[llvm-bugs] [Bug 43953] New: [SLP] Missed sum reduction of differences

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43953

            Bug ID: 43953
           Summary: [SLP] Missed sum reduction of differences
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm-dev at redking.me.uk
                CC: a.bataev at hotmail.com, dtemirbulatov at gmail.com,
                    llvm-bugs at lists.llvm.org, spatel+llvm at rotateright.com

Noticed in a from
https://devblogs.microsoft.com/cppblog/game-performance-improvements-in-visual-studio-2019-version-16-2/

While the original example does vectorize:

uint32_t TestVectorsEqual(float* Vec0, float* Vec1, float Tolerance = 1e7f)
{
    float sum = 0.f;
    for (int32_t Component = 0; Component < 4; Component++)
    {
        float Diff = Vec0[Component] - Vec1[Component];
        sum += (Diff >= 0.0f) ? Diff : -Diff;
    }
    return (sum <= Tolerance) ? 1 : 0;
}

This simpler version fails for some reason (probably costs....):

uint32_t TestVectorsEqual_alt(float* __restrict Vec0, float* __restrict Vec1,
float Tolerance = 1e7f)
{
    float sum = 0.f;
    for (int32_t Component = 0; Component < 4; Component++)
    {
        float Diff = Vec0[Component] - Vec1[Component];
        sum += Diff;
    }
    return (sum <= Tolerance) ? 1 : 0;
}

https://gcc.godbolt.org/z/hG7Ntp

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