[InstCombine] fold fsub+fsub with common operand

This is a sibling to the simplify from:
rL339171

llvm-svn: 339266

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 052cb4ab675..f3fe09f1cb0 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1907,6 +1907,14 @@ Instruction *InstCombiner::visitFSub(BinaryOperator &I) {
     return replaceInstUsesWith(I, V);
 
   if (I.hasAllowReassoc() && I.hasNoSignedZeros()) {
+    // (Y - X) - Y --> -X
+    if (match(Op0, m_FSub(m_Specific(Op1), m_Value(X))))
+      return BinaryOperator::CreateFNegFMF(X, &I);
+
+    // TODO: This performs reassociative folds for FP ops. Some fraction of the
+    // functionality has been subsumed by simple pattern matching here and in
+    // InstSimplify. We should let a dedicated reassociation pass handle more
+    // complex pattern matching and remove this from InstCombine.
     if (Value *V = FAddCombine(Builder).simplify(&I))
       return replaceInstUsesWith(I, V);
   }