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Re: Expose max_align_t unconditionally

On 26.02.2020 14:00, Michał Górny wrote:
> On Wed, 2020-02-26 at 12:37 +0100, Kamil Rytarowski wrote:
>> I propose to expose max_align_t unconditionally to C and C++ namespaces.
>> It was introduced in C11/C++11, but in practice it is used in C++ code
>> that formally builds in the C++03/older mode (llvm libc++ expects it
>> unconditionally).
>> Instead of adding fallback definitions in third party code like in llvm
>> libc++ (there was an attempt for this in libc++, but reverted) it is
>> easier to expose it in our headers as expected unconditionally.
> While we're already touching this, I wonder if we should extend it to
> cover alignment for AVX types.  In particular __m512i requires 512-bit
> alignment while our max_align_t currently has 128-bit alignment
> on amd64.

Actually it is 16byte alignment on amd64 (gcc and clang).

No other mainstream platform (Linux, Windows, MacOSX, FreeBSD) sets
max_align_t to a value covering SSE/cache-line/VM page allocations.

It looks like there are aligned_alloc(3)/posix_memalign(3) for these

There is no prior art and there would be ABI breakage so I recommend to
leave it as it is.

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