Re: PATCH: PR target/38736: [4.4 Regression] -mavx can change the ABI via BIGGEST_ALIGNMENT

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Wed, Jan 7, 2009 at 1:57 PM, Ian Lance Taylor <iant@google.com> wrote:
> Jakub Jelinek <jakub@redhat.com> writes:
>
>> That doesn't reflect what the patch does for i386 (where it doesn't
>> set the default alignment to the largest useful alignment, but to the
>> 4.3 and older default for compatibility) and I think it would be a very bad
>> idea to change it between compiler versions, that would make the aligned
>> attribute quite useless.  glibc uses it in its exported headers, so does
>> libstdc++ and so does unwind.h.
>
> The aligned attribute is certainly not useless even if it does change
> between compiler versions.
>
> We could decide to keep it the same between compiler versions, but
> that leaves us in a very awkward position: it makes it hard to write a
> generic memory allocator that works for all versions of the processor.
> This matters even if you don't use any special types, since
> auto-vectorization can introduce them.
>
> You're right, though: libstdc++ does appear to use attribute
> ((aligned)) in shared_ptr (although I'm not sure it uses it in a way
> which changes the ABI).  Of course, if we don't update the alignment,
> then we can't use some cases of shared_ptr with a type which requires
> a large alignment, which is weird and confusing.
>
> The only use I see in glibc is __pthread_unwind_buf_t, where as far as
> I can see it does not affect the ABI (I also don't see why that
> structure requires any particular alignment).
>
> Do you have any suggestions for how we should handle this issue?
>

Here is the updated patch with a new macro, __BIGGEST_ALIGNMENT__.
OK for trunk?

Thanks.

-- 
H.J.

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