R600: Implement getRsqrtEstimate

Only do for f32 since I'm unclear on both what this is expecting
for the refinement steps in terms of accuracy, and what
f64 instruction actually provides.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225827 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/R600/AMDGPUISelLowering.cpp

@@ -2567,6 +2567,24 @@ const char* AMDGPUTargetLowering::getTargetNodeName(unsigned Opcode) const {
   }
 }
 
+SDValue AMDGPUTargetLowering::getRsqrtEstimate(SDValue Operand,
+                                               DAGCombinerInfo &DCI,
+                                               unsigned &RefinementSteps,
+                                               bool &UseOneConstNR) const {
+  SelectionDAG &DAG = DCI.DAG;
+  EVT VT = Operand.getValueType();
+
+  if (VT == MVT::f32) {
+    RefinementSteps = 0;
+    return DAG.getNode(AMDGPUISD::RSQ, SDLoc(Operand), VT, Operand);
+  }
+
+  // TODO: There is also f64 rsq instruction, but the documentation is less
+  // clear on its precision.
+
+  return SDValue();
+}
+
 static void computeKnownBitsForMinMax(const SDValue Op0,
                                       const SDValue Op1,
                                       APInt &KnownZero,