标签:-o style assign variant ++ cte void 机制 compiler
返回值优化(Return Value Optimization,简称RVO)是一种编译器优化机制:当函数需要返回一个对象的时候,如果自己创建一个临时对象用于返回,那么这个临时对象会消耗一个构造函数(Constructor)的调用、一个复制构造函数的调用(Copy Constructor)以及一个析构函数(Destructor)的调用的代价。
经过返回值优化,就可以将成本降低到一个构造函数的代价。这样就省去了一次拷贝构造函数的调用和依次析构函数的调用。
例子如下:
class MyString { public: MyString() { _data = NULL; _len = 0; printf("Constructor is called!\n"); } MyString(const char* p) { _len = strlen (p); _init_data(p); cout << "Constructor is called! this->_data: " << (long)_data << endl; } MyString(const MyString& str) { _len = str._len; _init_data(str._data); cout << "Copy Constructor is called! src: " << (long)str._data << " dst: " << (long)_data << endl; } ~MyString() { if (_data) { cout << "DeConstructor is called! this->_data: " << (long)_data << endl; free(_data); } else { std::cout << "DeConstructor is called!" << std::endl; } } MyString& operator=(const MyString& str) { if (this != &str) { _len = str._len; _init_data(str._data); } cout << "Copy Assignment is called! src: " << (long)str._data << " dst" << (long)_data << endl; return *this; } operator const char *() const { return _data; } void display() const { if (_data) { cout << "str is " << _data << "(" << (long)_data << ")" << endl; } else { cout << "nothing" << endl; } } private: char *_data; size_t _len; void _init_data(const char *s) { _data = new char[_len+1]; memcpy(_data, s, _len); _data[_len] = ‘\0‘; } }; MyString foo1() { return MyString("123"); } MyString foo2() { MyString str1("456"); return str1; } int main() { foo1(); cout << "--------------------\n"; foo2(); cout << "--------------------\n"; MyString str1 = foo1(); cout << "--------------------\n"; MyString str2 = foo2(); cout << "--------------------\n"; return 0; }
函数foo1直接返回一个临时对象,而foo2返回一个局部变量。在没有RVO的情况下,不管是调用foo1还是foo2,实际上都是先调用构造函数,然后调用复制构造函数构造作为返回值的临时对象。而对于str1和str2的构造,还会再次调用一次复制构造函数。上述代码,使用的编译命令为:g++ -fno-elide-constructors -o rvo rvo.cpp
-fno-elide-constructors选项可以取消编译器的 copy-elision 优化策略。得到的结果如下:
Constructor is called! this->_data: 8949776
Copy Constructor is called! src: 8949776 dst: 8949808
DeConstructor is called! this->_data: 8949776
DeConstructor is called! this->_data: 8949808
--------------------
Constructor is called! this->_data: 8949808
Copy Constructor is called! src: 8949808 dst: 8949776
DeConstructor is called! this->_data: 8949808
DeConstructor is called! this->_data: 8949776
--------------------
Constructor is called! this->_data: 8949776
Copy Constructor is called! src: 8949776 dst: 8949808
DeConstructor is called! this->_data: 8949776
Copy Constructor is called! src: 8949808 dst: 8949776
DeConstructor is called! this->_data: 8949808
--------------------
Constructor is called! this->_data: 8949808
Copy Constructor is called! src: 8949808 dst: 8949840
DeConstructor is called! this->_data: 8949808
Copy Constructor is called! src: 8949840 dst: 8949808
DeConstructor is called! this->_data: 8949840
--------------------
DeConstructor is called! this->_data: 8949808
DeConstructor is called! this->_data: 8949776
如果编译时去掉了-fno-elide-constructors选项,则编译器开启RVO,结果如下:
Constructor is called! this->_data: 34054160
DeConstructor is called! this->_data: 34054160
--------------------
Constructor is called! this->_data: 34054160
DeConstructor is called! this->_data: 34054160
--------------------
Constructor is called! this->_data: 34054160
--------------------
Constructor is called! this->_data: 34054192
--------------------
DeConstructor is called! this->_data: 34054192
DeConstructor is called! this->_data: 34054160
可见开启了RVO之后,省略了不必要的复制拷贝,开启RVO之后,函数是直接在接收返回值的地方直接构造对象。
实际上,foo1和foo2分别对应了RVO和NRVO(Named Return Value Optimization)。具名返回值优化(NRVO),是对于按值返回“具名对象”(就是有名字的变量)时的优化手段,其实道理是一样的,但由于返回的值是具名变量,情况会复杂很多。所以,能执行优化的条件更苛刻。比如函数中,在不同的返回路径上返回不同名的对象,就不会执行NRVO。
比如下面的代码:
MyString bar1(int n) { if (n > 2) { return MyString("abc"); } else { return MyString("ABC"); } } MyString bar2(int n) { MyString str1("abc"); MyString str2("ABC"); if (n > 2) { return str1; } else { return str2; } } int main(int argc, char **argv) { bar1(1); cout << "--------------------\n"; bar2(1); cout << "--------------------\n"; MyString str1 = bar1(1); cout << "--------------------\n"; MyString str2 = bar2(1); cout << "--------------------\n"; return 0; }
函数bar1返回临时对象,bar2返回具名对象,也就是说,如果执行优化的话,bar1执行RVO,而bar2执行NRVO。
首先是加上-fno-elide-constructors选项后的运行结果:
Constructor is called! this->_data: 11149328
Copy Constructor is called! src: 11149328 dst: 11149360
DeConstructor is called! this->_data: 11149328
DeConstructor is called! this->_data: 11149360
--------------------
Constructor is called! this->_data: 11149360
Constructor is called! this->_data: 11149328
Copy Constructor is called! src: 11149328 dst: 11149392
DeConstructor is called! this->_data: 11149328
DeConstructor is called! this->_data: 11149360
DeConstructor is called! this->_data: 11149392
--------------------
Constructor is called! this->_data: 11149392
Copy Constructor is called! src: 11149392 dst: 11149360
DeConstructor is called! this->_data: 11149392
Copy Constructor is called! src: 11149360 dst: 11149392
DeConstructor is called! this->_data: 11149360
--------------------
Constructor is called! this->_data: 11149360
Constructor is called! this->_data: 11149328
Copy Constructor is called! src: 11149328 dst: 11149424
DeConstructor is called! this->_data: 11149328
DeConstructor is called! this->_data: 11149360
Copy Constructor is called! src: 11149424 dst: 11149360
DeConstructor is called! this->_data: 11149424
--------------------
DeConstructor is called! this->_data: 11149360
DeConstructor is called! this->_data: 11149392
加上-fno-elide-constructors选项后,运行结果如下:
Constructor is called! this->_data: 9449488
DeConstructor is called! this->_data: 9449488
--------------------
Constructor is called! this->_data: 9449488
Constructor is called! this->_data: 9449520
Copy Constructor is called! src: 9449520 dst: 9449552
DeConstructor is called! this->_data: 9449520
DeConstructor is called! this->_data: 9449488
DeConstructor is called! this->_data: 9449552
--------------------
Constructor is called! this->_data: 9449552
--------------------
Constructor is called! this->_data: 9449488
Constructor is called! this->_data: 9449520
Copy Constructor is called! src: 9449520 dst: 9449584
DeConstructor is called! this->_data: 9449520
DeConstructor is called! this->_data: 9449488
--------------------
DeConstructor is called! this->_data: 9449584
DeConstructor is called! this->_data: 9449552
对比上面的结果,可见返回临时对象的bar1函数的调用进行了优化。而bar2函数的调用,不管有没有-fno-elide-constructors选项,单独调用bar2返回结果都是一样的,说明没有执行NRVO。对比”MyString str2 = bar2(1);”语句的执行结果,发现加上-fno-elide-constructors选项选项之后,仅仅少了一次复制构造函数的调用,这是因为虽然bar2没有执行NRVO,但是使用bar2返回的临时对象初始化str2时,编译器依然有copy elision的优化策略。
有关copy elision的解释如下:
In C++ computer programming, copy elision refers to a compiler optimization technique that eliminates unnecessary copying of objects.
The standard also describes a few situations where copying can be eliminated even if this would alter the program‘s behavior, the most common being the return value optimization. Another widely implemented optimization, described in the C++ standard, is when a temporary object of class type is copied to an object of the same type.
(https://en.wikipedia.org/wiki/Copy_elision)
When a nameless temporary, not bound to any references, would be copied or moved (since C++11) into an object of the same type (ignoring top-level cv-qualification), the copy/move (since C++11) is omitted. When that temporary is constructed, it is constructed directly in the storage where it would otherwise be copied or moved (since C++11) to. When the nameless temporary is the argument of a return statement, this variant of copy elision is known as RVO, "return value optimization".
(http://en.cppreference.com/w/cpp/language/copy_elision)
注:以上所有代码的编译环境是:操作系统CentOS Linux release 7.3.1611;GCC版本:gcc version 4.8.5 20150623 (Red Hat 4.8.5-11) (GCC)
参考:
http://blog.csdn.net/gatieme/article/details/22650353
http://www.cnblogs.com/liyiwen/archive/2009/12/02/1615711.html
标签:-o style assign variant ++ cte void 机制 compiler
原文地址:http://www.cnblogs.com/gqtcgq/p/7308701.html