[InstCombine] mark ADD with nuw if no unsigned overflow

[oota-llvm.git] / lib / Transforms / InstCombine / InstCombineAddSub.cpp

diff --git a/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 63c440afcd71eed8a6765c4e41361f1153484286..f6658964ae851f82995180b058bab8aea56d63cf 100644 (file)
--- a/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -965,6 +965,21 @@ bool InstCombiner::WillNotOverflowSignedAdd(Value *LHS, Value *RHS) {
   return false;
 }
 
+/// WillNotOverflowUnsignedAdd - Return true if we can prove that:
+///    (zext (add LHS, RHS))  === (add (zext LHS), (zext RHS))
+bool InstCombiner::WillNotOverflowUnsignedAdd(Value *LHS, Value *RHS) {
+  // There are different heuristics we can use for this. Here is a simple one.
+  // If the sign bit of LHS and that of RHS are both zero, no unsigned wrap.
+  bool LHSKnownNonNegative, LHSKnownNegative;
+  bool RHSKnownNonNegative, RHSKnownNegative;
+  ComputeSignBit(LHS, LHSKnownNonNegative, LHSKnownNegative, DL, 0);
+  ComputeSignBit(RHS, RHSKnownNonNegative, RHSKnownNegative, DL, 0);
+  if (LHSKnownNonNegative && RHSKnownNonNegative)
+    return true;
+
+  return false;
+}
+
 Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
   bool Changed = SimplifyAssociativeOrCommutative(I);
   Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
@@ -1240,10 +1255,17 @@ Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
       return BinaryOperator::CreateOr(A, B);
   }
 
+  // TODO(jingyue): Consider WillNotOverflowSignedAdd and
+  // WillNotOverflowUnsignedAdd to reduce the number of invocations of
+  // computeKnownBits.
   if (!I.hasNoSignedWrap() && WillNotOverflowSignedAdd(LHS, RHS)) {
     Changed = true;
     I.setHasNoSignedWrap(true);
   }
+  if (!I.hasNoUnsignedWrap() && WillNotOverflowUnsignedAdd(LHS, RHS)) {
+    Changed = true;
+    I.setHasNoUnsignedWrap(true);
+  }
 
   return Changed ? &I : nullptr;
 }