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

[Joerg Sonnenberger]

On Thu, Jan 13, 2011 at 12:19:00PM -0500, Ryan Stone wrote:
> 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.

FreeBSD follows the original SYSV ABI. Linux at some point silently
decided to redefine the ABI to fit their mindset. I think you want to
use a combination of -mpreferred-stack-boundary=4 and
-mincoming-stack-boundary=2.

Joerg