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

[Alexander Best]

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

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