[Bug target/111107] New: i686-w64-mingw32 does not realign stack when __attribute__((aligned)) or __attribute__((vector_size)) are used

["zfigura at codeweavers dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111107

            Bug ID: 111107
           Summary: i686-w64-mingw32 does not realign stack when
                    __attribute__((aligned)) or
                    __attribute__((vector_size)) are used
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Keywords: ABI, wrong-code
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zfigura at codeweavers dot com
  Target Milestone: ---
            Target: i686-w64-mingw32

Minimal example:

typedef int myint[4] __attribute__((aligned(16)));

extern void g(void *);

void f(void)
{
    myint a;
    g(&a);
}

The same thing happens if __attribute__((aligned(16))) is applied to the
variable instead of the typedef.

This seems to also prevent __m128 from being aligned correctly (which uses the
"vector_size" attribute rather than "aligned", but I would assume that
"vector_size" implies "aligned").


-mincoming-stack-boundary=2 works as a workaround; so does -mstackrealign.
Neither should be necessary, though.

I've seen some disagreement [1] [2] as to whether the stack alignment for
i686-w64-mingw32 *should* be 16 or 4, but as far as I can tell it really should
be 4. It's explicitly called out in a code comment [3]; it shows up when -msse2
is used [4], and, well, it reflects the actual ABI of programs that exist in
the wild.

We do regularly come across programs in Wine that don't align the stack to a
16-byte boundary before calling win32 functions, and while -mstackrealign and
similar functions exist, they imply that we either waste time and space
unnecessarily aligning *every* function, or we manually align any function that
might use an aligned type, which is in general something that's treated as the
compiler's responsibility.

[1] https://github.com/mingw-w64/mingw-w64/issues/30#issuecomment-1685487779
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110273#c5
[3]
https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/config/i386/cygming.h;h=d539f8d0699d69b014e9d3378e78d690ea289f14;hb=HEAD#l34
[4] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110273#c6