https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84037
--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
So probably the big slowdown is because the vectorized loop body is so much
larger. Unvectorized:
.L61:
vmulss __solv_cap_MOD_d1(%rip), %xmm4, %xmm0
incl %ecx
vmulss (%rdx), %xmm0, %xmm0
vmovss %xmm0, (%rdx)
addq %rax, %rdx
cmpl %r12d, %ecx
jne .L61
vectorized (see how we hoist the load), with -march=haswell:
vmulss __solv_cap_MOD_d1(%rip), %xmm4, %xmm5
movq 144(%rsp), %rdi
leaq (%rbx,%r13), %rdx
xorl %r10d, %r10d
movq %rdx, %rsi
leaq 0(%r13,%rdi), %rcx
movq %rcx, %rdi
vbroadcastss %xmm5, %ymm5
.p2align 4,,10
.p2align 3
.L58:
vmovss (%rcx,%rax,2), %xmm1
vmovss (%rsi,%rax,2), %xmm0
incl %r10d
vinsertps $0x10, (%rcx,%r8), %xmm1, %xmm3
vinsertps $0x10, (%rsi,%r8), %xmm0, %xmm7
vmovss (%rcx), %xmm1
vmovss (%rsi), %xmm0
vinsertps $0x10, (%rcx,%rax), %xmm1, %xmm1
vinsertps $0x10, (%rsi,%rax), %xmm0, %xmm0
addq %r9, %rcx
addq %r9, %rsi
vmovlhps %xmm7, %xmm0, %xmm0
vmovlhps %xmm3, %xmm1, %xmm1
^^ not sure why we construct in such strange way - ICC simply
does a single vmovss and then 7 vinsertps
vinsertf128 $0x1, %xmm1, %ymm0, %ymm0
vmulps %ymm5, %ymm0, %ymm0
vmovss %xmm0, (%rdx)
vextractps $1, %xmm0, (%rdx,%rax)
vextractps $2, %xmm0, (%rdx,%rax,2)
vextractps $3, %xmm0, (%rdx,%r8)
vextractf128 $0x1, %ymm0, %xmm0
addq %r9, %rdx
vmovss %xmm0, (%rdi)
vextractps $1, %xmm0, (%rdi,%rax)
vextractps $2, %xmm0, (%rdi,%rax,2)
vextractps $3, %xmm0, (%rdi,%r8)
Similar here. But fixing this would only reduce this by a few stmts.
addq %r9, %rdi
cmpl %r10d, %r14d
jne .L58
Anyway, this size (140 bytes, 9 cache lines) probably blows any loop
stream cache limits (IIRC that was around 3 cache lines). Compared
to 26 bytes for the scalar version (2 cache lines).
Any such considerations would be best placed in the targets finish_cost
hook where the target knows all stmts that are going to be emitted and
can in theory also cost against the scalar variant (not easily available).
The SSE variant is smaller so measuring the slowdown with SSE only would
be interesting. Hmm, SSE variant is slower (for all of capacita) but
-fno-tree-vectorize is fastest.
