https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99638
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
.L4:
vmovups b(%rax), %ymm0
addq $32, %rax
vfmadd213ps aa+988(%rax), %ymm1, %ymm0
vmovups %ymm0, aa-32(%rax)
cmpq $996, %rax
jne .L4
vs.
.L4:
vmulps b(%rax), %ymm2, %ymm0
addq $32, %rax
vaddps aa+988(%rax), %ymm0, %ymm0
vmovups %ymm0, aa-32(%rax)
cmpq $996, %rax
jne .L4
I'm not sure we can explain the difference, can we?
On Zen2 -mfma doesn't make a difference btw. (but Zen3 should have FMA
with one cycle less latency even...)
The 2nd testcase has one more load uop in the loop. Both Zen2 and Zen3
should be able to issue two load uops per cycle. The 2nd testcase is not
vectorized, on Zen2 -mno-fma vs. -mfma is in the noise (-mfma looks slightly
faster).
