[PATCH] fold: sqrt(x * x * y) -> fabs(x) * sqrt(y)

[Sanjay Patel]

Hi beanz, hfinkel, mcrosier,

If a square root call has an FP multiplication argument that can be reassociated, 
then we can hoist a repeated factor out of the square root call and into a fabs().

In the simplest case, this:
   y = sqrt(x * x);
becomes this:
   y = fabs(x);

This patch relies on an earlier optimization in instcombine or reassociate to put the
multiplication tree into a canonical form, so we don't have to search over
every permutation of the multiplication tree.

http://reviews.llvm.org/D5787

Files:
  include/llvm/Transforms/Utils/SimplifyLibCalls.h
  lib/Transforms/Utils/SimplifyLibCalls.cpp
  test/Transforms/InstCombine/fast-math.ll
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