[PATCH] Define max_align_t in C++11 and C11 mode

[Arthur O'Dwyer]

On Fri, Oct 18, 2013 at 3:01 PM, Joerg Sonnenberger
<joerg at britannica.bec.de> wrote:
> On Fri, Oct 18, 2013 at 10:22:41AM -0700, David Majnemer wrote:
>>
>> This implementation of max_align_t disagrees with gcc
[...]
>> gcc's alignof(max_align_t): 8
>> clang's alignof(max_align_t): 4
>
> Well, what is it supposed to do in the face of SSE2 registers? For all
> basic types, the IA32 ABI mandates at most 32bit alignment. With the
> compiler extensions it can certainly be at least 128bit.

The Standard seems pretty clear on the subject:
alignof(std::max_align_t) should be the largest "fundamental
alignment" supported by the implementation. It's
implementation-defined whether "overaligned types" (such as Altivec
'vector' types, perhaps) exist, but certainly the invariant must be
maintained that alignof(std::max_align_t) >= alignof(long long).

It's also implied (7.6.2) that alignof(std::max_align_t) is the
largest value that can be reliably passed to alignas() in absolutely
any context. If Clang can't reliably align things to 8-byte
boundaries, then choosing alignof(max_align_t)==4 might actually be
defensible.

On a modern x86 compiler, I would expect alignof(max_align_t) to be at
least 16, since users will reasonably assume that if an address is
aligned to a max_align_t boundary, then it's safe to store any object
type there.

my $.02,
–Arthur