Re: [PATCH v3] x86: Disable SSE on unwind-c.c and unwind-dw2.c

[Jakub Jelinek <jakub@redhat.com>]

On Tue, Mar 08, 2022 at 07:37:17AM -0800, H.J. Lu wrote:
> > Though, perhaps it should be
> > #ifndef __x86_64__
> > #define LIBGCC2_UNWIND_ATTRIBUTE __attribute__((target ("no-sse")))
> > #endif
> > or something similar, on x86-64 one at least normally doesn't use lower
> > stack realignment unless avx or later.  Maybe we want to use
> > no-avx for the x86-64 case though.
> 
> I have verified that AVX and AVX512 have no issues on x86-64.  In 32-bit,
> -mstackrealign triggers the problem.

I bet it would be a problem if we started vectorizing something in there
using avx/avx2/avx512*.  But given the sorry, I think we'd find that out
immediately.

> > Disabling sse/sse2 might be a problem especially on mingw where we need to
> > restore SSE registers in the EH return, no?
> 
> No, it isn't needed.

I meant for 64-bit where I think the Windows ABI preserves some XMM regs
(low 128-bits of them).  So my earlier patch to just define
LIBGCC2_UNWIND_ATTRIBUTE unconditionally would be wrong for it.

> > Even better would be to make __builtin_eh_return work even with DRAP,
> > but I admit I haven't understood what exactly is the problem that prevents
> > it from working.
> 
> The EH return is a very special case.  Disable SSE in 32-bit is the simplest
> way to make the EH return to work.

Ok.  So, what do you think about replacing the libgcc/ part of your patch
with that
/* __builtin_eh_return can't handle stack realignment, so disable SSE in
   32-bit libgcc functions that call it.  */
#ifndef __x86_64__
#define LIBGCC2_UNWIND_ATTRIBUTE __attribute__((target ("no-sse")))
#endif
?
I'm bootstrapping/regtesting such a patch right now (because I needed some
quick fix for the gnat1 hangs).

	Jakub