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Re: PATCH: PR target/40838: gcc shouldn't assume that the stack is aligned
- From: Jakub Jelinek <jakub at redhat dot com>
- To: Mikulas Patocka <mikulas at artax dot karlin dot mff dot cuni dot cz>
- Cc: gcc-patches at gcc dot gnu dot org, ubizjak at gmail dot com
- Date: Fri, 7 Aug 2009 09:13:05 +0200
- Subject: Re: PATCH: PR target/40838: gcc shouldn't assume that the stack is aligned
- References: <Pine.LNX.4.64.0908070254030.30134@artax.karlin.mff.cuni.cz>
- Reply-to: Jakub Jelinek <jakub at redhat dot com>
On Fri, Aug 07, 2009 at 02:54:46AM +0200, Mikulas Patocka wrote:
> > > In 32bit, the incoming stack may not be 16 byte aligned. This patch
> > > assumes the incoming stack is 4 byte aligned and realigns stack if any
> > > SSE variable is put on stack. Any comments?
> >
> > IMHO this is wrong, I could live with a non-default option for those who
> > don't care about performance and think a SCO document from 1996 has any
> > relevance to Linux these days. In reality a Linux ABI for years assumes
> > 16 byte stack alignment for 32-bit code.
>
> Tell me which Linux distribution did you run with 16-byte stack alignment
> checking (as proposed in bug 40838) and what was the result?
>
> For me, the result was that 75% of binaries in /bin in Debian Lenny do not
> align the stack on 16-byte boundary.
Besides the obstack glibc bug which has been fixed since then you haven't
reported anything particular. It is true that parts of i?86 glibc is
compiled with -mpreferered-stack-boundary=2, but only parts that don't call
callbacks. Async signals AFAIK will align the stack properly.
I simply don't trust your 75% claim, lots of stuff would break if things
weren't aligned properly.
Jakub