[LLVMdev] Use CreateVectorSplat to create a vector value

[zhi chen]

I used CreateVectorSplat to create a vector for a double variable x, e.g.,
<x, x>. I expected that it would work like _mm_set1_pd for sse or
_mm256_broadcast_sd for avx. But it turned out that llc -O3 would create
the following assembly. I don't understand why it needs to move it to two
xmm registers and then unpack it. Is it because of alignment?

    movq    %rcx, %rbx
    vmovd   %esi, %xmm4
    vpbroadcastq    %xmm4, %xmm5
    vmovdqa %xmm5, 64(%rsp)         # 16-byte Spill
    vpextrq $1, %xmm5, %rax
    cltq
    vmovq   %rax, %xmm4
    vmovq   %xmm5, %rcx
    movslq  %ecx, %rcx
    vmovq   %rcx, %xmm5
    vpunpcklqdq %xmm4, %xmm5, %xmm4 # xmm4 = xmm5[0],xmm4[0]

Thanks,
Best
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