https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104582
--- 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;
>
> __attribute__((noipa)) void
> foo (unsigned long a, unsigned long b)
> {
> s.a = a;
> s.b = b;
> }
>
> int
> main ()
> {
> int i;
> for (i = 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_ = *a;
unsigned long b_ = *b;
s.a = a_;
s.b = 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
