InstSimplify: Simplify (sub 0, X) -> X if it's NUW

This is a generalization of the X - (0 - Y) -> X transform.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222611 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/InstructionSimplify.cpp

@@ -683,17 +683,9 @@ static Value *SimplifySubInst(Value *Op0, Value *Op1, bool isNSW, bool isNUW,
   if (Op0 == Op1)
     return Constant::getNullValue(Op0->getType());
 
-  // X - (0 - Y) -> X if the second sub is NUW.
-  // If Y != 0, 0 - Y is a poison value.
-  // If Y == 0, 0 - Y simplifies to 0.
-  if (BinaryOperator::isNeg(Op1)) {
-    if (const auto *BO = dyn_cast<BinaryOperator>(Op1)) {
-      assert(BO->getOpcode() == Instruction::Sub &&
-             "Expected a subtraction operator!");
-      if (BO->hasNoUnsignedWrap())
-        return Op0;
-    }
-  }
+  // 0 - X -> 0 if the sub is NUW.
+  if (isNUW && match(Op0, m_Zero()))
+    return Op0;
 
   // (X + Y) - Z -> X + (Y - Z) or Y + (X - Z) if everything simplifies.
   // For example, (X + Y) - Y -> X; (Y + X) - Y -> X

test/Transforms/InstSimplify/AndOrXor.ll

@@ -148,3 +148,10 @@ define i1 @or_of_icmps5(i32 %b) {
   ret i1 %cmp
 ; CHECK: ret i1 true
 }
+
+define i32 @neg_nuw(i32 %x) {
+; CHECK-LABEL: @neg_nuw(
+  %neg = sub nuw i32 0, %x
+  ret i32 %neg
+; CHECK: ret i32 0
+}