[InstCombine] add xor+not folds

This fold is needed to avoid a regression when we try
to recommit rL300977. 
We can't see the most basic win currently because 
demanded bits changes the patterns:
https://rise4fun.com/Alive/plpp

llvm-svn: 341559

1 files changed, 16 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index 9e6f09dab3c..bbe4b9b8556 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -2702,6 +2702,22 @@ Instruction *InstCombiner::visitXor(BinaryOperator &I) {
       return BinaryOperator::CreateSub(ConstantExpr::getNeg(AddOne(C)), X);
   }
 
+  // Use DeMorgan and reassociation to eliminate a 'not' op.
+  Constant *C1;
+  if (match(Op1, m_Constant(C1))) {
+    Constant *C2;
+    if (match(Op0, m_OneUse(m_Or(m_Not(m_Value(X)), m_Constant(C2))))) {
+      // (~X | C2) ^ C1 --> ((X & ~C2) ^ -1) ^ C1 --> (X & ~C2) ^ ~C1
+      Value *And = Builder.CreateAnd(X, ConstantExpr::getNot(C2));
+      return BinaryOperator::CreateXor(And, ConstantExpr::getNot(C1));
+    }
+    if (match(Op0, m_OneUse(m_And(m_Not(m_Value(X)), m_Constant(C2))))) {
+      // (~X & C2) ^ C1 --> ((X | ~C2) ^ -1) ^ C1 --> (X | ~C2) ^ ~C1
+      Value *Or = Builder.CreateOr(X, ConstantExpr::getNot(C2));
+      return BinaryOperator::CreateXor(Or, ConstantExpr::getNot(C1));
+    }
+  }
+
   // not (cmp A, B) = !cmp A, B
   CmpInst::Predicate Pred;
   if (match(&I, m_Not(m_OneUse(m_Cmp(Pred, m_Value(), m_Value()))))) {