https://llvm.org/bugs/show_bug.cgi?id=27854
Bug ID: 27854
Summary: [x86, SSE] insertps chosen when unpcklps would be
better
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Forking this off of bug 27826:
If we have a sequence of insertelements to fill in a vector:
define <4 x float> @goo(float %f0, float %f1, float %f2, float %f3) {
%ins0 = insertelement <4 x float> undef, float %f0, i32 0
%ins1 = insertelement <4 x float> %ins0, float %f1, i32 1
%ins2 = insertelement <4 x float> %ins1, float %f2, i32 2
%ins3 = insertelement <4 x float> %ins2, float %f3, i32 3
ret <4 x float> %ins3
}
We do the optimal thing with SSE2:
$ ./llc -o - inselts.ll
...
unpcklps %xmm3, %xmm1 ## xmm1 = xmm1[0],xmm3[0],xmm1[1],xmm3[1]
unpcklps %xmm2, %xmm0 ## xmm0 = xmm0[0],xmm2[0],xmm0[1],xmm2[1]
unpcklps %xmm1, %xmm0 ## xmm0 = xmm0[0],xmm1[0],xmm0[1],xmm1[1]
retq
The first two instructions are independent, so they can execute in parallel
given enough hardware.
But given the opportunity to use ever more shuffle instructions with each ISA
extension:
$ ./llc -o - inselts.ll -mattr=sse4.1
...
insertps $16, %xmm1, %xmm0 ## xmm0 = xmm0[0],xmm1[0],xmm0[2,3]
insertps $32, %xmm2, %xmm0 ## xmm0 = xmm0[0,1],xmm2[0],xmm0[3]
insertps $48, %xmm3, %xmm0 ## xmm0 = xmm0[0,1,2],xmm3[0]
retq
We now have a sequence of 3 dependent instructions, and each of those
instructions is larger in size too.
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