[InstCombine] not(sub X, Y) --> add (not X), Y

The tests with constants show a missing optimization.
Analysis for adds is better than subs, so this can also
help with other transforms. And codegen is better with 
adds for targets like x86 (destructive ops, no sub-from).

https://rise4fun.com/Alive/llK

llvm-svn: 338118

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index d1922ccd81d..372bc41f780 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -2545,6 +2545,10 @@ Instruction *InstCombiner::visitXor(BinaryOperator &I) {
       }
     }
 
+    // ~(X - Y) --> ~X + Y
+    if (match(NotVal, m_OneUse(m_Sub(m_Value(X), m_Value(Y)))))
+      return BinaryOperator::CreateAdd(Builder.CreateNot(X), Y);
+
     // ~(~X >>s Y) --> (X >>s Y)
     if (match(NotVal, m_AShr(m_Not(m_Value(X)), m_Value(Y))))
       return BinaryOperator::CreateAShr(X, Y);