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From: "zfigura at codeweavers dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/111107] i686-w64-mingw32 does not realign stack when
 __attribute__((aligned)) or __attribute__((vector_size)) are used
Date: Thu, 24 Aug 2023 20:29:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111107

--- Comment #6 from Zebediah Figura <zfigura at codeweavers dot com> ---
(In reply to Zebediah Figura from comment #4)
> (In reply to Andrew Pinski from comment #3)
> > https://inbox.sourceware.org/gcc-patches/5969976.Bvae8NF9fS@polaris/
>=20
> Again, I'm not sure what you're trying to communicate here. I'm aware that
> -mstackrealign exists (and its attribute equivalent). We *do* use that in
> Wine.

Ah, I'm sorry, I think I see what you're trying to say=E2=80=94that it was =
an
intentional choice to add -mstackrealign if -msse2 is used, so it's hard to
call this a "bug" per se.

(In reply to Richard Biener from comment #5)
> I'd say that
>=20
> config/i386/cygming.h:#define STACK_REALIGN_DEFAULT TARGET_SSE
>=20
> is a non-working "fix".  The appropriate default would be
> -mincoming-stack-boundary=3D2.  MIN_STACK_BOUNDARY should already be 4, so
> that leaves PREFERRED_STACK_BOUNDARY_DEFAULT is the way to go here.  I al=
so
> see
>=20
> /* It should be MIN_STACK_BOUNDARY.  But we set it to 128 bits for
>    both 32bit and 64bit, to support codes that need 128 bit stack
>    alignment for SSE instructions, but can't realign the stack.  */
> #define PREFERRED_STACK_BOUNDARY_DEFAULT \
>   (TARGET_IAMCU ? MIN_STACK_BOUNDARY : 128)
>=20
> which suggests there might be problems with SSE anyway.
>
> So does the following work?

But I would agree with this, yeah. If we're going to manually align for SSE
then we should also manually align for types that need to be manually align=
ed.
Which means that we should just have -mincoming-stack-boundary=3D2 everywhe=
re.

In theory that patch works, although I'll have to put together a gcc build =
to
be sure.

I do have one question, though... from reading the documentation, I have a =
hard
time understanding the difference, or intended difference, between
-mincoming-stack-boundary and -mpreferred-stack-boundary. Could you by chan=
ce
try to clarify?=