https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39821
--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
0x398f310 _2 * _4 1 times scalar_stmt costs 12 in body
...
0x392b3f0 _1 w* _3 2 times vec_promote_demote costs 8 in body
...
t4.c:4:12: note: Cost model analysis:
Vector inside of loop cost: 40
Vector prologue cost: 4
Vector epilogue cost: 108
Scalar iteration cost: 40
Scalar outside cost: 32
Vector outside cost: 112
prologue iterations: 0
epilogue iterations: 2
Calculated minimum iters for profitability: 3
so clearly the widening multiplication is not costed correctly. With SSE 4.2
we can do better:
.L4:
movdqu (%rcx,%rax), %xmm0
movdqu (%rsi,%rax), %xmm1
addq $16, %rax
movdqa %xmm0, %xmm3
movdqa %xmm1, %xmm4
punpckldq %xmm0, %xmm3
punpckldq %xmm1, %xmm4
punpckhdq %xmm0, %xmm0
pmuldq %xmm4, %xmm3
punpckhdq %xmm1, %xmm1
pmuldq %xmm1, %xmm0
paddq %xmm3, %xmm2
paddq %xmm0, %xmm2
cmpq %rdi, %rax
jne .L4
but even there the costing is imprecise. The vectorizer is unhelpful in
categorizing the widen mult as vec_promote_demote which then fails to
run into
case MULT_EXPR:
case WIDEN_MULT_EXPR:
case MULT_HIGHPART_EXPR:
stmt_cost = ix86_multiplication_cost (ix86_cost, mode);
break;
fixing that yields
0x392b3f0 _1 w* _3 2 times vector_stmt costs 136 in body
for both SSE2 and SSE4.2 and AVX2 so that's over-estimating cost then via
/* V*DImode is emulated with 5-8 insns. */
else if (mode == V2DImode || mode == V4DImode)
{
if (TARGET_XOP && mode == V2DImode)
return ix86_vec_cost (mode, cost->mulss * 2 + cost->sse_op * 3);
else
return ix86_vec_cost (mode, cost->mulss * 3 + cost->sse_op * 5);
}
with cost->mulss == 16. I suppose it is somehow failing to realize it's
doing a widening multiply.
