Split up subtracts into add+negate if they have a reassociable use or operand

that is also a subtract.  This implements PR2047 and Transforms/Reassociate/subtest2.ll 

llvm-svn: 47241

1 files changed, 8 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Scalar/Reassociate.cpp b/llvm/lib/Transforms/Scalar/Reassociate.cpp
index d0537ea8c43..bec91dbe67c 100644
--- a/llvm/lib/Transforms/Scalar/Reassociate.cpp
+++ b/llvm/lib/Transforms/Scalar/Reassociate.cpp
@@ -399,13 +399,16 @@ static bool ShouldBreakUpSubtract(Instruction *Sub) {
     return false;
   
   // Don't bother to break this up unless either the LHS is an associable add or
-  // if this is only used by one.
-  if (isReassociableOp(Sub->getOperand(0), Instruction::Add))
+  // subtract or if this is only used by one.
+  if (isReassociableOp(Sub->getOperand(0), Instruction::Add) ||
+      isReassociableOp(Sub->getOperand(0), Instruction::Sub))
     return true;
-  if (isReassociableOp(Sub->getOperand(1), Instruction::Add))
+  if (isReassociableOp(Sub->getOperand(1), Instruction::Add) ||
+      isReassociableOp(Sub->getOperand(0), Instruction::Sub))
     return true;
-  
-  if (Sub->hasOneUse() && isReassociableOp(Sub->use_back(), Instruction::Add))
+  if (Sub->hasOneUse() && 
+      (isReassociableOp(Sub->use_back(), Instruction::Add) ||
+       isReassociableOp(Sub->use_back(), Instruction::Sub)))
     return true;
     
   return false;