InstCombine: Annotate sub with nuw when we prove it's safe
We can prove that a 'sub' can be a 'sub nuw' if the left-hand side is
negative and the right-hand side is non-negative.
llvm-svn: 216045
diff --git a/llvm/lib/Transforms/InstCombine/InstCombine.h b/llvm/lib/Transforms/InstCombine/InstCombine.h
index 8cb634c8..9e1cc3c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombine.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombine.h
@@ -251,6 +251,7 @@
bool WillNotOverflowSignedAdd(Value *LHS, Value *RHS);
bool WillNotOverflowUnsignedAdd(Value *LHS, Value *RHS);
bool WillNotOverflowSignedSub(Value *LHS, Value *RHS);
+ bool WillNotOverflowUnsignedSub(Value *LHS, Value *RHS);
Value *EmitGEPOffset(User *GEP);
Instruction *scalarizePHI(ExtractElementInst &EI, PHINode *PN);
Value *EvaluateInDifferentElementOrder(Value *V, ArrayRef<int> Mask);
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 22566ce..e7e6cdd 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -988,6 +988,20 @@
return false;
}
+/// \brief Return true if we can prove that:
+/// (sub LHS, RHS) === (sub nuw LHS, RHS)
+bool InstCombiner::WillNotOverflowUnsignedSub(Value *LHS, Value *RHS) {
+ // If the LHS is negative and the RHS is non-negative, no unsigned wrap.
+ bool LHSKnownNonNegative, LHSKnownNegative;
+ bool RHSKnownNonNegative, RHSKnownNegative;
+ ComputeSignBit(LHS, LHSKnownNonNegative, LHSKnownNegative, DL, 0);
+ ComputeSignBit(RHS, RHSKnownNonNegative, RHSKnownNegative, DL, 0);
+ if (LHSKnownNegative && RHSKnownNonNegative)
+ return true;
+
+ return false;
+}
+
// Checks if any operand is negative and we can convert add to sub.
// This function checks for following negative patterns
// ADD(XOR(OR(Z, NOT(C)), C)), 1) == NEG(AND(Z, C))
@@ -1660,6 +1674,10 @@
Changed = true;
I.setHasNoSignedWrap(true);
}
+ if (!I.hasNoUnsignedWrap() && WillNotOverflowUnsignedSub(Op0, Op1)) {
+ Changed = true;
+ I.setHasNoUnsignedWrap(true);
+ }
return Changed ? &I : nullptr;
}