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Re: [PATCH] Define libc_max_align_t
- From: Rich Felker <dalias at libc dot org>
- To: pinskia at gmail dot com
- Cc: Andreas Schwab <schwab at linux-m68k dot org>, Florian Weimer <fweimer at redhat dot com>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Wed, 1 Apr 2015 22:32:45 -0400
- Subject: Re: [PATCH] Define libc_max_align_t
- Authentication-results: sourceware.org; auth=none
- References: <551C1A60 dot 4080507 at redhat dot com> <87h9szqzi9 dot fsf at igel dot home> <20150402004025 dot GQ6817 at brightrain dot aerifal dot cx> <666580F4-D816-4260-97EF-C75F785CFB20 at gmail dot com>
On Thu, Apr 02, 2015 at 09:36:56AM +0800, pinskia@gmail.com wrote:
> > On Apr 2, 2015, at 8:40 AM, Rich Felker <dalias@libc.org> wrote:
> >
> >> On Thu, Apr 02, 2015 at 12:01:34AM +0200, Andreas Schwab wrote:
> >> Florian Weimer <fweimer@redhat.com> writes:
> >>
> >>> +/* This mirrors the C11 max_align_t type provided by GCC, but it is
> >>> + also available in C99 mode. */
> >>> +typedef struct {
> >>> + long long ll __attribute__ ((__aligned__ (__alignof__ (long long))));
> >>> + long double ld __attribute__ ((__aligned__ (__alignof__ (long double))));
> >>> +} libc_max_align_t;
> >>
> >> Why aren't the attributes no-ops?
> >
> > Because gcc is insane.
>
> No. Just some abi's are insane.
I agree it was a bad ABI choice not to require alignment equal to size
for all types, but at least it's consistent. On the other hand gcc is
inconsistent here. The fact that Andreas had to ask why the above
attributes are not no-ops should be sufficient to establish that the
gcc behavior is inconsistent and confusing.
> > On gcc, __alignof__ returns the "preferred alignment for optimization"
> > that gcc would use allocating such objects individually, not the "ABI
> > alignment" that would be used in structs, etc.
>
> Except some abis talk about different alignment for where the field is located.
Someone can write meaningless text like that in the ABI, but it's not
ABI if it's not observable.
Rich