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Re: __HAVE_64B_ATOMICS and alignment
- From: Andreas Schwab <schwab at linux-m68k dot org>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: Torvald Riegel <triegel at redhat dot com>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Sat, 26 Nov 2016 21:52:43 +0100
- Subject: Re: __HAVE_64B_ATOMICS and alignment
- Authentication-results: sourceware.org; auth=none
- References: <email@example.com>
On Nov 26 2016, Florian Weimer <firstname.lastname@example.org> wrote:
> I'm describing the background for this question. The opaque sem_t
> definition looks like this:
> typedef union
> char __size[__SIZEOF_SEM_T];
> long int __align;
> } sem_t;
> But the __HAVE_64B_ATOMICS definition of the non-opaque version looks like
> struct new_sem
> uint64_t data;
> int private;
> int pad;
> This means that for an LP32 architecture such as i686 which could
> conceivable provide 64-bit atomics, we might try to perform an atomic
> operation on a potentially misaligned uint64_t value.
It has always been true that sem_t must have the same or higher
alignment as struct new_sem.
> Could this be a problem on other architectures? (IA-32 is generally fine
> with atomic operations on unaligned objects.)
That's why AArch64 ILP32 changes sem_t to use long long.
Andreas Schwab, email@example.com
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