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

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

llvm-svn: 222611

1 files changed, 3 insertions, 11 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index f151a3a33b4..7cbe6eff4ba 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/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