[llvm-bugs] [Bug 34110] New: Compiler not generating hadd/hsub instructions for simple pattern

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34110

            Bug ID: 34110
           Summary: Compiler not generating hadd/hsub instructions for
                    simple pattern
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: douglas_yung at playstation.sony.com
                CC: llvm-bugs at lists.llvm.org

After commit r310260, the following code no longer generates horizontal
add/subtract instructions when compiled with optimizations enabled:

__m128 hadd2ps(__m128 a, __m128 b)
{
    return (__m128){ a[0] + a[1], a[2] + a[3], b[0] + b[1], b[2] + b[3] };
}

Prior to r310260, the code above when compiled would generate the following x86
assembly:

clang -S -O2 repro.cpp -mavx -o -:

        .cfi_startproc
# BB#0:                                 # %entry
        vhaddps %xmm1, %xmm0, %xmm0
        retq


But starting with r310260, the compiler no longer generates the horizontal add
instruction:

clang -S -O2 repro.cpp -mavx -o -:

        .cfi_startproc
# BB#0:                                 # %entry
        vpermilps       $232, %xmm0, %xmm2 # xmm2 = xmm0[0,2,2,3]
        vpermilps       $237, %xmm0, %xmm0 # xmm0 = xmm0[1,3,2,3]
        vpermilpd       $1, %xmm1, %xmm3 # xmm3 = xmm1[1,0]
        vaddps  %xmm0, %xmm2, %xmm0
        vmovshdup       %xmm1, %xmm2    # xmm2 = xmm1[1,1,3,3]
        vaddss  %xmm2, %xmm1, %xmm2
        vpermilps       $231, %xmm1, %xmm1 # xmm1 = xmm1[3,1,2,3]
        vaddss  %xmm1, %xmm3, %xmm1
        vinsertps       $32, %xmm2, %xmm0, %xmm0 # xmm0 =
xmm0[0,1],xmm2[0],xmm0[3]
        vinsertps       $48, %xmm1, %xmm0, %xmm0 # xmm0 = xmm0[0,1,2],xmm1[0]
        retq


This suboptimal codegen also applies to the horizontal subtraction case.

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