[C PATCH] Don't reject valid code with _Alignas (PR c/61053)

[Marek Polacek]

On Mon, May 05, 2014 at 10:27:03PM +0200, Marek Polacek wrote:
> In this PR the issue is that we reject (valid) code such as
> _Alignas (long long) long long foo;
> with -m32, because we trip this condition:
> 
>    alignas_align = 1U << declspecs->align_log;
>    if (alignas_align < TYPE_ALIGN_UNIT (type))
>      {
>        if (name)
>          error_at (loc, "%<_Alignas%> specifiers cannot reduce "
>                    "alignment of %qE", name);
> 
> and error later on, since alignas_align is 4 (correct, see PR52023 for
> why), but TYPE_ALIGN_UNIT of long long is 8.  I think TYPE_ALIGN_UNIT
> is wrong here as that won't give us minimal alignment required.
> In c_sizeof_or_alignof_type we already have the code to compute such
> minimal alignment so I just moved the code to a separate function
> and used that instead of TYPE_ALIGN_UNIT.
> 
> Note that the test is run only on i?86 and x86_64, because we can't (?)
> easily determine which target requires what alignment.
> 
> Regtested/bootstrapped on x86_64-unknown-linux-gnu and
> powerpc64-unknown-linux-gnu, ok for trunk?

I should add that I checked what clang does on my testcase, and with this
patch both compilers accept and reject the same sort of testcases,
both with -m32 and -m64.

	Marek