[TargetLowering] Drop nuw/nsw flags in TargetLowering::SimplifyDemandedBits

Author: dtcxzyw

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -2598,14 +2598,23 @@ bool TargetLowering::SimplifyDemandedBits(
   }
   case ISD::TRUNCATE: {
     SDValue Src = Op.getOperand(0);
+    SDNodeFlags Flags = Op->getFlags();
 
     // Simplify the input, using demanded bit information, and compute the known
     // zero/one bits live out.
     unsigned OperandBitWidth = Src.getScalarValueSizeInBits();
     APInt TruncMask = DemandedBits.zext(OperandBitWidth);
     if (SimplifyDemandedBits(Src, TruncMask, DemandedElts, Known, TLO,
-                             Depth + 1))
+                             Depth + 1)) {
+      if (Flags.hasNoSignedWrap() || Flags.hasNoUnsignedWrap()) {
+        // Disable the nsw and nuw flags. We can no longer guarantee that we
+        // won't wrap after simplification.
+        Flags.setNoSignedWrap(false);
+        Flags.setNoUnsignedWrap(false);
+        Op->setFlags(Flags);
+      }
       return true;
+    }
     Known = Known.trunc(BitWidth);
 
     // Attempt to avoid multi-use ops if we don't need anything from them.

llvm/test/CodeGen/X86/trunc-nsw-nuw.ll

@@ -62,11 +62,11 @@ entry:
 define i32 @simplify_demanded_bits_drop_flag(i1 zeroext %x, i1 zeroext %y) nounwind {
 ; CHECK-LABEL: simplify_demanded_bits_drop_flag:
 ; CHECK:       # %bb.0: # %entry
-; CHECK-NEXT:    # kill: def $esi killed $esi def $rsi
 ; CHECK-NEXT:    negl %edi
 ; CHECK-NEXT:    shll $2, %esi
 ; CHECK-NEXT:    xorl %edi, %esi
-; CHECK-NEXT:    imulq $-1634202141, %rsi, %rax # imm = 0x9E980DE3
+; CHECK-NEXT:    movslq %esi, %rax
+; CHECK-NEXT:    imulq $-1634202141, %rax, %rax # imm = 0x9E980DE3
 ; CHECK-NEXT:    movq %rax, %rcx
 ; CHECK-NEXT:    shrq $63, %rcx
 ; CHECK-NEXT:    sarq $44, %rax