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

[Alexander Best]

On Thu Jan 13 11, Alexander Best wrote:
> On Thu Jan 13 11, 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.
> > 
> > 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.
> 
> i'm experiencing a similar issue on amd64 with mplayer (svn snapshot) and
> gcc46:
> 
> otaku% ./mplayer ~/filme/wiedhow.mkv
> MPlayer SVN-r32787-4.6.0 (C) 2000-2011 MPlayer Team
> 161 audio & 350 video codecs
> 
> Playing /home/arundel/filme/wiedhow.mkv.
> zsh: illegal hardware instruction (core dumped)  ./mplayer ~/filme/wiedhow.mkv
> otaku% echo $?                      
> 132

just wanted to report that the issue i was having with mplayer vanished. so it
seems this was in fact a bug in mplayer itself.

cheers.
alex

> 
> i'm not sure however, if both matters are related. gdb and 'bt' report:
> 
> #0  0x0000000805327f03 in sscanf (str=0xa <Address 0xa out of bounds>, fmt=0x7fffffbffe60 <Address 0x7fffffbffe60 out of bounds>) at /usr/subversion-src/lib/libc/stdio/sscanf.c:46
> 46	{
> [New Thread 80a407400 (LWP 105253/initial thread)]
> (gdb) bt
> #0  0x0000000805327f03 in sscanf (str=0xa <Address 0xa out of bounds>, fmt=0x7fffffbffe60 <Address 0x7fffffbffe60 out of bounds>) at /usr/subversion-src/lib/libc/stdio/sscanf.c:46
> #1  0x000000000062fa25 in vsscanf ()
> #2  0x0000000805327f84 in sscanf (str=Variable "str" is not available.
> ) at /usr/subversion-src/lib/libc/stdio/sscanf.c:51
> #3  0x000000000062fa25 in vsscanf ()
> 
> cheers.
> alex
> 
> -- 
> a13x

-- 
a13x