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++.
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.
LLVM supports handling exceptions produced by the Windows runtime, but it requires a very different intermediate representation. It is not based on the “landingpad” instruction like the other two models, and is described later in this document under Exception Handling using the Windows Runtime.
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.
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.
Languages that support exception handling typically provide a throw operation to initiate the exception process. Internally, a throw operation breaks down into two steps.
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:
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 looks for a reference to the personality function to be used for this try/catch sequence in the parent function’s attribute list. The instruction contains 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
filter <type> [<type> @ExcType1, ..., <type> @ExcTypeN]
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:
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.
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.
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.
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.
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.
i8 addrspace(N)* @llvm.eh.padparam.pNi8(token %catchpad)
This intrinsic retrieves a pointer to the exception caught by the given catchpad.
The llvm.eh.sjlj intrinsics are used internally within LLVM’s backend. Uses of them are generated by the backend’s SjLjEHPrepare pass.
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.
There are two tables that are used by the exception handling runtime to determine which actions should be taken when an exception is thrown.
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.
The format of this call frame information (CFI) is often platform-dependent, however. ARM, for example, defines their own format. Apple has their own compact unwind info format. On Windows, another format is used for all architectures since 32-bit x86. LLVM will emit whatever information is required by the target.
An exception table contains information about what actions to take when an exception is thrown in a particular part of a function’s code. This is typically referred to as the language-specific data area (LSDA). The format of the LSDA table is specific to the personality function, but the majority of personalities out there use a variation of the tables consumed by __gxx_personality_v0. 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.
Interacting with exceptions on Windows is significantly more complicated than on Itanium C++ ABI platforms. The fundamental difference between the two models is that Itanium EH is designed around the idea of “successive unwinding,” while Windows EH is not.
Under Itanium, throwing an exception typically involes allocating thread local memory to hold the exception, and calling into the EH runtime. The runtime identifies frames with appropriate exception handling actions, and successively resets the register context of the current thread to the most recently active frame with actions to run. In LLVM, execution resumes at a landingpad instruction, which produces register values provided by the runtime. If a function is only cleaning up allocated resources, the function is responsible for calling _Unwind_Resume to transition to the next most recently active frame after it is finished cleaning up. Eventually, the frame responsible for handling the exception calls __cxa_end_catch to destroy the exception, release its memory, and resume normal control flow.
The Windows EH model does not use these successive register context resets. Instead, the active exception is typically described by a frame on the stack. In the case of C++ exceptions, the exception object is allocated in stack memory and its address is passed to __CxxThrowException. General purpose structured exceptions (SEH) are more analogous to Linux signals, and they are dispatched by userspace DLLs provided with Windows. Each frame on the stack has an assigned EH personality routine, which decides what actions to take to handle the exception. There are a few major personalities for C and C++ code: the C++ personality (__CxxFrameHandler3) and the SEH personalities (_except_handler3, _except_handler4, and __C_specific_handler). All of them implement cleanups by calling back into a “funclet” contained in the parent function.
Funclets, in this context, are regions of the parent function that can be called as though they were a function pointer with a very special calling convention. The frame pointer of the parent frame is passed into the funclet either using the standard EBP register or as the first parameter register, depending on the architecture. The funclet implements the EH action by accessing local variables in memory through the frame pointer, and returning some appropriate value, continuing the EH process. No variables live in to or out of the funclet can be allocated in registers.
The C++ personality also uses funclets to contain the code for catch blocks (i.e. all user code between the braces in catch (Type obj) { ... }). The runtime must use funclets for catch bodies because the C++ exception object is allocated in a child stack frame of the function handling the exception. If the runtime rewound the stack back to frame of the catch, the memory holding the exception would be overwritten quickly by subsequent function calls. The use of funclets also allows __CxxFrameHandler3 to implement rethrow without resorting to TLS. Instead, the runtime throws a special exception, and then uses SEH (__try / __except) to resume execution with new information in the child frame.
In other words, the successive unwinding approach is incompatible with Visual C++ exceptions and general purpose Windows exception handling. Because the C++ exception object lives in stack memory, LLVM cannot provide a custom personality function that uses landingpads. Similarly, SEH does not provide any mechanism to rethrow an exception or continue unwinding. Therefore, LLVM must use the IR constructs described later in this document to implement compatible exception handling.
The SEH personality functions also use funclets to implement filter expressions, which allow executing arbitrary user code to decide which exceptions to catch. Filter expressions should not be confused with the filter clause of the LLVM landingpad instruction. Typically filter expressions are used to determine if the exception came from a particular DLL or code region, or if code faulted while accessing a particular memory address range. LLVM does not currently have IR to represent filter expressions because it is difficult to represent their control dependencies. Filter expressions run during the first phase of EH, before cleanups run, making it very difficult to build a faithful control flow graph. For now, the new EH instructions cannot represent SEH filter expressions, and frontends must outline them ahead of time. Local variables of the parent function can be escaped and accessed using the llvm.localescape and llvm.localrecover intrinsics.
The primary design goal of the new EH instructions is to support funclet generation while preserving information about the CFG so that SSA formation still works. As a secondary goal, they are designed to be generic across MSVC and Itanium C++ exceptions. They make very few assumptions about the data required by the personality, so long as it uses the familiar core EH actions: catch, cleanup, and terminate. However, the new instructions are hard to modify without knowing details of the EH personality. While they can be used to represent Itanium EH, the landingpad model is strictly better for optimization purposes.
The following new instructions are considered “exception handling pads”, in that they must be the first non-phi instruction of a basic block that may be the unwind destination of an EH flow edge: catchswitch, catchpad, and cleanuppad. As with landingpads, when entering a try scope, if the frontend encounters a call site that may throw an exception, it should emit an invoke that unwinds to a catchswitch block. Similarly, inside the scope of a C++ object with a destructor, invokes should unwind to a cleanuppad.
New instructions are also used to mark the points where control is transferred out of a catch/cleanup handler (which will correspond to exits from the generated funclet). A catch handler which reaches its end by normal execution executes a catchret instruction, which is a terminator indicating where in the function control is returned to. A cleanup handler which reaches its end by normal execution executes a cleanupret instruction, which is a terminator indicating where the active exception will unwind to next.
Each of these new EH pad instructions has a way to identify which action should be considered after this action. The catchswitch instruction is a terminator and has an unwind destination operand analogous to the unwind destination of an invoke. The cleanuppad instruction is not a terminator, so the unwind destination is stored on the cleanupret instruction instead. Successfully executing a catch handler should resume normal control flow, so neither catchpad nor catchret instructions can unwind. All of these “unwind edges” may refer to a basic block that contains an EH pad instruction, or they may unwind to the caller. Unwinding to the caller has roughly the same semantics as the resume instruction in the landingpad model. When inlining through an invoke, instructions that unwind to the caller are hooked up to unwind to the unwind destination of the call site.
Putting things together, here is a hypothetical lowering of some C++ that uses all of the new IR instructions:
struct Cleanup {
Cleanup();
~Cleanup();
int m;
};
void may_throw();
int f() noexcept {
try {
Cleanup obj;
may_throw();
} catch (int e) {
may_throw();
return e;
}
return 0;
}
define i32 @f() nounwind personality i32 (...)* @__CxxFrameHandler3 {
entry:
%obj = alloca %struct.Cleanup, align 4
%e = alloca i32, align 4
%call = invoke %struct.Cleanup* @"\01??0Cleanup@@QEAA@XZ"(%struct.Cleanup* nonnull %obj)
to label %invoke.cont unwind label %lpad.catch
invoke.cont: ; preds = %entry
invoke void @"\01?may_throw@@YAXXZ"()
to label %invoke.cont.2 unwind label %lpad.cleanup
invoke.cont.2: ; preds = %invoke.cont
call void @"\01??_DCleanup@@QEAA@XZ"(%struct.Cleanup* nonnull %obj) nounwind
br label %return
return: ; preds = %invoke.cont.3, %invoke.cont.2
%retval.0 = phi i32 [ 0, %invoke.cont.2 ], [ %3, %invoke.cont.3 ]
ret i32 %retval.0
lpad.cleanup: ; preds = %invoke.cont.2
%0 = cleanuppad within none []
call void @"\01??1Cleanup@@QEAA@XZ"(%struct.Cleanup* nonnull %obj) nounwind
cleanupret %0 unwind label %lpad.catch
lpad.catch: ; preds = %lpad.cleanup, %entry
%1 = catchswitch within none [label %catch.body] unwind label %lpad.terminate
catch.body: ; preds = %lpad.catch
%catch = catchpad within %1 [%rtti.TypeDescriptor2* @"\01??_R0H@8", i32 0, i32* %e]
invoke void @"\01?may_throw@@YAXXZ"()
to label %invoke.cont.3 unwind label %lpad.terminate
invoke.cont.3: ; preds = %catch.body
%3 = load i32, i32* %e, align 4
catchret from %catch to label %return
lpad.terminate: ; preds = %catch.body, %lpad.catch
cleanuppad within none []
call void @"\01?terminate@@YAXXZ"
unreachable
}
In order to produce tables for EH personalities that use funclets, it is necessary to recover the nesting that was present in the source. This funclet parent relationship is encoded in the IR using tokens produced by the new “pad” instructions. The token operand of a “pad” or “ret” instruction indicates which funclet it is in, or “none” if it is not nested within another funclet.
The catchpad and cleanuppad instructions establish new funclets, and their tokens are consumed by other “pad” instructions to establish membership. The catchswitch instruction does not create a funclet, but it produces a token that is always consumed by its immediate successor catchpad instructions. This ensures that every catch handler modelled by a catchpad belongs to exactly one catchswitch, which models the dispatch point after a C++ try.
Here is an example of what this nesting looks like using some hypothetical C++ code:
void f() {
try {
throw;
} catch (...) {
try {
throw;
} catch (...) {
}
}
}
define void @f() #0 personality i8* bitcast (i32 (...)* @__CxxFrameHandler3 to i8*) {
entry:
invoke void @_CxxThrowException(i8* null, %eh.ThrowInfo* null) #1
to label %unreachable unwind label %catch.dispatch
catch.dispatch: ; preds = %entry
%0 = catchswitch within none [label %catch] unwind to caller
catch: ; preds = %catch.dispatch
%1 = catchpad within %0 [i8* null, i32 64, i8* null]
invoke void @_CxxThrowException(i8* null, %eh.ThrowInfo* null) #1
to label %unreachable unwind label %catch.dispatch2
catch.dispatch2: ; preds = %catch
%2 = catchswitch within %1 [label %catch3] unwind to caller
catch3: ; preds = %catch.dispatch2
%3 = catchpad within %2 [i8* null, i32 64, i8* null]
catchret from %3 to label %try.cont
try.cont: ; preds = %catch3
catchret from %1 to label %try.cont6
try.cont6: ; preds = %try.cont
ret void
unreachable: ; preds = %catch, %entry
unreachable
}
The “inner” catchswitch consumes %1 which is produced by the outer catchswitch.
The EH tables for personalities that use funclets make implicit use of the funclet nesting relationship to encode unwind destinations, and so are constrained in the set of funclet transitions they can represent. The related LLVM IR instructions accordingly have constraints that ensure encodability of the EH edges in the flow graph.
A catchswitch, catchpad, or cleanuppad is said to be “entered” when it executes. It may subsequently be “exited” by any of the following means:
Note that the ret instruction is not a valid way to exit a funclet pad; it is undefined behavior to execute a ret when a pad has been entered but not exited.
A single unwind edge may exit any number of pads (with the restrictions that the edge from a catchswitch must exit at least itself, and the edge from a cleanupret must exit at least its cleanuppad), and then must enter exactly one pad, which must be distinct from all the exited pads. The parent of the pad that an unwind edge enters must be the most-recently-entered not-yet-exited pad (after exiting from any pads that the unwind edge exits), or “none” if there is no such pad. This ensures that the stack of executing funclets at run-time always corresponds to some path in the funclet pad tree that the parent tokens encode.
All unwind edges which exit any given funclet pad (including cleanupret edges exiting their cleanuppad and catchswitch edges exiting their catchswitch) must share the same unwind destination. Similarly, any funclet pad which may be exited by unwind to caller must not be exited by any exception edges which unwind anywhere other than the caller. This ensures that each funclet as a whole has only one unwind destination, which EH tables for funclet personalities may require. Note that any unwind edge which exits a catchpad also exits its parent catchswitch, so this implies that for any given catchswitch, its unwind destination must also be the unwind destination of any unwind edge that exits any of its constituent catchpads. Because catchswitch has no nounwind variant, and because IR producers are not required to annotate calls which will not unwind as nounwind, it is legal to nest a call or an “unwind to caller” catchswitch within a funclet pad that has an unwind destination other than caller; it is undefined behavior for such a call or catchswitch to unwind.
Finally, the funclet pads’ unwind destinations cannot form a cycle. This ensures that EH lowering can construct “try regions” with a tree-like structure, which funclet-based personalities may require.