eh.sjlj.setjmp and llvm.eh.sjlj.longjmp to handle control flow for exception handling.

For each function which does exception processing — be it try/catch blocks or cleanups — that function registers itself on a global frame list. When exceptions are unwinding, the runtime uses this list to identify which functions need processing.

Landing pad selection is encoded in the call site entry of the function context. The runtime returns to the function via llvm.eh.sjlj.longjmp, where a switch table transfers control to the appropriate landing pad based on the index stored in the function context.

In contrast to DWARF exception handling, which encodes exception regions and frame information in out-of-line tables, SJLJ exception handling builds and removes the unwind frame context at runtime. This results in faster exception handling at the expense of slower execution when no exceptions are thrown. As exceptions are, by their nature, intended for uncommon code paths, DWARF exception handling is generally preferred to SJLJ.

When an exception is thrown in LLVM code, the runtime does its best to find a handler suited to processing the circumstance.

The runtime first attempts to find an exception frame corresponding to the function where the exception was thrown. If the programming language supports exception handling (e.g. C++), the exception frame contains a reference to an exception table describing how to process the exception. If the language does not support exception handling (e.g. C), or if the exception needs to be forwarded to a prior activation, the exception frame contains information about how to unwind the current activation and restore the state of the prior activation. This process is repeated until the exception is handled. If the exception is not handled and no activations remain, then the application is terminated with an appropriate error message.

Because different programming languages have different behaviors when handling exceptions, the exception handling ABI provides a mechanism for supplying personalities. An exception handling personality is defined by way of a personality function (e.g. __gxx_personality_v0 in C++), which receives the context of the exception, an exception structure containing the exception object type and value, and a reference to the exception table for the current function. The personality function for the current compile unit is specified in a common exception frame.

The organization of an exception table is language dependent. For C++, an exception table is organized as a series of code ranges defining what to do if an exception occurs in that range. Typically, the information associated with a range defines which types of exception objects (using C++ type info) that are handled in that range, and an associated action that should take place. Actions typically pass control to a landing pad.

A landing pad corresponds roughly to the code found in the catch portion of a try/catch sequence. When execution resumes at a landing pad, it receives an exception structure and a selector value corresponding to the type of exception thrown. The selector is then used to determine which catch should actually process the exception.

Languages that support exception handling typically provide a throw operation to initiate the exception process. Internally, a throw operation breaks down into two steps.

1. A request is made to allocate exception space for an exception structure. This structure needs to survive beyond the current activation. This structure will contain the type and value of the object being thrown.
2. A call is made to the runtime to raise the exception, passing the exception structure as an argument.

In C++, the allocation of the exception structure is done by the __cxa_allocate_exception runtime function. The exception raising is handled by __cxa_throw. The type of the exception is represented using a C++ RTTI structure.

A call within the scope of a try statement can potentially raise an exception. In those circumstances, the LLVM C++ front-end replaces the call with an invoke instruction. Unlike a call, the invoke has two potential continuation points:

1. where to continue when the call succeeds as per normal, and
2. where to continue if the call raises an exception, either by a throw or the unwinding of a throw

The term used to define a the place where an invoke continues after an exception is called a landing pad. LLVM landing pads are conceptually alternative function entry points where an exception structure reference and a type info index are passed in as arguments. The landing pad saves the exception structure reference and then proceeds to select the catch block that corresponds to the type info of the exception object.

The LLVM landingpad instruction is used to convey information about the landing pad to the back end. For C++, the landingpad instruction returns a pointer and integer pair corresponding to the pointer to the exception structure and the selector value respectively.

The landingpad instruction takes a reference to the personality function to be used for this try/catch sequence. The remainder of the instruction is a list of cleanup, catch, and filter clauses. The exception is tested against the clauses sequentially from first to last. The selector value is a positive number if the exception matched a type info, a negative number if it matched a filter, and zero if it matched a cleanup. If nothing is matched, the behavior of the program is undefined. If a type info matched, then the selector value is the index of the type info in the exception table, which can be obtained using the llvm.eh.typeid.for intrinsic.

Once the landing pad has the type info selector, the code branches to the code for the first catch. The catch then checks the value of the type info selector against the index of type info for that catch. Since the type info index is not known until all the type infos have been gathered in the backend, the catch code must call the llvm.eh.typeid.for intrinsic to determine the index for a given type info. If the catch fails to match the selector then control is passed on to the next catch.

Finally, the entry and exit of catch code is bracketed with calls to __cxa_begin_catch and __cxa_end_catch.

• __cxa_begin_catch takes an exception structure reference as an argument and returns the value of the exception object.
• __cxa_end_catch takes no arguments. This function:
  
  
  1. Locates the most recently caught exception and decrements its handler count,
  2. Removes the exception from the caught stack if the handler count goes to zero, and
  3. Destroys the exception if the handler count goes to zero and the exception was not re-thrown by throw.

  Note: a rethrow from within the catch may replace this call with a __cxa_rethrow.

A cleanup is extra code which needs to be run as part of unwinding a scope. C++ destructors are a typical example, but other languages and language extensions provide a variety of different kinds of cleanups. In general, a landing pad may need to run arbitrary amounts of cleanup code before actually entering a catch block. To indicate the presence of cleanups, a landingpad instruction should have a cleanup clause. Otherwise, the unwinder will not stop at the landing pad if there are no catches or filters that require it to.

Note: Do not allow a new exception to propagate out of the execution of a cleanup. This can corrupt the internal state of the unwinder. Different languages describe different high-level semantics for these situations: for example, C++ requires that the process be terminated, whereas Ada cancels both exceptions and throws a third.

When all cleanups are finished, if the exception is not handled by the current function, resume unwinding by calling the resume instruction, passing in the result of the landingpad instruction for the original landing pad.

C++ allows the specification of which exception types may be thrown from a function. To represent this, a top level landing pad may exist to filter out invalid types. To express this in LLVM code the landingpad instruction will have a filter clause. The clause consists of an array of type infos. landingpad will return a negative value if the exception does not match any of the type infos. If no match is found then a call to __cxa_call_unexpected should be made, otherwise _Unwind_Resume. Each of these functions requires a reference to the exception structure. Note that the most general form of a landingpad instruction can have any number of catch, cleanup, and filter clauses (though having more than one cleanup is pointless). The LLVM C++ front-end can generate such landingpad instructions due to inlining creating nested exception handling scopes.

The unwinder delegates the decision of whether to stop in a call frame to that call frame's language-specific personality function. Not all unwinders guarantee that they will stop to perform cleanups. For example, the GNU C++ unwinder doesn't do so unless the exception is actually caught somewhere further up the stack.

In order for inlining to behave correctly, landing pads must be prepared to handle selector results that they did not originally advertise. Suppose that a function catches exceptions of type A, and it's inlined into a function that catches exceptions of type B. The inliner will update the landingpad instruction for the inlined landing pad to include the fact that B is also caught. If that landing pad assumes that it will only be entered to catch an A, it's in for a rude awakening. Consequently, landing pads must test for the selector results they understand and then resume exception propagation with the resume instruction if none of the conditions match.

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

This intrinsic returns the type info index in the exception table of the current function. This value can be used to compare against the result of landingpad instruction. The single argument is a reference to a type info.

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

For SJLJ based exception handling, this intrinsic forces register saving for the current function and stores the address of the following instruction for use as a destination address by llvm.eh.sjlj.longjmp. The buffer format and the overall functioning of this intrinsic is compatible with the GCC __builtin_setjmp implementation allowing code built with the clang and GCC to interoperate.

The single parameter is a pointer to a five word buffer in which the calling context is saved. The front end places the frame pointer in the first word, and the target implementation of this intrinsic should place the destination address for a llvm.eh.sjlj.longjmp in the second word. The following three words are available for use in a target-specific manner.

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

For SJLJ based exception handling, the llvm.eh.sjlj.longjmp intrinsic is used to implement __builtin_longjmp(). The single parameter is a pointer to a buffer populated by llvm.eh.sjlj.setjmp. The frame pointer and stack pointer are restored from the buffer, then control is transferred to the destination address.

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

For SJLJ based exception handling, the llvm.eh.sjlj.lsda intrinsic returns the address of the Language Specific Data Area (LSDA) for the current function. The SJLJ front-end code stores this address in the exception handling function context for use by the runtime.

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

For SJLJ based exception handling, the llvm.eh.sjlj.callsite intrinsic identifies the callsite value associated with the following invoke instruction. This is used to ensure that landing pad entries in the LSDA are generated in matching order.

An exception handling frame eh_frame is very similar to the unwind frame used by DWARF debug info. The frame contains all the information necessary to tear down the current frame and restore the state of the prior frame. There is an exception handling frame for each function in a compile unit, plus a common exception handling frame that defines information common to all functions in the unit.

An exception table contains information about what actions to take when an exception is thrown in a particular part of a function's code. There is one exception table per function, except leaf functions and functions that have calls only to non-throwing functions. They do not need an exception table.