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

These are reassociated versions of the same pattern and
similar transforms as in rL338200 and rL338118.

The motivation is identical to those commits:
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: 338221

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index e79bdf75d40..aa66b442b07 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1614,6 +1614,14 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
   if (match(Op0, m_Not(m_Value(X))) && match(Op1, m_Not(m_Value(Y))))
     return BinaryOperator::CreateSub(Y, X);
 
+  // (X + -1) - Y --> ~Y + X
+  if (match(Op0, m_OneUse(m_Add(m_Value(X), m_AllOnes()))))
+    return BinaryOperator::CreateAdd(Builder.CreateNot(Op1), X);
+
+  // Y - (X + 1) --> ~X + Y
+  if (match(Op1, m_OneUse(m_Add(m_Value(X), m_One()))))
+    return BinaryOperator::CreateAdd(Builder.CreateNot(X), Op0);
+
   if (Constant *C = dyn_cast<Constant>(Op0)) {
     bool IsNegate = match(C, m_ZeroInt());
     Value *X;