Exception Handling in LLVM

Introduction

This document is the central repository for all information pertaining to exception handling in LLVM. It describes the format that LLVM exception handling information takes, which is useful for those interested in creating front-ends or dealing directly with the information. Further, this document provides specific examples of what exception handling information is used for in C and C++.

Itanium ABI Zero-cost Exception Handling

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 Exception Handling

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.

Windows Runtime Exception Handling

Windows runtime based exception handling uses the same basic IR structure as Itanium ABI based exception handling, but it relies on the personality functions provided by the native Windows runtime library, __CxxFrameHandler3 for C++ exceptions: __C_specific_handler for 64-bit SEH or _frame_handler3/4 for 32-bit SEH. This results in a very different execution model and requires some minor modifications to the initial IR representation and a significant restructuring just before code generation.

General information about the Windows x64 exception handling mechanism can be found at MSDN Exception Handling (x64).

Overview

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.

LLVM Code Generation

From a C++ developer’s perspective, exceptions are defined in terms of the throw and try/catch statements. In this section we will describe the implementation of LLVM exception handling in terms of C++ examples.

Throw

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.

Try/Catch

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 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 clauses have the following meanings:

  • catch <type> @ExcType

    • This clause means that the landingpad block should be entered if the exception being thrown is of type @ExcType or a subtype of @ExcType. For C++, @ExcType is a pointer to the std::type_info object (an RTTI object) representing the C++ exception type.
    • If @ExcType is null, any exception matches, so the landingpad should always be entered. This is used for C++ catch-all blocks (“catch (...)”).
    • When this clause is matched, the selector value will be equal to the value returned by “@llvm.eh.typeid.for(i8* @ExcType)”. This will always be a positive value.
  • filter <type> [<type> @ExcType1, ..., <type> @ExcTypeN]

    • This clause means that the landingpad should be entered if the exception being thrown does not match any of the types in the list (which, for C++, are again specified as std::type_info pointers).
    • C++ front-ends use this to implement C++ exception specifications, such as “void foo() throw (ExcType1, ..., ExcTypeN) { ... }”.
    • When this clause is matched, the selector value will be negative.
    • The array argument to filter may be empty; for example, “[0 x i8**] undef”. This means that the landingpad should always be entered. (Note that such a filter would not be equivalent to “catch i8* null”, because filter and catch produce negative and positive selector values respectively.)
  • cleanup

    • This clause means that the landingpad should always be entered.

    • C++ front-ends use this for calling objects’ destructors.

    • When this clause is matched, the selector value will be zero.

    • The runtime may treat “cleanup” differently from “catch <type> null”.

      In C++, if an unhandled exception occurs, the language runtime will call std::terminate(), but it is implementation-defined whether the runtime unwinds the stack and calls object destructors first. For example, the GNU C++ unwinder does not call object destructors when an unhandled exception occurs. The reason for this is to improve debuggability: it ensures that std::terminate() is called from the context of the throw, so that this context is not lost by unwinding the stack. A runtime will typically implement this by searching for a matching non-cleanup clause, and aborting if it does not find one, before entering any landingpad blocks.

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.

Cleanups

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.

Throw Filters

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.

Restrictions

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.

C++ Exception Handling using the Windows Runtime

(Note: Windows C++ exception handling support is a work in progress and is
not yet fully implemented. The text below describes how it will work when completed.)

The Windows runtime function for C++ exception handling uses a multi-phase approach. When an exception occurs it searches the current callstack for a frame that has a handler for the exception. If a handler is found, it then calls the cleanup handler for each frame above the handler which has a cleanup handler before calling the catch handler. These calls are all made from a stack context different from the original frame in which the handler is defined. Therefore, it is necessary to outline these handlers from their original context before code generation.

Catch handlers are called with a pointer to the handler itself as the first argument and a pointer to the parent function’s stack frame as the second argument. The catch handler uses the llvm.recoverframe to get a pointer to a frame allocation block that is created in the parent frame using the llvm.allocateframe intrinsic. The WinEHPrepare pass will have created a structure definition for the contents of this block. The first two members of the structure will always be (1) a 32-bit integer that the runtime uses to track the exception state of the parent frame for the purposes of handling chained exceptions and (2) a pointer to the object associated with the exception (roughly, the parameter of the catch clause). These two members will be followed by any frame variables from the parent function which must be accessed in any of the functions unwind or catch handlers. The catch handler returns the address at which execution should continue.

Cleanup handlers perform any cleanup necessary as the frame goes out of scope, such as calling object destructors. The runtime handles the actual unwinding of the stack. If an exception occurs in a cleanup handler the runtime manages termination of the process. Cleanup handlers are called with the same arguments as catch handlers (a pointer to the handler and a pointer to the parent stack frame) and use the same mechanism described above to access frame variables in the parent function. Cleanup handlers do not return a value.

The IR generated for Windows runtime based C++ exception handling is initially very similar to the landingpad mechanism described above. Calls to libc++abi functions (such as __cxa_begin_catch/__cxa_end_catch and __cxa_throw_exception are replaced with calls to intrinsics or Windows runtime functions (such as llvm.eh.begincatch/llvm.eh.endcatch and __CxxThrowException).

During the WinEHPrepare pass, the handler functions are outlined into handler functions and the original landing pad code is replaced with a call to the llvm.eh.actions intrinsic that describes the order in which handlers will be processed from the logical location of the landing pad and an indirect branch to the return value of the llvm.eh.actions intrinsic. The llvm.eh.actions intrinsic is defined as returning the address at which execution will continue. This is a temporary construct which will be removed before code generation, but it allows for the accurate tracking of control flow until then.

A typical landing pad will look like this after outlining:

lpad:
  %vals = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)* @__CxxFrameHandler3 to i8*)
          cleanup
      catch i8* bitcast (i8** @_ZTIi to i8*)
      catch i8* bitcast (i8** @_ZTIf to i8*)
  %recover = call i8* (...)* @llvm.eh.actions(
      i32 3, i8* bitcast (i8** @_ZTIi to i8*), i8* (i8*, i8*)* @_Z4testb.catch.1)
      i32 2, i8* null, void (i8*, i8*)* @_Z4testb.cleanup.1)
      i32 1, i8* bitcast (i8** @_ZTIf to i8*), i8* (i8*, i8*)* @_Z4testb.catch.0)
      i32 0, i8* null, void (i8*, i8*)* @_Z4testb.cleanup.0)
  indirectbr i8* %recover, [label %try.cont1, label %try.cont2]

In this example, the landing pad represents an exception handling context with two catch handlers and a cleanup handler that have been outlined. If an exception is thrown with a type that matches _ZTIi, the _Z4testb.catch.1 handler will be called an no clean-up is needed. If an exception is thrown with a type that matches _ZTIf, first the _Z4testb.cleanup.1 handler will be called to perform unwind-related cleanup, then the _Z4testb.catch.1 handler will be called. If an exception is throw which does not match either of these types and the exception is handled by another frame further up the call stack, first the _Z4testb.cleanup.1 handler will be called, then the _Z4testb.cleanup.0 handler (which corresponds to a different scope) will be called, and exception handling will continue at the next frame in the call stack will be called. One of the catch handlers will return the address of %try.cont1 in the parent function and the other will return the address of %try.cont2, meaning that execution continues at one of those blocks after an exception is caught.

Exception Handling Intrinsics

In addition to the landingpad and resume instructions, LLVM uses several intrinsic functions (name prefixed with llvm.eh) to provide exception handling information at various points in generated code.

llvm.eh.typeid.for

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.

Uses of this intrinsic are generated by the C++ front-end.

llvm.eh.begincatch

void @llvm.eh.begincatch(i8* %ehptr, i8* %ehobj)

This intrinsic marks the beginning of catch handling code within the blocks following a landingpad instruction. The exact behavior of this function depends on the compilation target and the personality function associated with the landingpad instruction.

The first argument to this intrinsic is a pointer that was previously extracted from the aggregate return value of the landingpad instruction. The second argument to the intrinsic is a pointer to stack space where the exception object should be stored. The runtime handles the details of copying the exception object into the slot. If the second parameter is null, no copy occurs.

Uses of this intrinsic are generated by the C++ front-end. Many targets will use implementation-specific functions (such as __cxa_begin_catch) instead of this intrinsic. The intrinsic is provided for targets that require a more abstract interface.

When used in the native Windows C++ exception handling implementation, this intrinsic serves as a placeholder to delimit code before a catch handler is outlined. When the handler is is outlined, this intrinsic will be replaced by instructions that retrieve the exception object pointer from the frame allocation block.

llvm.eh.endcatch

void @llvm.eh.endcatch()

This intrinsic marks the end of catch handling code within the current block, which will be a successor of a block which called llvm.eh.begincatch''. The exact behavior of this function depends on the compilation target and the personality function associated with the corresponding ``landingpad instruction.

There may be more than one call to llvm.eh.endcatch for any given call to llvm.eh.begincatch with each llvm.eh.endcatch call corresponding to the end of a different control path. All control paths following a call to llvm.eh.begincatch must reach a call to llvm.eh.endcatch.

Uses of this intrinsic are generated by the C++ front-end. Many targets will use implementation-specific functions (such as __cxa_begin_catch) instead of this intrinsic. The intrinsic is provided for targets that require a more abstract interface.

When used in the native Windows C++ exception handling implementation, this intrinsic serves as a placeholder to delimit code before a catch handler is outlined. After the handler is outlined, this intrinsic is simply removed.

llvm.eh.actions

void @llvm.eh.actions()

This intrinsic represents the list of actions to take when an exception is thrown. It is typically used by Windows exception handling schemes where cleanup outlining is required by the runtime. The arguments are a sequence of i32 sentinels indicating the action type followed by some pre-determined number of arguments required to implement that action.

A code of i32 0 indicates a cleanup action, which expects one additional argument. The argument is a pointer to a function that implements the cleanup action.

A code of i32 1 indicates a catch action, which expects three additional arguments. Different EH schemes give different meanings to the three arguments, but the first argument indicates whether the catch should fire, the second is a pointer to stack object where the exception object should be stored, and the third is the code to run to catch the exception.

For Windows C++ exception handling, the first argument for a catch handler is a pointer to the RTTI type descriptor for the object to catch. The third argument is a pointer to a function implementing the catch. This function returns the address of the basic block where execution should resume after handling the exception.

For Windows SEH, the first argument is a pointer to the filter function, which indicates if the exception should be caught or not. The second argument is typically null. The third argument is the address of a basic block where the exception will be handled. In other words, catch handlers are not outlined in SEH. After running cleanups, execution immediately resumes at this PC.

In order to preserve the structure of the CFG, a call to ‘llvm.eh.actions‘ must be followed by an ‘indirectbr‘ instruction that jumps to the result of the intrinsic call.

llvm.eh.unwindhelp

void @llvm.eh.unwindhelp(i8*)

This intrinsic designates the provided static alloca as the unwind help object. This object is used by Windows native exception handling on non-x86 platforms where xdata unwind information is used. It is typically an 8 byte chunk of memory treated as two 32-bit integers.

llvm.eh.parentframe

void @llvm.eh.parentframe(i8*)

This intrinsic designates the provided static alloca as the object which holds the address of the parent frame. This object is used by Windows native exception handling on non-x86 platforms where xdata unwind information is used.

SJLJ Intrinsics

The llvm.eh.sjlj intrinsics are used internally within LLVM’s backend. Uses of them are generated by the backend’s SjLjEHPrepare pass.

llvm.eh.sjlj.setjmp

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.

llvm.eh.sjlj.longjmp

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.

llvm.eh.sjlj.lsda

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.

llvm.eh.sjlj.callsite

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.

Asm Table Formats

There are two tables that are used by the exception handling runtime to determine which actions should be taken when an exception is thrown.

Exception Handling Frame

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.

Exception Tables

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.