i386: Make xmm16-xmm31 call used even in ms ABI [PR65782]

On Tue, Feb 04, 2020 at 11:16:06AM +0100, Uros Bizjak wrote:
> I guess that Comment #9 patch form the PR should be trivially correct,
> but althouhg it looks obvious, I don't want to propose the patch since
> I have no means of testing it.

I don't have means of testing it either.
https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention?view=vs-2019
is quite explicit that [xyz]mm16-31 are call clobbered and only xmm6-15 (low
128-bits only) are call preserved.

We are talking e.g. about
/* { dg-options "-O2 -mabi=ms -mavx512vl" } */

typedef double V __attribute__((vector_size (16)));
void foo (void);
V bar (void);
void baz (V);
void
qux (void)
{
  V c;
  {
    register V a __asm ("xmm18");
    V b = bar ();
    asm ("" : "=x" (a) : "0" (b));
    c = a;
  }
  foo ();
  {
    register V d __asm ("xmm18");
    V e;
    d = c;
    asm ("" : "=x" (e) : "0" (d));
    baz (e);
  }
}
where according to the MSDN doc gcc incorrectly holds the c value
in xmm18 register across the foo call; if foo is compiled by some Microsoft
compiler (or LLVM), then it could clobber %xmm18.
If all xmm18 occurrences are changed to say xmm15, then it is valid to hold
the 128-bit value across the foo call (though, surprisingly, LLVM saves it
into stack anyway).

The other parts are I guess mainly about SEH.  Consider e.g.
void
foo (void)
{
  register double x __asm ("xmm14");
  register double y __asm ("xmm18");
  asm ("" : "=x" (x));
  asm ("" : "=v" (y));
  x += y;
  y += x;
  asm ("" : : "x" (x));
  asm ("" : : "v" (y));
}
looking at cross-compiler output, with -O2 -mavx512f this emits
	.file	"abcdeq.c"
	.text
	.align 16
	.globl	foo
	.def	foo;	.scl	2;	.type	32;	.endef
	.seh_proc	foo
foo:
	subq	$40, %rsp
	.seh_stackalloc	40
	vmovaps %xmm14,	(%rsp)
	.seh_savexmm	%xmm14, 0
	vmovaps %xmm18,	16(%rsp)
	.seh_savexmm	%xmm18, 16
	.seh_endprologue
	vaddsd	%xmm18, %xmm14, %xmm14
	vaddsd	%xmm18, %xmm14, %xmm18
	vmovaps	(%rsp), %xmm14
	vmovaps	16(%rsp), %xmm18
	addq	$40, %rsp
	ret
	.seh_endproc
	.ident	"GCC: (GNU) 10.0.1 20200207 (experimental)"
Does whatever assembler mingw64 uses even assemble this (I mean the
.seh_savexmm %xmm16, 16 could be problematic)?
I can find e.g.
https://stackoverflow.com/questions/43152633/invalid-register-for-seh-savexmm-in-cygwin/43210527
which then links to
https://gcc.gnu.org/PR65782

2020-02-08  Uroš Bizjak  <ubizjak@gmail.com>
	    Jakub Jelinek  <jakub@redhat.com>

	PR target/65782
	* config/i386/i386.h (CALL_USED_REGISTERS): Make
	xmm16-xmm31 call-used even in 64-bit ms-abi.

	* gcc.target/i386/pr65782.c: New test.

Co-authored-by: Uroš Bizjak <ubizjak@gmail.com>