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[libcxxabi] Align unwindHeader on a double-word boundary. 

r276215 made a change to annotate _Unwind_Exception with attribute
"aligned" so that an exception object following field __cxa_exception
is sufficiently aligned. This fix hasn't been incorporated to unwind.h
on Darwin since it is an ABI breaking change.

Instead of annotating struct _Unwind_Exception with the attribute, this
commit annotates field unwindHeader of __cxa_exception. This ensures the
exception object is sufficiently aligned without breaking the ABI.

This recommits r302978 and r302981, which were reverted in r303016
because a libcxx test was failing on an AArch64 bot. I also modified the
libcxxabi test case to check the alignment of the pointer returned by
__cxa_allocate_exception rather than compiling the test with -O1 and
checking whether it segfaults.

rdar://problem/25364625

Differential Revision: https://reviews.llvm.org/D33030

llvm-svn: 303175
This commit is contained in:
Akira Hatanaka 2017-05-16 15:19:08 +00:00
parent b7b5d55c38
commit 0c1016a337
2 changed files with 53 additions and 0 deletions
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20
libcxxabi/src/cxa_exception.hpp
View File

@ -61,7 +61,21 @@ struct _LIBCXXABI_HIDDEN __cxa_exception {
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
@ -96,7 +110,13 @@ struct _LIBCXXABI_HIDDEN __cxa_dependent_exception {
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 {

33
libcxxabi/test/exception_object_alignment.pass.cpp Normal file
View File

@ -0,0 +1,33 @@
//===---------------- exception_object_alignment.pass.cpp -----------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is dual licensed under the MIT and the University of Illinois Open
// Source Licenses. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
// UNSUPPORTED: libcxxabi-no-exceptions
// Check that the pointer __cxa_allocate_exception returns is aligned to the
// default alignment for the target architecture.
#include <cassert>
#include <cstdint>
#include <cxxabi.h>
#include <type_traits>
#include <__cxxabi_config.h>
struct S {
int a[4];
} __attribute__((aligned));
int main() {
#if !defined(_LIBCXXABI_ARM_EHABI)
void *p = __cxxabiv1::__cxa_allocate_exception(16);
auto i = reinterpret_cast<uintptr_t>(p);
auto a = std::alignment_of<S>::value;
assert(i % a == 0);
#endif
return 0;
}