Use rsqrt (X86) to speed up reciprocal square root calcs

This is a first step for generating SSE rsqrt instructions for
reciprocal square root calcs when fast-math is allowed.

For now, be conservative and only enable this for AMD btver2
where performance improves significantly - for example, 29%
on llvm/projects/test-suite/SingleSource/Benchmarks/BenchmarkGame/n-body.c
(if we convert the data type to single-precision float).

This patch adds a two constant version of the Newton-Raphson
refinement algorithm to DAGCombiner that can be selected by any target
via a parameter returned by getRsqrtEstimate()..

See PR20900 for more details:
http://llvm.org/bugs/show_bug.cgi?id=20900

Differential Revision: http://reviews.llvm.org/D5658

llvm-svn: 220570

1 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
index ef357f4b7f2..72d3a59192b 100644
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -7466,7 +7466,8 @@ PPCTargetLowering::EmitInstrWithCustomInserter(MachineInstr *MI,
 
 SDValue PPCTargetLowering::getRsqrtEstimate(SDValue Operand,
                                             DAGCombinerInfo &DCI,
-                                            unsigned &RefinementSteps) const {
+                                            unsigned &RefinementSteps,
+                                            bool &UseOneConstNR) const {
   EVT VT = Operand.getValueType();
   if ((VT == MVT::f32 && Subtarget.hasFRSQRTES()) ||
       (VT == MVT::f64 && Subtarget.hasFRSQRTE())  ||
@@ -7479,6 +7480,7 @@ SDValue PPCTargetLowering::getRsqrtEstimate(SDValue Operand,
     RefinementSteps = Subtarget.hasRecipPrec() ? 1 : 3;
     if (VT.getScalarType() == MVT::f64)
       ++RefinementSteps;
+    UseOneConstNR = true;
     return DCI.DAG.getNode(PPCISD::FRSQRTE, SDLoc(Operand), VT, Operand);
   }
   return SDValue();