Re: [PATCH] add definition of max_align_t to stddef.h

["Paweł Dziepak" <pdziepak@quarnos.org>]

2014-05-08 19:41 GMT+02:00 Rich Felker <dalias@libc.org>:
> On Thu, May 08, 2014 at 06:41:19PM +0200, Paweł Dziepak wrote:
>> > As I see it, we have a choice whether to use the "8" definition on
>> > i386 or use the natural definition, which would yield "4" on i386.
>> > This is not an ABI issue (it does not affect the ability to use
>> > glibc-built object files/binaries/shared libraries with musl, nor the
>> > C++ name mangling ABI) but an API issue.
>>
>> What about something like this?
>>
>> struct foobar {
>>     char foo;
>>     alignas(max_align_t) char bar;
>> };
>>
>> The binary representation of this structure depends on the definition
>> of max_align_t and binaries compiled with different
>> alignof(max_align_t) won't be compatible.
>
> Indeed. This is not an ABI issue with libc.so, but it's an API
> difference that translates into an ABI difference between third-party
> translation units if they use max_align_t as you've described. Whether
> we care, I'm not sure. At least historically there were other cases
> like this in musl where type sizes differed in ways that didn't affect
> libc ABI but did affect ABI between third-party programs (jmp_buf
> comes to mind) but most if not all of these were changed.
>
> I'm not strongly opposed to imposing the 8-byte alignment requirement
> on malloc (it would be hard to make malloc work on smaller granularity
> anyway, and most archs need 8-byte alignment anyway for long long and
> for double) but I generally dislike the inconsistency of defining
> max_align_t in a semi-absurd way on i386, as well as the issue of
> having to use non-portable definitions and/or arch-specific ones.

I agree. The question remains, though, whether the consequences of
defining max_align_t differently are acceptable.

> BTW in your above example, it's not even clear to me if that use of
> alignas is valid. It makes no sense for an object to have an alignment
> that does not divide its type (imagine if you added [2] to the end of
> bar) and in other places (like the contract of aligned_alloc)
> alignments that do not divide the size are explicitly illegal. I'd
> like to understand this before making a decision.

6.7.5 doesn't mention such requirement. _Alignas, obviously, cannot
reduce the alignment requirement and the specified alignment has to
has to be either a valid fundamental alignment or valid extended
alignment supported by the implementation. Moreover, 6.2.8 requires
that valid alignment is a nonnegative integral power of two. As for
the additional requirement in contract of aligned_alloc 7.22.3.1
states that the requested alignment has to be valid and divide size of
the requested memory block. I don't see how that would disallow using
in alignas alignment larger than the size of the object.

Paweł