http://llvm.org/bugs/show_bug.cgi?id=21975
Bug ID: 21975
Summary: recognize dot products [x86/SSE: dpps, dppd]
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
This probably qualifies as a Heroic / Stupid Compiler Trick, but we should
recognize dot product operations and generate the SSE specialized instructions
('vdpps' / 'vdppd') for them.
I just noticed this pattern in test-suite/MultiSource/Benchmarks/Bullet.
'dpps' probably doesn't execute any faster than a sequence using horizontal
vector adds on any recent hardware, but it is smaller code at least.
$ cat dpps.c
float dpps(float *v1, float *v2) {
float mul0 = v1[0] * v2[0];
float mul1 = v1[1] * v2[1];
float mul2 = v1[2] * v2[2];
float mul3 = v1[3] * v2[3];
return mul0 + mul1 + mul2 + mul3;
}
$ ./clang -O2 -ffast-math -march=corei7-avx dpps.c -S -o -
...
vmovss (%rdi), %xmm0
vmovss 4(%rdi), %xmm1
vmulss (%rsi), %xmm0, %xmm0
vmulss 4(%rsi), %xmm1, %xmm1
vmovss 8(%rdi), %xmm2
vmulss 8(%rsi), %xmm2, %xmm2
vmovss 12(%rdi), %xmm3
vmulss 12(%rsi), %xmm3, %xmm3
vaddss %xmm1, %xmm0, %xmm0
vaddss %xmm2, %xmm0, %xmm0
vaddss %xmm3, %xmm0, %xmm0
popq %rbp
retq
This could be (as icc 15 does):
vmovups (%rsi), %xmm0
vmovups (%rdi), %xmm1
vdpps $241, %xmm1, %xmm0, %xmm0
ret
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