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

[Kostik Belousov]

On Thu, Jan 13, 2011 at 10:57:15PM +0100, Joerg Sonnenberger wrote:
> 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.

I think gcc [*] requires 16-byte alignment. Also, it follows the
policy of not changing the stack alignment through the calls.

What you see is a plain bug in FreeBSD, when a developer (me) tried to
adopt to newer ABI but failed.

* Might be not gcc in our tree, but definitely newer gcc releases.
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