[DAG] disable nsw/nuw for add/sub/mul when simplifying based on demanded bits (PR30841)

This bug was exposed by using nsw/nuw for more aggressive folds in:
https://reviews.llvm.org/rL284844

The changes mimic the IR demanded bits logic in InstCombiner::SimplifyDemandedUseBits(),
but we can't just flip flag bits in the DAG; we have to create a new node that has the
bits cleared.

This should fix:
https://llvm.org/bugs/show_bug.cgi?id=30841 

llvm-svn: 285656

1 files changed, 18 insertions, 7 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 12282988129..487af807ab9 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -1216,14 +1216,25 @@ bool TargetLowering::SimplifyDemandedBits(SDValue Op,
     APInt LoMask = APInt::getLowBitsSet(BitWidth,
                                         BitWidth - NewMask.countLeadingZeros());
     if (SimplifyDemandedBits(Op.getOperand(0), LoMask, KnownZero2,
-                             KnownOne2, TLO, Depth+1))
-      return true;
-    if (SimplifyDemandedBits(Op.getOperand(1), LoMask, KnownZero2,
-                             KnownOne2, TLO, Depth+1))
-      return true;
-    // See if the operation should be performed at a smaller bit width.
-    if (TLO.ShrinkDemandedOp(Op, BitWidth, NewMask, dl))
+                             KnownOne2, TLO, Depth+1) ||
+        SimplifyDemandedBits(Op.getOperand(1), LoMask, KnownZero2,
+                             KnownOne2, TLO, Depth+1) ||
+        // See if the operation should be performed at a smaller bit width.
+        TLO.ShrinkDemandedOp(Op, BitWidth, NewMask, dl)) {
+      const SDNodeFlags *Flags = Op.getNode()->getFlags();
+      if (Flags->hasNoSignedWrap() || Flags->hasNoUnsignedWrap()) {
+        // Disable the nsw and nuw flags. We can no longer guarantee that we
+        // won't wrap after simplification.
+        SDNodeFlags NewFlags = *Flags;
+        NewFlags.setNoSignedWrap(false);
+        NewFlags.setNoUnsignedWrap(false);
+        SDValue NewOp = TLO.DAG.getNode(Op.getOpcode(), dl, Op.getValueType(),
+                                        Op.getOperand(0), Op.getOperand(1),
+                                        &NewFlags);
+        return TLO.CombineTo(Op, NewOp);
+      }
       return true;
+    }
     LLVM_FALLTHROUGH;
   }
   default: