[LLVMbugs] [Bug 20578] New: missed optimization: recognize fnabs (FP negative absolute value) (x86-64)

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20578

            Bug ID: 20578
           Summary: missed optimization: recognize fnabs (FP negative
                    absolute value) (x86-64)
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Ouch - 32 bytes of constant pool data to turn on a single bit.

Using llc built from r214897:

$ cat fnabs.c
#include <math.h>
float foo(float a) {
  return -fabs(a);
}

or in LLVM IR:

$ cat fnabs.ll
define float @foo(float %a) {
  %fabsf = tail call float @fabsf(float %a) #1
  %fsub = fsub float -0.0, %fabsf
  ret float %fsub
}

declare float @fabsf(float)
attributes #1 = { readnone }

$ ./llc -mtriple=x86_64-unknown-unknown fnabs.ll -o -
    .text
    .file    "fnabs.ll"
    .section    .rodata.cst16,"aM", at progbits,16
    .align    16
.LCPI0_0:
    .long    2147483647              # float nan
    .long    2147483647              # float nan
    .long    2147483647              # float nan
    .long    2147483647              # float nan
.LCPI0_1:
    .long    2147483648              # float -0.000000e+00
    .long    2147483648              # float -0.000000e+00
    .long    2147483648              # float -0.000000e+00
    .long    2147483648              # float -0.000000e+00
    .text
    .globl    foo
    .align    16, 0x90
    .type    foo, at function
foo:                                    # @foo
    .cfi_startproc
# BB#0:
    andps    .LCPI0_0(%rip), %xmm0
    xorps    .LCPI0_1(%rip), %xmm0
    retq

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