Re: [PATCH] Fix alignment of sem_t for AArch64 ILP32

[Chris Metcalf <cmetcalf at ezchip dot com>]

On 05/27/2015 07:00 AM, Andreas Schwab wrote:
> sem_t must have the same alignment as struct new_sem.
>
> Andreas.
>
> 	* sysdeps/aarch64/nptl/bits/semaphore.h (sem_t) [!__LP64__]: Use
> 	long long for __align.
> ---
>   sysdeps/aarch64/nptl/bits/semaphore.h | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/sysdeps/aarch64/nptl/bits/semaphore.h b/sysdeps/aarch64/nptl/bits/semaphore.h
> index 047dd4e..d9f4ab6 100644
> --- a/sysdeps/aarch64/nptl/bits/semaphore.h
> +++ b/sysdeps/aarch64/nptl/bits/semaphore.h
> @@ -31,5 +31,9 @@
>   typedef union
>   {
>     char __size[__SIZEOF_SEM_T];
> +#ifdef __LP64__
>     long int __align;
> +#else
> +  long long int __align;
> +#endif
>   } sem_t;

Why not unconditionally use "long long" as the type?  Or if you really 
prefer
the different types, just use "__INT64_TYPE__ __align" instead? Either way
seems like it makes the code a little more readable, though I'd prefer
the former (and it is more friendly to non-gcc compilers).

More generally, I'm not sure why we don't just use 
__attribute__((aligned(8)))
on a simple struct holding the __size[] array, rather than playing union 
games.
Obviously this still would have to be per-platform due to 
__HAVE_64B_ATOMICS,
but just explicitly specifying the alignment does seem cleaner.  I guess 
it is
arguably more friendly to non-gcc compilers using the public headers to
do it the way it has been done traditionally.

None of these suggestions change C++ type mangling as far as I can see,
so are presumably all safe from an ABI perspective.

--
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com