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

llvm/lib/Transforms/InstCombine/InstCombine.h

@@ -251,6 +251,7 @@ private:
   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);

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -988,6 +988,20 @@ bool InstCombiner::WillNotOverflowSignedSub(Value *LHS, Value *RHS) {
   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 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
     Changed = true;
     I.setHasNoSignedWrap(true);
   }
+  if (!I.hasNoUnsignedWrap() && WillNotOverflowUnsignedSub(Op0, Op1)) {
+    Changed = true;
+    I.setHasNoUnsignedWrap(true);
+  }
 
   return Changed ? &I : nullptr;
 }

llvm/test/Transforms/InstCombine/sub.ll

@@ -506,6 +506,18 @@ define i4 @test42(i4 %x, i4 %y) {
 ; CHECK-LABEL: @test42(
 ; CHECK-NEXT: [[AND:%.*]] = and i4 %y, 7
 ; CHECK-NEXT: [[AND1:%.*]] = and i4 %x, 7
-; CHECK-NEXT: [[RET:%.*]] = sub nsw i4 %a, %b
+; CHECK-NEXT: [[RET:%.*]] = sub nsw i4 [[AND]], [[AND1]]
+; CHECK: ret i4 [[RET]]
+}
+
+define i4 @test43(i4 %x, i4 %y) {
+  %a = or i4 %x, -8
+  %b = and i4 %y, 7
+  %c = sub i4 %a, %b
+  ret i4 %c
+; CHECK-LABEL: @test43(
+; CHECK-NEXT: [[OR:%.*]] = or i4 %x, -8
+; CHECK-NEXT: [[AND:%.*]] = and i4 %y, 7
+; CHECK-NEXT: [[RET:%.*]] = sub nuw i4 [[OR]], [[AND]]
 ; CHECK: ret i4 [[RET]]
 }