Simplify n-ary adds by reassociation

Summary:
This transformation reassociates a n-ary add so that the add can partially reuse
existing instructions. For example, this pass can simplify

  void foo(int a, int b) {
    bar(a + b);
    bar((a + 2) + b);
  }

to

  void foo(int a, int b) {
    int t = a + b;
    bar(t);
    bar(t + 2);
  }

saving one add instruction.

Fixes PR22357 (https://llvm.org/bugs/show_bug.cgi?id=22357).

Test Plan: nary-add.ll

Reviewers: broune, dberlin, hfinkel, meheff, sanjoy, atrick

Reviewed By: sanjoy, atrick

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8950

llvm-svn: 234855

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/Scalar.cpp b/llvm/lib/Transforms/Scalar/Scalar.cpp
index b2ab40fef7e..42095ae261e 100644
--- a/llvm/lib/Transforms/Scalar/Scalar.cpp
+++ b/llvm/lib/Transforms/Scalar/Scalar.cpp
@@ -59,6 +59,7 @@ void llvm::initializeScalarOpts(PassRegistry &Registry) {
   initializeLowerExpectIntrinsicPass(Registry);
   initializeMemCpyOptPass(Registry);
   initializeMergedLoadStoreMotionPass(Registry);
+  initializeNaryReassociatePass(Registry);
   initializePartiallyInlineLibCallsPass(Registry);
   initializeReassociatePass(Registry);
   initializeRegToMemPass(Registry);