From: Hannes Roth <hannesr@fb.com>
Date: Tue, 19 Aug 2014 22:26:47 +0000 (-0700)
Subject: (Wangle) Optimize empty continuation
X-Git-Tag: v0.22.0~393
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=a8218a39511e55b8d2c869b5daf3f26519f73cec;p=folly.git

(Wangle) Optimize empty continuation

Summary: It should remove one malloc as per the task.

Test Plan: `fbconfig folly/wangle && fbmake runtests` Also, `--clang`.

Reviewed By: davejwatson@fb.com

Subscribers: fugalh

FB internal diff: D1498331

Tasks: 4913277
---

diff --git a/folly/wangle/detail/State.h b/folly/wangle/detail/State.h
index 1e98e4e3..b93e9ea6 100644
--- a/folly/wangle/detail/State.h
+++ b/folly/wangle/detail/State.h
@@ -29,6 +29,11 @@
 
 namespace folly { namespace wangle { namespace detail {
 
+// As of GCC 4.8.1, the std::function in libstdc++ optimizes only for pointers
+// to functions, using a helper avoids a call to malloc.
+template<typename T>
+void empty_callback(Try<T>&&) { }
+
 /** The shared state object for Future and Promise. */
 template<typename T>
 class State {
@@ -107,7 +112,7 @@ class State {
   // Called by a destructing Future
   void detachFuture() {
     if (!callback_) {
-      setCallback([](Try<T>&&) {});
+      setCallback(empty_callback<T>);
     }
     activate();
     detachOne();