[PATCH] Refactor reciprocal and reciprocal square root estimate into target-independent functions (part 2).

[Sanjay Patel]

Hi chandlerc, hfinkel,

This is purely refactoring. No functional changes intended.

The ultimate goal is to allow targets other than PowerPC (certainly X86 and Aarch64) to turn this:
   z = y / sqrt(x)
into:
   z = y * rsqrte(x)

And:
   z = y / x
into:
   z = y * rcpe(x)

using whatever HW magic they can use. See http://llvm.org/bugs/show_bug.cgi?id=20900 .

In part 1 ( http://reviews.llvm.org/D5425 ) of this refactoring, I moved just the wrapper portion of the square root estimate out of the PPC backend and into DAGCombiner. In this patch, I've moved everything that I can out of PPCISelLowering and into DAGCombiner.

It turns out that we might as well grab the reciprocal estimate code too because I think that any hardware that provides a rsqrt estimate is also going to provide a recip estimate. And PPC even uses rcpe to generate sqrt from rsqrte! I added a visitFSQRT() to DAGCombiner to keep that functionality.

There are small hooks in TargetLowering to get the target-specific opcode for each estimate instruction and a function to tell DAGCombiner how many times it needs to run the Newton-Raphson refinement loop.

This will allow any target to generate the estimate code by implementing these methods:
   virtual SDValue getRecipEst(SDValue Op, DAGCombinerInfo &DCI) const;
   virtual SDValue getRSqrtEst(SDValue Op, DAGCombinerInfo &DCI) const;
   virtual unsigned getNRSteps(EVT VT) const;

http://reviews.llvm.org/D5484

Files:
  include/llvm/Target/TargetLowering.h
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  lib/Target/PowerPC/PPCISelLowering.cpp
  lib/Target/PowerPC/PPCISelLowering.h
  test/CodeGen/PowerPC/recipest.ll
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