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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190624 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/PowerPC/PPCISelLowering.cpp b/lib/Target/PowerPC/PPCISelLowering.cpp
index d3410749d1a5375b323b11fb8df561ddf5ad87d7..b4ba5274454e4f793aa4726f851fef4e9593ebee 100644 (file)
--- a/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/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;
+      }
     }
 
     }

diff --git a/test/CodeGen/PowerPC/recipest.ll b/test/CodeGen/PowerPC/recipest.ll
index 38d76820bb7b0a3b586fe8a5bf5ba9c79e8df1de..891e801dd3b5814bd87c7f2a203edcb454028ad3 100644 (file)
--- a/test/CodeGen/PowerPC/recipest.ll
+++ b/test/CodeGen/PowerPC/recipest.ll
@@ -169,6 +169,7 @@ entry:
   ret double %r
 
 ; CHECK: @foo3
+; CHECK: fcmpu
 ; CHECK-DAG: frsqrte
 ; CHECK-DAG: fnmsub
 ; CHECK: fmul
@@ -195,6 +196,7 @@ entry:
   ret float %r
 
 ; CHECK: @goo3
+; CHECK: fcmpu
 ; CHECK-DAG: frsqrtes
 ; CHECK-DAG: fnmsubs
 ; CHECK: fmuls
@@ -217,7 +219,8 @@ entry:
 
 ; CHECK: @hoo3
 ; CHECK: vrsqrtefp
-; CHECK: vrefp
+; CHECK-DAG: vrefp
+; CHECK-DAG: vcmpeqfp
 
 ; CHECK-SAFE: @hoo3
 ; CHECK-SAFE-NOT: vrsqrtefp