__builtin_alloca always uses __BIGGEST_ALIGNMENT__ for the alignment of
the allocation. __builtin_alloca_with_align allows the programmer to
specify the alignment of the allocation.
This fixes PR30658.
llvm-svn: 285544
GCC documents __builtin_alloca as aligning the storage to at least
__BIGGEST_ALIGNMENT__.
MSVC documents essentially the same for the x64 ABI:
https://msdn.microsoft.com/en-us/library/x9sx5da1.aspx
The 32-bit ABI follows the same rule: it emits a call to _alloca_probe_16
Differential Revision: https://reviews.llvm.org/D24378
llvm-svn: 285316