diff options
| -rw-r--r-- | src/cxa_exception.cpp | 40 | ||||
| -rw-r--r-- | src/cxa_exception.hpp | 22 |
2 files changed, 36 insertions, 26 deletions
diff --git a/src/cxa_exception.cpp b/src/cxa_exception.cpp index e6ed093..d5230cd 100644 --- a/src/cxa_exception.cpp +++ b/src/cxa_exception.cpp @@ -137,6 +137,28 @@ static _LIBCXXABI_NORETURN void failed_throw(__cxa_exception* exception_header) std::__terminate(exception_header->terminateHandler); } +// Return the offset of the __cxa_exception header from the start of the +// allocated buffer. If __cxa_exception's alignment is smaller than the maximum +// useful alignment for the target machine, padding has to be inserted before +// the header to ensure the thrown object that follows the header is +// sufficiently aligned. This happens if _Unwind_exception isn't double-word +// aligned (on Darwin, for example). +static size_t get_cxa_exception_offset() { + struct S { + } __attribute__((aligned)); + + // Compute the maximum alignment for the target machine. + constexpr size_t alignment = std::alignment_of<S>::value; + constexpr size_t excp_size = sizeof(__cxa_exception); + constexpr size_t aligned_size = + (excp_size + alignment - 1) / alignment * alignment; + constexpr size_t offset = aligned_size - excp_size; + static_assert((offset == 0 || + std::alignment_of<_Unwind_Exception>::value < alignment), + "offset is non-zero only if _Unwind_Exception isn't aligned"); + return offset; +} + extern "C" { // Allocate a __cxa_exception object, and zero-fill it. @@ -146,10 +168,16 @@ extern "C" { // user's exception object. void *__cxa_allocate_exception(size_t thrown_size) throw() { size_t actual_size = cxa_exception_size_from_exception_thrown_size(thrown_size); - __cxa_exception *exception_header = - static_cast<__cxa_exception *>(__aligned_malloc_with_fallback(actual_size)); - if (NULL == exception_header) + + // Allocate extra space before the __cxa_exception header to ensure the + // start of the thrown object is sufficiently aligned. + size_t header_offset = get_cxa_exception_offset(); + char *raw_buffer = + (char *)__aligned_malloc_with_fallback(header_offset + actual_size); + if (NULL == raw_buffer) std::terminate(); + __cxa_exception *exception_header = + static_cast<__cxa_exception *>((void *)(raw_buffer + header_offset)); std::memset(exception_header, 0, actual_size); return thrown_object_from_cxa_exception(exception_header); } @@ -157,7 +185,11 @@ void *__cxa_allocate_exception(size_t thrown_size) throw() { // Free a __cxa_exception object allocated with __cxa_allocate_exception. void __cxa_free_exception(void *thrown_object) throw() { - __aligned_free_with_fallback(cxa_exception_from_thrown_object(thrown_object)); + // Compute the size of the padding before the header. + size_t header_offset = get_cxa_exception_offset(); + char *raw_buffer = + ((char *)cxa_exception_from_thrown_object(thrown_object)) - header_offset; + __aligned_free_with_fallback((void *)raw_buffer); } diff --git a/src/cxa_exception.hpp b/src/cxa_exception.hpp index b9f74e3..c8b0fb1 100644 --- a/src/cxa_exception.hpp +++ b/src/cxa_exception.hpp @@ -60,22 +60,7 @@ struct _LIBCXXABI_HIDDEN __cxa_exception { // previously adding padded to 64-bit align unwindHeader. size_t referenceCount; #endif - - // This field is annotated with attribute aligned so that the exception - // object following the field is sufficiently aligned and there is no - // gap between the field and the exception object. r276215 made a change to - // annotate _Unwind_Exception in unwind.h with __attribute__((aligned)), but - // we cannot incorporate the fix on Darwin since it is an ABI-breaking - // change, which is why we need the attribute on this field. - // - // For ARM EHABI, we do not align this field since _Unwind_Exception is an - // alias of _Unwind_Control_Block, which is not annotated with - // __attribute__((aligned). -#if defined(_LIBCXXABI_ARM_EHABI) _Unwind_Exception unwindHeader; -#else - _Unwind_Exception unwindHeader __attribute__((aligned)); -#endif }; // http://sourcery.mentor.com/archives/cxx-abi-dev/msg01924.html @@ -109,14 +94,7 @@ struct _LIBCXXABI_HIDDEN __cxa_dependent_exception { #if !defined(__LP64__) && !defined(_LIBCXXABI_ARM_EHABI) void* primaryException; #endif - - // See the comment in __cxa_exception as to why this field has attribute - // aligned. -#if defined(_LIBCXXABI_ARM_EHABI) _Unwind_Exception unwindHeader; -#else - _Unwind_Exception unwindHeader __attribute__((aligned)); -#endif }; struct _LIBCXXABI_HIDDEN __cxa_eh_globals { |