From 9b35c8275c6e25442915a25648a1b0bade40a43e Mon Sep 17 00:00:00 2001
From: Howard Hinnant <hhinnant@apple.com>
Date: Tue, 16 Nov 2010 21:33:17 +0000
Subject: [PATCH] Dave Zarzycki showed how the efficiency of shared_ptr could
 be significantly increased.  The following program is running 49% faster:

#include <iostream>
#include <memory>
#include <chrono>
#include <vector>
#include "chrono_io"

int main()
{
    typedef std::chrono::high_resolution_clock Clock;
    Clock::time_point t0 = Clock::now();
    {
        std::shared_ptr<int> p(new int (1));
        std::vector<std::shared_ptr<int> > v(1000000, p);
        v.insert(v.begin(), p);
        v.insert(v.begin(), p);
        v.insert(v.begin(), p);
        v.insert(v.begin(), p);
    }
    Clock::time_point t1 = Clock::now();
    std::cout << (t1-t0) << '\n';
}

llvm-svn: 119388
---
 libcxx/include/memory |  2 +-
 libcxx/src/memory.cpp | 11 +++++++----
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/libcxx/include/memory b/libcxx/include/memory
index c01615469bf9..4752ba0007d1 100644
--- a/libcxx/include/memory
+++ b/libcxx/include/memory
@@ -2535,7 +2535,7 @@ public:
         : __shared_owners_(__refs) {}
 
     void __add_shared();
-    void __release_shared();
+    bool __release_shared();
     _LIBCPP_INLINE_VISIBILITY
     long use_count() const {return __shared_owners_ + 1;}
 };
diff --git a/libcxx/src/memory.cpp b/libcxx/src/memory.cpp
index 9918531acc06..5bbf48e442cc 100644
--- a/libcxx/src/memory.cpp
+++ b/libcxx/src/memory.cpp
@@ -50,11 +50,15 @@ __shared_count::__add_shared()
     increment(__shared_owners_);
 }
 
-void
+bool
 __shared_count::__release_shared()
 {
     if (decrement(__shared_owners_) == -1)
+    {
         __on_zero_shared();
+        return true;
+    }
+    return false;
 }
 
 __shared_weak_count::~__shared_weak_count()
@@ -65,7 +69,6 @@ void
 __shared_weak_count::__add_shared()
 {
     __shared_count::__add_shared();
-    __add_weak();
 }
 
 void
@@ -77,8 +80,8 @@ __shared_weak_count::__add_weak()
 void
 __shared_weak_count::__release_shared()
 {
-    __shared_count::__release_shared();
-    __release_weak();
+    if (__shared_count::__release_shared())
+        __release_weak();
 }
 
 void