[llvm] r370497 - [TargetLowering] SimplifyDemandedBits ADD/SUB/MUL - correctly inherit SDNodeFlags from the original node.

[Simon Pilgrim via llvm-commits]

Author: rksimon
Date: Fri Aug 30 10:58:55 2019
New Revision: 370497

URL: http://llvm.org/viewvc/llvm-project?rev=370497&view=rev
Log:
[TargetLowering] SimplifyDemandedBits ADD/SUB/MUL - correctly inherit SDNodeFlags from the original node.

Just disable NSW/NUW flags. This matches what we're already doing for the other situations for these nodes, it was just missed for the demanded constant case.

Noticed by inspection - confirmed in offline discussion with @spatel. I've checked we have test coverage in the x86 extract-bits.ll and extract-lowbits.ll tests

Modified:
    llvm/trunk/lib/CodeGen/SelectionDAG/TargetLowering.cpp

Modified: llvm/trunk/lib/CodeGen/SelectionDAG/TargetLowering.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/TargetLowering.cpp?rev=370497&r1=370496&r2=370497&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/TargetLowering.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/TargetLowering.cpp Fri Aug 30 10:58:55 2019
@@ -1953,10 +1953,8 @@ bool TargetLowering::SimplifyDemandedBit
     if (C && !C->isAllOnesValue() && !C->isOne() &&
         (C->getAPIntValue() | HighMask).isAllOnesValue()) {
       SDValue Neg1 = TLO.DAG.getAllOnesConstant(dl, VT);
-      // We can't guarantee that the new math op doesn't wrap, so explicitly
-      // clear those flags to prevent folding with a potential existing node
-      // that has those flags set.
-      SDNodeFlags Flags;
+      // Disable the nsw and nuw flags. We can no longer guarantee that we
+      // won't wrap after simplification.
       Flags.setNoSignedWrap(false);
       Flags.setNoUnsignedWrap(false);
       SDValue NewOp = TLO.DAG.getNode(Op.getOpcode(), dl, VT, Op0, Neg1, Flags);