[TargetLowering] fix SETCC SETLT folding with FP types

The bug was introduced with:
https://reviews.llvm.org/rL294863

...and manifests as a selection failure in x86, but that's actually
another bug. This fix prevents wrong codegen with -0.0, but in the
more common case when we have NSZ and NNAN (-ffast-math), we should 
still be able to fold this setcc/compare.

llvm-svn: 294924

1 files changed, 13 insertions, 9 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 3db7c8202d6..3785f402c3e 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -751,25 +751,29 @@ bool TargetLowering::SimplifyDemandedBits(SDValue Op,
     KnownOne &= KnownOne2;
     KnownZero &= KnownZero2;
     break;
-  case ISD::SETCC:
+  case ISD::SETCC: {
+    SDValue Op0 = Op.getOperand(0);
+    SDValue Op1 = Op.getOperand(1);
+    ISD::CondCode CC = cast<CondCodeSDNode>(Op.getOperand(2))->get();
     // If (1) we only need the sign-bit, (2) the setcc operands are the same
     // width as the setcc result, and (3) the result of a setcc conforms to 0 or
     // -1, we may be able to bypass the setcc.
-    if (NewMask.isSignBit() &&
-        Op.getOperand(0).getScalarValueSizeInBits() == BitWidth &&
+    if (NewMask.isSignBit() && Op0.getScalarValueSizeInBits() == BitWidth &&
         getBooleanContents(Op.getValueType()) ==
             BooleanContent::ZeroOrNegativeOneBooleanContent) {
-      ISD::CondCode CC = cast<CondCodeSDNode>(Op.getOperand(2))->get();
-      // If we're testing if X < 0, then this compare isn't needed - just use X!
-      if (CC == ISD::SETLT &&
-          (isNullConstant(Op.getOperand(1)) ||
-           ISD::isBuildVectorAllZeros(Op.getOperand(1).getNode())))
-        return TLO.CombineTo(Op, Op.getOperand(0));
+      // If we're testing X < 0, then this compare isn't needed - just use X!
+      // FIXME: We're limiting to integer types here, but this should also work
+      // if we don't care about FP signed-zero. The use of SETLT with FP means
+      // that we don't care about NaNs.
+      if (CC == ISD::SETLT && Op1.getValueType().isInteger() &&
+          (isNullConstant(Op1) || ISD::isBuildVectorAllZeros(Op1.getNode())))
+        return TLO.CombineTo(Op, Op0);
 
       // TODO: Should we check for other forms of sign-bit comparisons?
       // Examples: X <= -1, X >= 0
     }
     break;
+  }
   case ISD::SHL:
     if (ConstantSDNode *SA = dyn_cast<ConstantSDNode>(Op.getOperand(1))) {
       unsigned ShAmt = SA->getZExtValue();