[LLVMbugs] [Bug 9019] New: __builtin_fabs shouldn't lower to libm call

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=9019

           Summary: __builtin_fabs shouldn't lower to libm call
           Product: new-bugs
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: nlewycky at google.com
                CC: llvmbugs at cs.uiuc.edu


This program:

  float f(float x) {
    return __builtin_fabsf(x);
  }

requires -lm when built by clang -O0 but not when built by gcc or clang -O2.
GCC emits this code at -O2 levels (-O0 has no call either):

  f:
  .LFB0:
          .cfi_startproc
          movss   .LC0(%rip), %xmm1
          andps   %xmm1, %xmm0
          ret
          .cfi_endproc
[...]
  .LC0:
          .long   2147483647
          .long   0
          .long   0
          .long   0

without -fno-errno or -ffast-math or any other such flags. Clang -O2 will emit
this IR:

  define float @f(float %x) nounwind readnone {
  entry:
    %call = tail call float @fabsf(float %x)
    ret float %call
  }

but in llc -O0 we emit the call as written and in llc -O2 we replace it with an
andps like gcc.

Either llc -O0 should learn to lower fabs/fabsf calls to 'and', or clang should
lower __builtin_fabs differently. I think I prefer the latter, but the backend
would need to be improved first. The lowering in LLVM IR would be:

  define float @f(float %x) {
    %cast = bitcast float %x to i32
    %abs.i = and i32 %cast, 2147483647
    %abs = bitcast i32 %abs.i to float
    ret float %abs
  }

which produces this sub-optimal code through llc -O2:

        movd    %xmm0, %eax
        andl    $2147483647, %eax       # imm = 0x7FFFFFFF
        movd    %eax, %xmm0
        ret

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