[Bug libc/206] malloc does not align memory correctly for sse capable systems

["wg at malloc dot de" <sourceware-bugzilla at sources dot redhat dot com>]

------- Additional Comments From wg at malloc dot de  2004-06-05 22:24 -------
Subject: Re:  malloc does not align memory correctly for sse capable systems

Hello,

> The alignment size is defined as MALLOC_ALIGNMENT defined as
> (2*sizeof(INTERNAL_SIZE_T)). INTERNAL_SIZE_T is size_t (4 byte),
> so MALLOC_ALIGNMENT is 8 bytes.

Yes.  I believe setting MALLOC_ALIGNMENT to 16 would work in the
source, but _should not_ be done in the general case, because of the
significant performance drop (many more objects /about half would need
to be rounded up to a multiple of 16 in size).

>  Bug#206 says it's not suitable
> for SSE instruction that needs 16 byte alignment as follows:
>    http://sources.redhat.com/bugzilla/show_bug.cgi?id=206
>    http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15795 
> 
> I don't know that changing memory alignment size is acceptable
> on i386.  IMHO, SSE is special instruction, so I think in this
> case using posix_memalign() is safe.  Please check this bug?

I am well aware that the C standard says that alignment needs to be
"suitable for any type of object".  However, SSE instructions are IMHO
clearly out of the scope of the C standard -- at least it is not a
clear case of non-conformity to the standard.

Best would be to use posix_memalign() in the applications only for the
allocations where the alignment is really required, because that would
give optimal performance and least memory waste.

Second best (if you don't want/can't change the apps _at all_) would
be to have an additional shared library (libmalloc16?) where malloc is
compiled with MALLOC_ALIGNMENT=16, and link that library only into the
SSE-using applications.  A bit tricky because of interdependency with
libpthread, but most probably doable.

Worst and IMHO unacceptable would be to make MALLOC_ALIGNMENT dynamic;
malloc would become much slower.

What do you think?  How do other systems handle this?

Regards,
Wolfram.



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http://sources.redhat.com/bugzilla/show_bug.cgi?id=206

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