InstCombine: Don't unconditionally preserve 'nuw' when shrinking constants

Consider:
  %add = add nuw i32 %a, -16777216
  %and = and i32 %add, 255

Regardless of whether or not we demand the sign bit of %add, we cannot
replace -16777216 with 2130706432 without also removing 'nuw' from the
instruction.

llvm-svn: 216273

2 files changed, 24 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index c26766a95ac..f0c96bdf768 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -44,12 +44,18 @@ static bool ShrinkDemandedConstant(Instruction *I, unsigned OpNo,
   Demanded &= OpC->getValue();
   I->setOperand(OpNo, ConstantInt::get(OpC->getType(), Demanded));
 
-  // If 'nsw' is set and the constant is negative, removing *any* bits from the
-  // constant could make overflow occur.  Remove 'nsw' from the instruction in
-  // this case.
-  if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(I))
-    if (OBO->hasNoSignedWrap() && OpC->getValue().isNegative())
-      cast<BinaryOperator>(OBO)->setHasNoSignedWrap(false);
+  // If either 'nsw' or 'nuw' is set and the constant is negative,
+  // removing *any* bits from the constant could make overflow occur.
+  // Remove 'nsw' and 'nuw' from the instruction in this case.
+  if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(I)) {
+    assert(OBO->getOpcode() == Instruction::Add);
+    if (OBO->hasNoSignedWrap() || OBO->hasNoUnsignedWrap()) {
+      if (OpC->getValue().isNegative()) {
+        cast<BinaryOperator>(OBO)->setHasNoSignedWrap(false);
+        cast<BinaryOperator>(OBO)->setHasNoUnsignedWrap(false);
+      }
+    }
+  }
 
   return true;
 }
diff --git a/llvm/test/Transforms/InstCombine/cast.ll b/llvm/test/Transforms/InstCombine/cast.ll
index 81b0d7eff52..47f7e73f998 100644
--- a/llvm/test/Transforms/InstCombine/cast.ll
+++ b/llvm/test/Transforms/InstCombine/cast.ll
@@ -1038,3 +1038,15 @@ define i8 @test84(i32 %a) {
 ; CHECK: [[SHR:%.*]] = lshr exact i32 [[ADD]], 23
 ; CHECK: [[CST:%.*]] = trunc i32 [[SHR]] to i8
 }
+
+define i8 @test85(i32 %a) {
+  %add = add nuw i32 %a, -16777216
+  %shr = lshr exact i32 %add, 23
+  %trunc = trunc i32 %shr to i8
+  ret i8 %trunc
+
+; CHECK-LABEL: @test85(
+; CHECK: [[ADD:%.*]] = add i32 %a, 2130706432
+; CHECK: [[SHR:%.*]] = lshr exact i32 [[ADD]], 23
+; CHECK: [[CST:%.*]] = trunc i32 [[SHR]] to i8
+}