[PATCH] D11172: LegalizeDAG: Fix and improve FCOPYSIGN/FABS legalization

[Matthias Braun]

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- Factor out code to query and modify the sign bit of a floatingpoint
  value as an integer. This also works if none of the targets integer
  types is big enough to hold all bits of the floatingpoint value.

- Legalize FABS(x) as FCOPYSIGN(x, 0.0) if FCOPYSIGN is available,
  otherwise perform bit manipulation on the sign bit. The previous code
  used "x >u 0 ? x : -x" which is incorrect for x being -0.0! It also
  takes 34 instructions on ARM Cortex-M4. With this patch we only
  require 5:
    vldr d0, LCPI0_0
    vmov r2, r3, d0
    lsrs r2, r3, #31
    bfi r1, r2, #31, #1
    bx lr
  (This could be further improved if the compiler would recognize that
   r2, r3 is zero).

- Only lower FCOPYSIGN(x, y) = sign(x) ? -FABS(x) : FABS(x) if FABS is
  available otherwise perform bit manipulation on the sign bit.

- Perform the sign(x) test by masking out the sign bit and comparing
  with 0 rather than shifting the sign bit to the highest position and
  testing for "<s 0". For x86 copysignl (on 80bit values) this gets us:
    testl $32768, %eax
  rather than:
    shlq $48, %rax
    sets %al
    testb %al, %al

This fixes http://llvm.org/PR24091

Repository:
  rL LLVM

http://reviews.llvm.org/D11172

Files:
  lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
  test/CodeGen/Thumb2/float-intrinsics-double.ll
  test/CodeGen/X86/pr13577.ll

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