[InstCombine] Use a more targeted fix instead of r235544

Only clear out the NSW/NUW flags if we are optimizing 'add'/'sub' while
taking advantage that the sign bit is not set.  We do this optimization
to further shrink the mask but shrinking the mask isn't NSW/NUW
preserving in this case.

llvm-svn: 235558

2 files changed, 20 insertions, 9 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index 0695ec17e36..3dbb1b190be 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -88,13 +88,6 @@ bool InstCombiner::SimplifyDemandedBits(Use &U, APInt DemandedMask,
                                           KnownOne, Depth, UserI);
   if (!NewVal) return false;
   U = NewVal;
-
-  // Shrinking a constant might cause a nsw/nuw violation to occur in
-  // instructions which are themselves demanded.
-  if (auto *UserOBO = dyn_cast<OverflowingBinaryOperator>(UserI)) {
-    cast<BinaryOperator>(UserOBO)->setHasNoSignedWrap(false);
-    cast<BinaryOperator>(UserOBO)->setHasNoUnsignedWrap(false);
-  }
   return true;
 }
 
@@ -607,8 +600,11 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
         if (SimplifyDemandedBits(I->getOperandUse(0), DemandedFromOps,
                                  LHSKnownZero, LHSKnownOne, Depth + 1) ||
             SimplifyDemandedBits(I->getOperandUse(1), DemandedFromOps,
-                                 LHSKnownZero, LHSKnownOne, Depth + 1))
+                                 LHSKnownZero, LHSKnownOne, Depth + 1)) {
+          cast<BinaryOperator>(I)->setHasNoSignedWrap(false);
+          cast<BinaryOperator>(I)->setHasNoUnsignedWrap(false);
           return I;
+        }
       }
     }
     break;
@@ -624,8 +620,11 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
       if (SimplifyDemandedBits(I->getOperandUse(0), DemandedFromOps,
                                LHSKnownZero, LHSKnownOne, Depth + 1) ||
           SimplifyDemandedBits(I->getOperandUse(1), DemandedFromOps,
-                               LHSKnownZero, LHSKnownOne, Depth + 1))
+                               LHSKnownZero, LHSKnownOne, Depth + 1)) {
+        cast<BinaryOperator>(I)->setHasNoSignedWrap(false);
+        cast<BinaryOperator>(I)->setHasNoUnsignedWrap(false);
         return I;
+      }
     }
 
     // Otherwise just hand the sub off to computeKnownBits to fill in
diff --git a/llvm/test/Transforms/InstCombine/cast.ll b/llvm/test/Transforms/InstCombine/cast.ll
index 7bf4a6047f2..8a4a60e8753 100644
--- a/llvm/test/Transforms/InstCombine/cast.ll
+++ b/llvm/test/Transforms/InstCombine/cast.ll
@@ -1125,3 +1125,15 @@ define i1 @PR23309(i32 %A, i32 %B) {
   %trunc = trunc i32 %sub to i1
   ret i1 %trunc
 }
+
+define i1 @PR23309v2(i32 %A, i32 %B) {
+; CHECK-LABEL: @PR23309v2(
+; CHECK-NEXT: %[[sub:.*]] = add i32 %A, %B
+; CHECK-NEXT: %[[and:.*]] = and i32 %[[sub]], 1
+; CHECK-NEXT: %[[cmp:.*]] = icmp ne i32 %[[and]], 0
+; CHECK-NEXT: ret i1 %[[cmp]]
+  %add = add i32 %A, -4
+  %sub = add nuw i32 %add, %B
+  %trunc = trunc i32 %sub to i1
+  ret i1 %trunc
+}