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 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47241 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 21 insertions, 5 deletions

diff --git a/lib/Transforms/Scalar/Reassociate.cpp b/lib/Transforms/Scalar/Reassociate.cpp
index d0537ea8c4..bec91dbe67 100644
--- a/lib/Transforms/Scalar/Reassociate.cpp
+++ b/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;
diff --git a/test/Transforms/Reassociate/subtest2.ll b/test/Transforms/Reassociate/subtest2.ll
new file mode 100644
index 0000000000..c81e9b38de
--- /dev/null
+++ b/test/Transforms/Reassociate/subtest2.ll
@@ -0,0 +1,13 @@
+; With sub reassociation, constant folding can eliminate the uses of %a.
+;
+; RUN: llvm-as < %s | opt -reassociate -instcombine | llvm-dis | grep %a | count 1
+; PR2047
+
+define i32 @test(i32 %a, i32 %b, i32 %c) nounwind  {
+entry:
+	%tmp3 = sub i32 %a, %b		; <i32> [#uses=1]
+	%tmp5 = sub i32 %tmp3, %c		; <i32> [#uses=1]
+	%tmp7 = sub i32 %tmp5, %a		; <i32> [#uses=1]
+	ret i32 %tmp7
+}
+