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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/104582] [11/12 Regression] Unoptimal code for
 __negdi2 (and others) from libgcc2 due to unwanted vectorization
Date: Fri, 18 Feb 2022 08:48:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104582

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #8)
> Just trying a dumb microbenchmark:
> struct S { unsigned long a, b; } s;
>=20
> __attribute__((noipa)) void
> foo (unsigned long a, unsigned long b)
> {
>   s.a =3D a;
>   s.b =3D b;
> }
>=20
> int
> main ()
> {
>   int i;
>   for (i =3D 0; i < 1000000000; i++)
>     foo (42, 43);
>   return 0;
> }
> the GCC 11 vs. GCC 12 code:
> -	movq	%rdi, s(%rip)
> -	movq	%rsi, s+8(%rip)
> +	movq	%rdi, %xmm0
> +	movq	%rsi, %xmm1
> +	punpcklqdq	%xmm1, %xmm0
> +	movaps	%xmm0, s(%rip)
> seems to be exactly the same speed (on i9-7960X) and the GCC 11 code is 7
> bytes smaller.

The GCC 12 code is 30% slower on Zen 2 (the gpr -> xmm move is comparatively
more costly there).  As said we fail to account for that.  But as I said
the cost is not there if it's

struct S { unsigned long a, b; } s;

__attribute__((noipa)) void
foo (unsigned long *a, unsigned long *b)
{
  unsigned long a_ =3D *a;
  unsigned long b_ =3D *b;
  s.a =3D a_;
  s.b =3D b_;
}

which vectorizes to

        movq    (%rdi), %xmm0
        movhps  (%rsi), %xmm0
        movaps  %xmm0, s(%rip)
        ret

which is _smaller_ than the scalar code.  So it's important to be able
to distinguish those cases.  The above is also

a__3 1 times scalar_store costs 12 in body
b__5 1 times scalar_store costs 12 in body
a__3 1 times vector_store costs 12 in body
<unknown> 1 times vec_construct costs 8 in prologue=