[CodeGen] Don't blindly combine (fp_round (fp_round x)) to (fp_round x).
We used to do this DAG combine, but it's not always correct:
If the first fp_round isn't a value preserving truncation, it might
introduce a tie in the second fp_round, that wouldn't occur in the
single-step fp_round we want to fold to.
In other words, double rounding isn't the same as rounding.
Differential Revision: http://reviews.llvm.org/D7571
llvm-svn: 228911
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index c8b723a..06bd6eb 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7890,11 +7890,16 @@
// fold (fp_round (fp_round x)) -> (fp_round x)
if (N0.getOpcode() == ISD::FP_ROUND) {
- // This is a value preserving truncation if both round's are.
- bool IsTrunc = N->getConstantOperandVal(1) == 1 &&
- N0.getNode()->getConstantOperandVal(1) == 1;
- return DAG.getNode(ISD::FP_ROUND, SDLoc(N), VT, N0.getOperand(0),
- DAG.getIntPtrConstant(IsTrunc));
+ const bool NIsTrunc = N->getConstantOperandVal(1) == 1;
+ const bool N0IsTrunc = N0.getNode()->getConstantOperandVal(1) == 1;
+ // If the first fp_round isn't a value preserving truncation, it might
+ // introduce a tie in the second fp_round, that wouldn't occur in the
+ // single-step fp_round we want to fold to.
+ // In other words, double rounding isn't the same as rounding.
+ // Also, this is a value preserving truncation iff both fp_round's are.
+ if (DAG.getTarget().Options.UnsafeFPMath || N0IsTrunc)
+ return DAG.getNode(ISD::FP_ROUND, SDLoc(N), VT, N0.getOperand(0),
+ DAG.getIntPtrConstant(NIsTrunc && N0IsTrunc));
}
// fold (fp_round (copysign X, Y)) -> (copysign (fp_round X), Y)