[Bug c/47842] New: gcc forces 16-byte stack alignment on Solaris i386, when SYSV requires word alignment

[Bug c/47842] New: gcc forces 16-byte stack alignment on Solaris i386, when SYSV requires word alignment

[yuri at tsoft dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47842

           Summary: gcc forces 16-byte stack alignment on Solaris i386,
                    when SYSV requires word alignment
           Product: gcc
           Version: 4.5.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: yuri@tsoft.com


Linux made a decision to switch to 16-byte alignment, but not others.

I know for a fact that Solaris is affected, I am not sure, but FreeBSD i386
might also be affected.

Please turn this behavior back to the one prescribed by specification for
Solaris (and FreeBSD if affected):
http://www.sco.com/developers/devspecs/abi386-4.pdf
section 3-10: "The stack is word aligned. Although the architecture does not
require any alignment of the stack, software convention and the operating
system requires that the stack be aligned on a word boundary."

Or please refer to the Sun/Oracle decision to change this behavior.

[Bug target/47842] gcc forces 16-byte stack alignment on Solaris i386, when SYSV requires word alignment

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47842

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2011.02.22 10:58:29
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-02-22 10:58:29 UTC ---
"forces"?  GCC re-aligns the stack if it thinks it is a good idea for
performance.  Or do you mean something else?  Please be more specific.

[Bug target/47842] gcc forces 16-byte stack alignment on Solaris i386, when SYSV requires word alignment

[schwab@linux-m68k.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47842

--- Comment #2 from Andreas Schwab <schwab@linux-m68k.org> 2011-02-22 11:06:41 UTC ---
A 16-byte aligned stack is still word aligned.

[Bug target/47842] gcc forces 16-byte stack alignment on Solaris i386, when SYSV requires word alignment

[yuri at tsoft dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47842

--- Comment #3 from Yuri <yuri at tsoft dot com> 2011-02-22 19:23:45 UTC ---
If gcc would only set 16 byte alignment this wouldn't be that bad since, as you
mentioned, it is still word aligned.

The problem is that gcc assumes that stack is 16 aligned and creates code based
on this. For example it places instruction like this 'movdqa %xmm0, 0x10(%esp)'
which assumes 16 byte alignment. As a result, gcc compiled procedure crashes
when called by ABI compliant code.

gcc can't assume 16 byte alignment.

[Bug target/47842] gcc forces 16-byte stack alignment on Solaris i386, when SYSV requires word alignment

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47842

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
         Resolution|---                         |INVALID

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Yuri from comment #3)
> The problem is that gcc assumes that stack is 16 aligned and creates code
> based on this. For example it places instruction like this 'movdqa %xmm0,
> 0x10(%esp)' which assumes 16 byte alignment. As a result, gcc compiled
> procedure crashes when called by ABI compliant code.

We need a testcase and one has not been forth coming in the last 2 years so
closing as invalid.