[llvm-bugs] [Bug 51068] New: [X86][SSE] Failure to merge scalar float comparison results

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51068

            Bug ID: 51068
           Summary: [X86][SSE] Failure to merge scalar float comparison
                    results
           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, pengfei.wang at intel.com,
                    spatel+llvm at rotateright.com

https://simd.godbolt.org/z/vWb57hqzx

#include <x86intrin.h>

bool cmp2_eq(float x, float y, float z, float w) {
    return (x == y) == (z == w);
}

define zeroext i1 @cmp2_eq(float %0, float %1, float %2, float %3) {
  %5 = fcmp une float %0, %1
  %6 = fcmp oeq float %2, %3
  %7 = xor i1 %5, %6
  ret i1 %7
}

cmp2:
        ucomiss %xmm1, %xmm0
        setp    %al
        setne   %cl
        orb     %al, %cl
        ucomiss %xmm3, %xmm2
        setnp   %dl
        sete    %al
        andb    %dl, %al
        xorb    %cl, %al
        retq

We should be able to use cmpss (or cmpsd for doubles) and merge the results to
reduce fpu->gpr traffic and branch density.

Compare to vectors:

__v4si cmp2_eq(__v4sf x, __v4sf y, __v4sf z, __v4sf w) {
    return (x == y) == (z == w);
}

define <4 x i32> @cmp2_(<4 x float> %0, <4 x float> %1, <4 x float> %2, <4 x
float> %3) {
  %5 = fcmp une <4 x float> %0, %1
  %6 = fcmp oeq <4 x float> %2, %3
  %7 = xor <4 x i1> %6, %5
  %8 = sext <4 x i1> %7 to <4 x i32>
  ret <4 x i32> %8
}

cmp2_:
        cmpneqps        %xmm1, %xmm0
        cmpeqps %xmm3, %xmm2
        xorps   %xmm2, %xmm0
        retq

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