https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91333
--- Comment #5 from Marc Glisse <glisse at gcc dot gnu.org> ---
With trunk (master?), compiling with -O3, h gives
movapd %xmm1, %xmm3
addsd %xmm3, %xmm1
movapd %xmm0, %xmm2
addsd %xmm2, %xmm0
addsd %xmm1, %xmm0
which looks good (the asm prevents from doing addsd %xmm1 %xmm1 directly).
However, if I add -mavx, I get
vmovapd %xmm0, %xmm2
vmovapd %xmm1, %xmm4
vmovapd %xmm1, %xmm0
vaddsd %xmm0, %xmm4, %xmm0
vmovapd %xmm2, %xmm3
vaddsd %xmm2, %xmm3, %xmm2
vaddsd %xmm0, %xmm2, %xmm0
That's 2 extra moves compared to the non-avx version, which seems wrong since
AVX gives more freedom to the RA.
Those initial moves look quite similar to the ones I get for f with gcc-9 -O3
-mno-avx, so the optimization looks fragile.
