What does the FreeBSD/i386 ABI say about stack alignment?

[Ryan Stone]

I've been trying to get an application compiled with gcc 4.5.1 running
on FreeBSD 8.1, but it's been crashing during startup with a SIGBUS.
It turns out that the problem is that gcc is issuing SSE
instructions(in my case, a movdqa) that assume that the stack will be
aligned to a 16-byte boundary.  It seems that Linux/i386 guarantees
this, and I worry that gcc has extended this assumption to all i386
architectures.  I'm assuming that FreeBSD doesn't make any such
promises based on the fact that I'm getting crashes.

There does seem to be a flag (-mstackrealign) that you can set to
force gcc to align the stack to what it wants, but that pessimizes the
generated code a bit.  Some googling would seem to indicate that
-mpreferred-stack-boundary won't always handle this problem correctly.

Any ideas?  My inclination, at least for our local source tree here at
$WORK, would be to accommodate gcc and guarantee the stack alignment
that it wants rather than pessimize our application.  It seems we have
an old local patch/hack in our FreeBSD 6.1 tree(apparently based on
this: http://www.freebsd.org/cgi/getmsg.cgi?fetch=438552+0+/usr/local/www/db/text/2000/freebsd-current/20000507.freebsd-current).
 I believe that this patch is the reason why we haven't seen the
problem when running on 6.1, but the patch doesn't seem to work
anymore on 8.1.