[llvm-bugs] [Bug 31872] New: Complex division is not optimised with -ffast-math

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31872

            Bug ID: 31872
           Summary: Complex division is not optimised with -ffast-math
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: drraph at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Consider:

#include <complex.h>
complex float f(complex float x, complex float y) {
  return x/y;
}

clang trunk with -O3 -march=core-avx2 but with or without -ffast-math gives:

f:                                      # @f
        vmovaps xmm2, xmm1
        vmovshdup       xmm1, xmm0      # xmm1 = xmm0[1,1,3,3]
        vmovshdup       xmm3, xmm2      # xmm3 = xmm2[1,1,3,3]
        jmp     __divsc3                # TAILCALL


However both gcc and ICC attempt to optimise this code when -ffast-math (or
equivalent) is enabled.

ICC appears to give the fastest code which is:

f:
        vcvtps2pd xmm2, xmm1                                    #3.12
        vcvtps2pd xmm4, xmm0                                    #3.12
        vmulpd    xmm8, xmm2, xmm2                              #3.12
        vunpckhpd xmm3, xmm2, xmm2                              #3.12
        vmulpd    xmm6, xmm3, xmm4                              #3.12
        vmovddup  xmm7, xmm2                                    #3.12
        vshufpd   xmm5, xmm4, xmm4, 1                           #3.12
        vshufpd   xmm9, xmm8, xmm8, 1                           #3.12
        vfmaddsub213pd xmm7, xmm5, xmm6                         #3.12
        vaddpd    xmm11, xmm8, xmm9                             #3.12
        vshufpd   xmm10, xmm7, xmm7, 1                          #3.12
        vdivpd    xmm12, xmm10, xmm11                           #3.12
        vcvtpd2ps xmm0, xmm12                                   #3.12
        ret

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