Make the PPC fast-math sqrt expansion safe at 0

In fast-math mode sqrt(x) is calculated using the fast expansion of the
reciprocal of the reciprocal sqrt expansion. The reciprocal and reciprocal
sqrt expansions use the associated estimate instructions along with some Newton
iterations. Unfortunately, as a result, sqrt(0) was being calculated as NaN,
which is not correct. Now we explicitly return a result of zero if the input is
zero.

llvm-svn: 190624

1 files changed, 21 insertions, 1 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
index d3410749d1a..b4ba5274454 100644
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -7021,8 +7021,28 @@ SDValue PPCTargetLowering::PerformDAGCombine(SDNode *N,
     if (RV.getNode() != 0) {
       DCI.AddToWorklist(RV.getNode());
       RV = DAGCombineFastRecip(RV, DCI);
-      if (RV.getNode() != 0)
+      if (RV.getNode() != 0) {
+	// Unfortunately, RV is now NaN if the input was exactly 0. Select out
+	// this case and force the answer to 0.
+
+        EVT VT = RV.getValueType();
+
+        SDValue Zero = DAG.getConstantFP(0.0, VT.getScalarType());
+        if (VT.isVector()) {
+          assert(VT.getVectorNumElements() == 4 && "Unknown vector type");
+          Zero = DAG.getNode(ISD::BUILD_VECTOR, dl, VT, Zero, Zero, Zero, Zero);
+        }
+
+        SDValue ZeroCmp =
+          DAG.getSetCC(dl, getSetCCResultType(*DAG.getContext(), VT),
+                       N->getOperand(0), Zero, ISD::SETEQ);
+        DCI.AddToWorklist(ZeroCmp.getNode());
+        DCI.AddToWorklist(RV.getNode());
+
+        RV = DAG.getNode(VT.isVector() ? ISD::VSELECT : ISD::SELECT, dl, VT,
+                         ZeroCmp, Zero, RV);
         return RV;
+      }
     }
 
     }