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
