[InstCombine] try to fold 'sub' to 'not'

https://rise4fun.com/Alive/jDd

Patterns with add/sub combos can be improved using
'not' ops. This is better for analysis and may lead
to follow-on transforms because 'xor' and 'add' are 
commutative/associative. It can also help codegen.  

llvm-svn: 338200

1 files changed, 7 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index aa31e0d850d..e79bdf75d40 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -926,7 +926,13 @@ Instruction *InstCombiner::foldAddWithConstant(BinaryOperator &Add) {
   if (Instruction *NV = foldBinOpIntoSelectOrPhi(Add))
     return NV;
 
-  Value *X;
+  Value *X, *Y;
+
+  // add (sub X, Y), -1 --> add (not Y), X
+  if (match(Op0, m_OneUse(m_Sub(m_Value(X), m_Value(Y)))) &&
+      match(Op1, m_AllOnes()))
+    return BinaryOperator::CreateAdd(Builder.CreateNot(Y), X);
+
   // zext(bool) + C -> bool ? C + 1 : C
   if (match(Op0, m_ZExt(m_Value(X))) &&
       X->getType()->getScalarSizeInBits() == 1)