Exception Handling in LLVM

Introduction Itanium ABI Zero-cost Exception Handling Setjmp/Longjmp Exception Handling Overview LLVM Code Generation Throw Try/Catch Cleanups Throw Filters Restrictions Exception Handling Intrinsics `llvm.eh.exception` `llvm.eh.selector` `llvm.eh.typeid.for` `llvm.eh.sjlj.setjmp` `llvm.eh.sjlj.longjmp` `llvm.eh.sjlj.lsda` `llvm.eh.sjlj.callsite` Asm Table Formats Exception Handling Frame Exception Tables ToDo

Written by Jim Laskey

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/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 3 specification at DWARF 3 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 being unwound, 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.

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 (e.g. C++) supports exception handling, the exception frame contains a reference to an exception table describing how to process the exception. If the language (e.g. C) does not support exception handling, 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 to the code found in the catch portion of a try/catch sequence. When execution resumes at a landing pad, it receives the exception structure and a selector corresponding to the type of exception thrown. The selector is then used to determine which catch should actually process the exception.

LLVM Code Generation

At the time of this writing, only C++ exception handling support is available in LLVM. So the remainder of this document will be somewhat C++-centric.

From the C++ developers 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. First, 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. Second, 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: where to continue when the call succeeds as per normal; and 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.

Two LLVM intrinsic functions are used to convey information about the landing pad to the back end.

1. `llvm.eh.exception` takes no arguments and returns a pointer to the exception structure. This only returns a sensible value if called after an `invoke` has branched to a landing pad. Due to code generation limitations, it must currently be called in the landing pad itself.
2. `llvm.eh.selector` takes a minimum of three arguments. The first argument is the reference to the exception structure. The second argument is a reference to the personality function to be used for this `try`/`catch` sequence. Each of the remaining arguments is either a reference to the type info for a `catch` statement, a filter expression, or the number zero (`0`) representing a cleanup. The exception is tested against the arguments sequentially from first to last. The result of the `llvm.eh.selector` 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 behaviour of the program is undefined. This only returns a sensible value if called after an `invoke` has branched to a landing pad. Due to codegen limitations, it must currently be called in the landing pad itself. 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 info have been gathered in the backend, the catch code will 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. Note: Since the landing pad will not be used if there is no match in the list of type info on the call to `llvm.eh.selector`, then neither the last catch nor catch all need to perform the check against the selector.

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

- `__cxa_begin_catch` takes a 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

To handle destructors and cleanups in `try` code, control may not run directly from a landing pad to the first catch. Control may actually flow from the landing pad to clean up code and then to the first catch. Since the required clean up for each `invoke` in a `try` may be different (e.g. intervening constructor), there may be several landing pads for a given try. If cleanups need to be run, an `i32 0` should be passed as the last `llvm.eh.selector` argument. However, when using DWARF exception handling with C++, a `i8* null` must be passed instead.

Throw Filters

C++ allows the specification of which exception types can 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 landing pad will call `llvm.eh.selector`. The arguments are a reference to the exception structure, a reference to the personality function, the length of the filter expression (the number of type infos plus one), followed by the type infos themselves. `llvm.eh.selector` 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 an `llvm.eh.selector` call can contain any number of type infos, filter expressions and cleanups (though having more than one cleanup is pointless). The LLVM C++ front-end can generate such `llvm.eh.selector` calls due to inlining creating nested exception handling scopes.

Restrictions

The semantics of the invoke instruction require that any exception that unwinds through an invoke call should result in a branch to the invoke's unwind label. However such a branch will only happen if the `llvm.eh.selector` matches. Thus in order to ensure correct operation, the front-end must only generate `llvm.eh.selector` calls that are guaranteed to always match whatever exception unwinds through the invoke. For most languages it is enough to pass zero, indicating the presence of a cleanup, as the last `llvm.eh.selector` argument. However for C++ this is not sufficient, because the C++ personality function will terminate the program if it detects that unwinding the exception only results in matches with cleanups. For C++ a `null i8*` should be passed as the last `llvm.eh.selector` argument instead. This is interpreted as a catch-all by the C++ personality function, and will always match.

Exception Handling Intrinsics

LLVM uses several intrinsic functions (name prefixed with "llvm.eh") to provide exception handling information at various points in generated code.

llvm.eh.exception

      i8* %llvm.eh.exception( )

This intrinsic returns a pointer to the exception structure.

llvm.eh.selector

      i32 %llvm.eh.selector(i8*, i8*, i8*, ...)

This intrinsic is used to compare the exception with the given type infos, filters and cleanups.

`llvm.eh.selector` takes a minimum of three arguments. The first argument is the reference to the exception structure. The second argument is a reference to the personality function to be used for this try catch sequence. Each of the remaining arguments is either a reference to the type info for a catch statement, a filter expression, or the number zero representing a cleanup. The exception is tested against the arguments sequentially from first to last. The result of the `llvm.eh.selector` 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 behaviour 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.

llvm.eh.typeid.for

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

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 `llvm.eh.selector`. The single argument is a reference to a type info.

llvm.eh.sjlj.setjmp

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

The SJLJ exception handling uses this intrinsic to force register saving for the current function and to store 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 two compilers 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.lsda

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

Used 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)

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 the matching order.

Asm Table Formats

There are two tables that are used by the exception handling runtime to determine which actions should take place 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.

Todo - Table details here.

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 routines and functions that have only calls to non-throwing functions will not need an exception table.

Todo - Table details here.

ToDo

1. Testing/Testing/Testing.