[llvm-bugs] [Bug 46531] New: [X86][SSE] Failure to convert (x&1) bittest to a shift to drive a blendvps

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=46531

            Bug ID: 46531
           Summary: [X86][SSE] Failure to convert (x&1) bittest to a shift
                    to drive a blendvps
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm-dev at redking.me.uk
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

https://c.godbolt.org/z/cvDB4B

#include <x86intrin.h>

inline unsigned opt(unsigned x, unsigned y) {
    unsigned val = x | y;
    if (y & 1) { val = x ^ y; }
    return val;
}

void opt( unsigned * __restrict dst, const unsigned *x, const unsigned *y )
{
    *dst++ = opt(*x++, *y++);
    *dst++ = opt(*x++, *y++);
    *dst++ = opt(*x++, *y++);
    *dst++ = opt(*x++, *y++);
}

.LCPI0_0:
        .long   1                       # 0x1
opt:
        vmovdqu (%rdx), %xmm1
        vpbroadcastd    .LCPI0_0(%rip), %xmm3 # xmm3 = [1,1,1,1]
        vmovdqu (%rsi), %xmm0
        vpxor   %xmm4, %xmm4, %xmm4
        vpand   %xmm3, %xmm1, %xmm3
        vpor    %xmm0, %xmm1, %xmm2
        vpxor   %xmm0, %xmm1, %xmm0
        vpcmpeqd        %xmm4, %xmm3, %xmm3
        vblendvps       %xmm3, %xmm2, %xmm0, %xmm0
        vmovups %xmm0, (%rdi)
        retq

but we should be able to replace the vpcmpeqd(vpand(y, 1), 0) with vpslli(y,31)
to drive the vblendvps (with inverted selections):

opt:
        vmovdqu (%rdx), %xmm1
        vmovdqu (%rsi), %xmm0
        vpslld  $31, %xmm1, %xmm3
        vpor    %xmm0, %xmm1, %xmm2
        vpxor   %xmm0, %xmm1, %xmm0
        vblendvps       %xmm3, %xmm0, %xmm2, %xmm0
        vmovups %xmm0, (%rdi)
        retq

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