Re: [PATCH] Implement new hook for max_align_t_align

[John David Anglin <dave dot anglin at bell dot net>]

On 2016-10-11 4:11 PM, Jason Merrill wrote:
> On Tue, Oct 11, 2016 at 2:59 PM, DJ Delorie <dj@redhat.com> wrote:
> > Jason Merrill <jason@redhat.com> writes:
> > > If PA malloc doesn't actually provide 16-byte alignment, this change
> > > seems problematic; it will mean any type that wants 16-byte alignment
> > > will silently get 8-byte alignment instead.
> > Should such cases be calling memalign (or posix_memalign) instead of
> > malloc?
> We're talking about this in the context of C++17 aligned new, which
> uses one of those functions (or C aligned_alloc) under the hood.
> Currently on PA, allocating one of these types with 'new' in C++14
> mode gives a warning because the compiler doesn't think the allocation
> will actually provide the 16-byte alignment that the type wants.  This
> warning seems to be correct.  This patch would silence that warning by
> pretending that malloc will provide 16-byte alignment, which is not
> actually true.
But the check isn't directly about malloc.  It's between the alignment 
for max_align_t
and the alignment that the type wants.  Joseph indicated that 
max_align_t should have an
alignment as large as the POSIX types (e.g., pthread_mutex_t).  So, I 
think setting it to 16
when pthread_mutex_t wants an alignment of 16 is correct.

--
John David Anglin  dave.anglin@bell.net