Fold (-x + -y) -> -(x+y) which promotes better association, fixing

the second half of PR2047


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47244 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 22 insertions, 2 deletions

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index c9ff01b907..1ecefeb82b 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -2090,8 +2090,16 @@ Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
   }
 
   // -A + B  -->  B - A
-  if (Value *V = dyn_castNegVal(LHS))
-    return BinaryOperator::createSub(RHS, V);
+  // -A + -B  -->  -(A + B)
+  if (Value *LHSV = dyn_castNegVal(LHS)) {
+    if (Value *RHSV = dyn_castNegVal(RHS)) {
+      Instruction *NewAdd = BinaryOperator::createAdd(LHSV, RHSV, "sum");
+      InsertNewInstBefore(NewAdd, I);
+      return BinaryOperator::createNeg(NewAdd);
+    }
+    
+    return BinaryOperator::createSub(RHS, LHSV);
+  }
 
   // A + -B  -->  A - B
   if (!isa<Constant>(RHS))
diff --git a/test/Transforms/InstCombine/addnegneg.ll b/test/Transforms/InstCombine/addnegneg.ll
new file mode 100644
index 0000000000..f3b9565a51
--- /dev/null
+++ b/test/Transforms/InstCombine/addnegneg.ll
@@ -0,0 +1,12 @@
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep { sub } | count 1
+; PR2047
+
+define i32 @l(i32 %a, i32 %b, i32 %c, i32 %d) {
+entry:
+	%b.neg = sub i32 0, %b		; <i32> [#uses=1]
+	%c.neg = sub i32 0, %c		; <i32> [#uses=1]
+	%sub4 = add i32 %c.neg, %b.neg		; <i32> [#uses=1]
+	%sub6 = add i32 %sub4, %d		; <i32> [#uses=1]
+	ret i32 %sub6
+}
+