InstCombine: Annotate sub with nsw when we prove it's safe
We can prove that a 'sub' can be a 'sub nsw' under certain conditions:
- The sign bits of the operands is the same.
- Both operands have more than 1 sign bit.
The subtraction cannot be a signed overflow in either case.
llvm-svn: 216037
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 22cc34d..22566ce 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -956,6 +956,38 @@
return false;
}
+/// \brief Return true if we can prove that:
+/// (sub LHS, RHS) === (sub nsw LHS, RHS)
+/// This basically requires proving that the add in the original type would not
+/// overflow to change the sign bit or have a carry out.
+/// TODO: Handle this for Vectors.
+bool InstCombiner::WillNotOverflowSignedSub(Value *LHS, Value *RHS) {
+ // If LHS and RHS each have at least two sign bits, the subtraction
+ // cannot overflow.
+ if (ComputeNumSignBits(LHS) > 1 && ComputeNumSignBits(RHS) > 1)
+ return true;
+
+ if (IntegerType *IT = dyn_cast<IntegerType>(LHS->getType())) {
+ unsigned BitWidth = IT->getBitWidth();
+ APInt LHSKnownZero(BitWidth, 0);
+ APInt LHSKnownOne(BitWidth, 0);
+ computeKnownBits(LHS, LHSKnownZero, LHSKnownOne);
+
+ APInt RHSKnownZero(BitWidth, 0);
+ APInt RHSKnownOne(BitWidth, 0);
+ computeKnownBits(RHS, RHSKnownZero, RHSKnownOne);
+
+ // Subtraction of two 2's compliment numbers having identical signs will
+ // never overflow.
+ if ((LHSKnownOne[BitWidth - 1] && RHSKnownOne[BitWidth - 1]) ||
+ (LHSKnownZero[BitWidth - 1] && RHSKnownZero[BitWidth - 1]))
+ return true;
+
+ // TODO: implement logic similar to checkRippleForAdd
+ }
+ 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))
@@ -1623,7 +1655,13 @@
return ReplaceInstUsesWith(I, Res);
}
- return nullptr;
+ bool Changed = false;
+ if (!I.hasNoSignedWrap() && WillNotOverflowSignedSub(Op0, Op1)) {
+ Changed = true;
+ I.setHasNoSignedWrap(true);
+ }
+
+ return Changed ? &I : nullptr;
}
Instruction *InstCombiner::visitFSub(BinaryOperator &I) {