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From: "dg at cowlark dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/52670] New: -mabi=ms generates bad code
Date: Thu, 22 Mar 2012 15:52:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52670

             Bug #: 52670
           Summary: -mabi=ms generates bad code
    Classification: Unclassified
           Product: gcc
           Version: 4.6.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: dg@cowlark.com


If I take this test case:

  int z(int i) { return i; }

...and compile it with:

  gcc-4.6 -Os -m32 -mabi=sysv -o test.s -c test.c -S -fomit-frame-pointer

...I get this (trimmed):

    movl    4(%esp), %eax
    ret

However, if I compile it with this:

  gcc-4.6 -Os -m32 -mabi=ms -o test.s -c test.c -S -fomit-frame-pointer

...then I get this, which is wrong:

    movl    36(%esp), %eax
    ret

Compiling with a copy of the Mingw compiler or with Visual C produces (the
equivalent of) the 4 result. The -fomit-frame-pointer is there purely to reduce
the size of the output; the bug persists without it. This appears to affect all
stack accesses. It *seems* to only apply to ia32 code. amd64 code appears to be
fine, but my target environment is ia32 only so I haven't looked into amd64
much.

This was discussed on gcc-help here:

  http://gcc.gnu.org/ml/gcc-help/2012-03/msg00281.html

This patch appears to fix the issue:

  http://gcc.gnu.org/ml/gcc-patches/2011-03/txt00118.txt

(although the patch needs a bit of work to apply).