性能优化篇（2）：小心“STL 低效率用法”所带来的性能开销
性能优化篇（2）：小心“STL 低效率用法”所带来的性能开销
Author:stormQ
Sunday, 17. November 2019 1 03:53PM
-
目录
-
善用 reserve 预分配内存
-
善用 emplace_back 避免不必要的开销
-
善用 std::move 避免不必要的拷贝开销
-
善用 reserve 预分配内存
-
reserve 函数的作用
将容器的容量（即可以容纳的最大元素数量）调整为指定的大小
n，如果n小于容器的当前容量，那么直接忽略此操作。 -
适用场景
- 如果
std::vector中要添加的元素数量已知，那么在添加元素前（通常是在循环中添加元素前）使用reserve函数预分配std::vector需要的内存。这样，可以避免由于std::vector内部发生数据迁移而带来的不必要的元素拷贝开销、创建新内存开销和释放旧内存开销，从而改善程序性能。
- 如果
-
代码示例
源码：
// main.cc
#include
#define N 1980*1024
class TestObject
{
public:
TestObject(int a, int b, int c) : a_(a), b_(b), c_(c) {}
int a_;
int b_;
int c_;
};
std::vector test_data_1;
std::vector test_data_2;
void poor(std::vector &data)
{
for (int i = 0; i < N; i++)
{
data.push_back(TestObject(i, i+1, i+2));
}
}
void better(std::vector &data)
{
data.reserve(N);
for (int i = 0; i < N; i++)
{
data.push_back(TestObject(i, i+1, i+2));
}
}
int main()
{
poor(test_data_1);
better(test_data_2);
return 0;
}
编译：
$ g++ -std=c++11 -g -Og -o main_Og main.cpp
函数耗时统计：
| 启动程序方式 | 第一次执行耗时(us) | 第二次执行耗时(us) | 第三次执行耗时(us) | 第四次执行耗时(us) | 第五次执行耗时(us) |
|---|---|---|---|---|---|
| ./main_Og |
从统计结果中可以看出，示例中better()函数的执行速度比poor()函数至少快 2倍。
- 代码调试
分析poor()函数添加前 9 个元素时，std::vector内部数据迁移情况。具体调试过程如下：
(gdb) l 25
20
21 void poor(std::vector &data)
22 {
23 for (int i = 0; i < N; i++)
24 {
25 data.push_back(TestObject(i, i+1, i+2));
26 }
27 }
28
29 void better(std::vector &data)
(gdb) b 25
Breakpoint 3 at 0x401065: file m.cpp, line 25.
(gdb) c
Continuing.
Breakpoint 3, poor (data=std::vector of length 0, capacity 0) at m.cpp:25
25 data.push_back(TestObject(i, i+1, i+2));
(gdb) c
Continuing.
Breakpoint 3, poor (data=std::vector of length 1, capacity 1 = {...}) at m.cpp:25
25 data.push_back(TestObject(i, i+1, i+2));
(gdb) display data.size()
1: data.size() = 1
(gdb) display &data[0]
2: &data[0] = (TestObject *) 0x614c20
(gdb) c
Continuing.
Breakpoint 3, poor (data=std::vector of length 2, capacity 2 = {...}) at m.cpp:25
25 data.push_back(TestObject(i, i+1, i+2));
1: data.size() = 2
2: &data[0] = (TestObject *) 0x614c40
(gdb)
Continuing.
Breakpoint 3, poor (data=std::vector of length 3, capacity 4 = {...}) at m.cpp:25
25 data.push_back(TestObject(i, i+1, i+2));
1: data.size() = 3
2: &data[0] = (TestObject *) 0x614c60
(gdb)
Continuing.
Breakpoint 3, poor (data=std::vector of length 4, capacity 4 = {...}) at m.cpp:25
25 data.push_back(TestObject(i, i+1, i+2));
1: data.size() = 4
2: &data[0] = (TestObject *) 0x614c60
(gdb)
Continuing.
Breakpoint 3, poor (data=std::vector of length 5, capacity 8 = {...}) at m.cpp:25
25 data.push_back(TestObject(i, i+1, i+2));
1: data.size() = 5
2: &data[0] = (TestObject *) 0x614ca0
(gdb)
Continuing.
Breakpoint 3, poor (data=std::vector of length 6, capacity 8 = {...}) at m.cpp:25
25 data.push_back(TestObject(i, i+1, i+2));
1: data.size() = 6
2: &data[0] = (TestObject *) 0x614ca0
(gdb)
Continuing.
Breakpoint 3, poor (data=std::vector of length 7, capacity 8 = {...}) at m.cpp:25
25 data.push_back(TestObject(i, i+1, i+2));
1: data.size() = 7
2: &data[0] = (TestObject *) 0x614ca0
(gdb)
Continuing.
Breakpoint 3, poor (data=std::vector of length 8, capacity 8 = {...}) at m.cpp:25
25 data.push_back(TestObject(i, i+1, i+2));
1: data.size() = 8
2: &data[0] = (TestObject *) 0x614ca0
(gdb)
Continuing.
Breakpoint 3, poor (data=std::vector of length 9, capacity 16 = {...}) at m.cpp:25
25 data.push_back(TestObject(i, i+1, i+2));
1: data.size() = 9
2: &data[0] = (TestObject *) 0x614d10
可以看出，poor()函数在添加第 2、3、5、9 个元素时，第一个元素的地址——&data[0]发生了变化。即在这添加这些元素时，std::vector内部发生了数据迁移。在添加后面的某些元素时，也会导致std::vector内部数据迁移，这里不再赘述。
分析better()函数添加前 9 个元素时，std::vector内部是否有数据迁移情况。具体调试过程如下：
(gdb) l 34
29 void better(std::vector &data)
30 {
31 data.reserve(N);
32 for (int i = 0; i < N; i++)
33 {
34 data.push_back(TestObject(i, i+1, i+2));
35 }
36 }
37
38 int main()
(gdb) b 34
Breakpoint 4 at 0x400ffe: file m.cpp, line 34.
(gdb) c
Continuing.
Breakpoint 4, better (data=std::vector of length 0, capacity 2027520) at m.cpp:34
34 data.push_back(TestObject(i, i+1, i+2));
(gdb) c
Continuing.
Breakpoint 4, better (data=std::vector of length 1, capacity 2027520 = {...}) at m.cpp:34
34 data.push_back(TestObject(i, i+1, i+2));
(gdb) display data.size()
3: data.size() = 1
(gdb) display &data[0]
4: &data[0] = (TestObject *) 0x7ffff2ba6010
(gdb) c
Continuing.
Breakpoint 4, better (data=std::vector of length 2, capacity 2027520 = {...}) at m.cpp:34
34 data.push_back(TestObject(i, i+1, i+2));
3: data.size() = 2
4: &data[0] = (TestObject *) 0x7ffff2ba6010
(gdb)
Continuing.
Breakpoint 4, better (data=std::vector of length 3, capacity 2027520 = {...}) at m.cpp:34
34 data.push_back(TestObject(i, i+1, i+2));
3: data.size() = 3
4: &data[0] = (TestObject *) 0x7ffff2ba6010
(gdb)
Continuing.
Breakpoint 4, better (data=std::vector of length 4, capacity 2027520 = {...}) at m.cpp:34
34 data.push_back(TestObject(i, i+1, i+2));
3: data.size() = 4
4: &data[0] = (TestObject *) 0x7ffff2ba6010
(gdb)
Continuing.
Breakpoint 4, better (data=std::vector of length 5, capacity 2027520 = {...}) at m.cpp:34
34 data.push_back(TestObject(i, i+1, i+2));
3: data.size() = 5
4: &data[0] = (TestObject *) 0x7ffff2ba6010
(gdb)
Continuing.
Breakpoint 4, better (data=std::vector of length 6, capacity 2027520 = {...}) at m.cpp:34
34 data.push_back(TestObject(i, i+1, i+2));
3: data.size() = 6
4: &data[0] = (TestObject *) 0x7ffff2ba6010
(gdb)
Continuing.
Breakpoint 4, better (data=std::vector of length 7, capacity 2027520 = {...}) at m.cpp:34
34 data.push_back(TestObject(i, i+1, i+2));
3: data.size() = 7
4: &data[0] = (TestObject *) 0x7ffff2ba6010
(gdb)
Continuing.
Breakpoint 4, better (data=std::vector of length 8, capacity 2027520 = {...}) at m.cpp:34
34 data.push_back(TestObject(i, i+1, i+2));
3: data.size() = 8
4: &data[0] = (TestObject *) 0x7ffff2ba6010
(gdb)
Continuing.
Breakpoint 4, better (data=std::vector of length 9, capacity 2027520 = {...}) at m.cpp:34
34 data.push_back(TestObject(i, i+1, i+2));
3: data.size() = 9
4: &data[0] = (TestObject *) 0x7ffff2ba6010
可以看出，better()函数在添加前 9 个元素时，第一个元素的地址——&data[0]都一直不变。即在这添加这些元素时，std::vector内部未发生数据迁移。在添加后面的其他元素时，也不会导致std::vector内部数据迁移，有兴趣的可以使用条件断点——b 34 if &data[0]!=0x7ffff2ba6010（0x7ffff2ba6010 为此处调试时 &data[0] 的值）验证这一点。这正是使用 reserve 预分配内存带来的效果。
上面示例中还有一处可以优化的地方，下文会详细说明。
善用 emplace_back 避免不必要的开销
-
emplace_back 函数的作用
相比与
push_back函数，emplace_back函数可以直接构造元素，从而避免不必要的临时对象构造/析构开销。 -
适用场景
- 如果
std::vector中要添加的元素需要传入参数构造时，用emplace_back而不是push_back函数来直接构造元素以避免不必要的开销，从而改善程序性能。
- 如果
-
代码示例
源码：
// main.cc
#include
#define N 1980*1024
class TestObject
{
public:
TestObject(int a, int b, int c) : a_(a), b_(b), c_(c) {}
int a_;
int b_;
int c_;
};
std::vector test_data_1;
std::vector test_data_2;
void poor(std::vector &data)
{
data.reserve(N);
for (int i = 0; i < N; i++)
{
data.push_back(TestObject(i, i+1, i+2));
}
}
void better(std::vector &data)
{
data.reserve(N);
for (int i = 0; i < N; i++)
{
data.emplace_back(i, i+1, i+2);
}
}
int main()
{
poor(test_data_1);
better(test_data_2);
return 0;
}
编译：
$ g++ -std=c++11 -g -Og -o main_Og main.cpp
函数耗时统计：
| 启动程序方式 | 第一次执行耗时(us) | 第二次执行耗时(us) | 第三次执行耗时(us) | 第四次执行耗时(us) | 第五次执行耗时(us) |
|---|---|---|---|---|---|
| ./main_Og |
从统计结果中可以看出，示例中better()函数的执行速度比poor()函数至少快 1.5倍。
- 代码调试
为了验证emplace_back函数的作用，引入三个全局变量g_count_1、g_count_2和g_count_3用于统计构造函数、拷贝构造函数、析构函数被调用的次数。修改后的源码为：
// main.cc
#include
#define N 1980*1024
int g_count_1 = 0;
int g_count_2 = 0;
int g_count_3 = 0;
class TestObject
{
public:
TestObject(int a, int b, int c) : a_(a), b_(b), c_(c)
{
g_count_1++;
}
TestObject(const TestObject &other)
{
if (this != &other)
{
this->a_ = other.a_;
this->b_ = other.b_;
this->c_ = other.c_;
}
g_count_2++;
}
~TestObject()
{
g_count_3++;
}
int a_;
int b_;
int c_;
};
std::vector test_data_1;
std::vector test_data_2;
void poor(std::vector &data)
{
data.reserve(N);
for (int i = 0; i < N; i++)
{
data.push_back(TestObject(i, i+1, i+2));
}
}
void better(std::vector &data)
{
data.reserve(N);
for (int i = 0; i < N; i++)
{
data.emplace_back(i, i+1, i+2);
}
}
int main()
{
g_count_1 = 0;
g_count_2 = 0;
g_count_3 = 0;
poor(test_data_1);
g_count_1 = 0;
g_count_2 = 0;
g_count_3 = 0;
better(test_data_2);
g_count_1 = 0;
return 0;
}
具体调试过程如下：
(gdb) l
61
62 int main()
63 {
64 g_count_1 = 0;
65 g_count_2 = 0;
66 g_count_3 = 0;
67 poor(test_data_1);
68
69 g_count_1 = 0;
70 g_count_2 = 0;
71 g_count_3 = 0;
72 better(test_data_2);
73 g_count_1 = 0;
74
75 return 0;
76 }
(gdb) b 69
Breakpoint 2 at 0x400b3e: file main.cpp, line 69.
(gdb) b 73
Breakpoint 3 at 0x400b66: file main.cpp, line 73.
(gdb) display g_count_1
1: g_count_1 = 0
(gdb) display g_count_2
2: g_count_2 = 0
(gdb) display g_count_3
3: g_count_3 = 0
(gdb) c
Continuing.
Breakpoint 2, main () at main.cpp:69
69 g_count_1 = 0;
1: g_count_1 = 2027520
2: g_count_2 = 2027520
3: g_count_3 = 2027520
(gdb) n
70 g_count_2 = 0;
1: g_count_1 = 0
2: g_count_2 = 2027520
3: g_count_3 = 2027520
(gdb)
71 g_count_3 = 0;
1: g_count_1 = 0
2: g_count_2 = 0
3: g_count_3 = 2027520
(gdb)
72 better(test_data_2);
1: g_count_1 = 0
2: g_count_2 = 0
3: g_count_3 = 0
(gdb) n
Breakpoint 3, main () at main.cpp:73
73 g_count_1 = 0;
1: g_count_1 = 2027520
2: g_count_2 = 0
3: g_count_3 = 0
(gdb) n
76 }
1: g_count_1 = 0
2: g_count_2 = 0
3: g_count_3 = 0
可以看出，poor函数在执行过程中调用容器元素构造函数、析构函数和拷贝构造函数的次数都是 2027520（2027520 = 1980 x 1024），验证了push_back函数的三种开销：临时对象的构造/析构开销和拷贝开销。better函数在执行过程中调用容器元素构造函数、析构函数和拷贝构造函数的次数分别是 2027520、0、0，验证了emplace_back函数只有必要的构造元素开销，不会引入临时对象，避免了不必要的临时对象构造/析构开销，从而改善程序性能。
善用 std::move 避免不必要的拷贝开销
-
std::move 函数的作用
移动而非拷贝数据以避免不必要的拷贝开销，从而改善程序性能。
-
适用场景
- 构造函数的参数列表中有类型为
std::vector的参数，并且该参数只在构造过程中使用。
- 构造函数的参数列表中有类型为
-
代码示例
源码：
// main.cpp
#include
#include
#define N 1980*1024
class TestObject
{
public:
TestObject(int a, int b, int c) : a_(a), b_(b), c_(c) {}
int a_;
int b_;
int c_;
};
class Element
{
public:
explicit Element(const std::vector &data)
: data_(data)
{
}
explicit Element(std::vector &&data)
: data_(std::move(data))
{
}
private:
std::vector data_;
};
std::list test_data_1;
std::list test_data_2;
Element *g_ele = nullptr;
void poor(std::list &data)
{
std::vector vec;
vec.reserve(N);
for (int i = 0; i < N; i++)
{
vec.push_back(TestObject(i, i+1, i+2));
}
data.emplace_back(vec);
g_ele = &data.front();
}
void better(std::list &data)
{
std::vector vec;
vec.reserve(N);
for (int i = 0; i < N; i++)
{
vec.emplace_back(i, i+1, i+2);
}
data.emplace_back(std::move(vec));
g_ele = &data.front();
}
int main()
{
poor(test_data_1);
better(test_data_2);
return 0;
}
编译：
$ g++ -std=c++11 -g -Og -o main_Og main.cpp
函数耗时统计：
| 启动程序方式 | 第一次执行耗时(us) | 第二次执行耗时(us) | 第三次执行耗时(us) | 第四次执行耗时(us) | 第五次执行耗时(us) |
|---|---|---|---|---|---|
| ./main_Og |
从统计结果中可以看出，示例中better()函数的执行速度比poor()函数至少快 2倍。
- 代码调试
验证data.emplace_back(vec); 会引发拷贝开销，具体调试过程如下：
(gdb) l 50
35
36 std::list test_data_1;
37 std::list test_data_2;
38 Element *g_ele = nullptr;
39
40 void poor(std::list &data)
41 {
42 std::vector vec;
43 vec.reserve(N);
44 for (int i = 0; i < N; i++)
45 {
46 vec.push_back(TestObject(i, i+1, i+2));
47 }
48
49 data.emplace_back(vec);
50 g_ele = &data.front();
51 }
52
53 void better(std::list &data)
54 {
55 std::vector vec;
56 vec.reserve(N);
57 for (int i = 0; i < N; i++)
58 {
59 vec.emplace_back(i, i+1, i+2);
60 }
61
62 data.emplace_back(std::move(vec));
63 g_ele = &data.front();
64 }
(gdb) b 50
Breakpoint 3 at 0x40131c: file p.cpp, line 50.
(gdb) c
Continuing.
Breakpoint 3, poor (data=std::__cxx11::list = {...}) at p.cpp:50
50 g_ele = &data.front();
(gdb) display vec._M_impl
1: vec._M_impl = {> = {<__gnu_cxx::new_allocator> = {}, }, _M_start = 0x7ffff5a2b010,
_M_finish = 0x7ffff715f010, _M_end_of_storage = 0x7ffff715f010}
(gdb) n
42 std::vector vec;
1: vec._M_impl = {> = {<__gnu_cxx::new_allocator> = {}, }, _M_start = 0x7ffff5a2b010,
_M_finish = 0x7ffff715f010, _M_end_of_storage = 0x7ffff715f010}
(gdb) display g_ele->data_._M_impl
2: g_ele->data_._M_impl = {> = {<__gnu_cxx::new_allocator> = {}, }, _M_start = 0x7ffff42f6010,
_M_finish = 0x7ffff5a2a010, _M_end_of_storage = 0x7ffff5a2a010}
可以看出，poor函数中的临时对象vec的内存地址范围为0x7ffff5a2b010~0x7ffff715f010（vec._M_impl._M_start 的值为 0x7ffff5a2b010，vec._M_impl.M_end_of_storage 的值为 0x7ffff715f010）。全局变量test_data_1首元素的数据成员data_的内存地址范围为0x7ffff42f6010~0x7ffff5a2a010（g_ele->data._M_impl.M_start 的值为 0x7ffff42f6010，g_ele->data._M_impl._M_end_of_storage 的值为 0x7ffff5a2a010）。两者的内存地址范围不相同。因此，全局变量test_data_1首元素的数据成员data_是从临时对象vec拷贝而来的，从而验证了data.emplace_back(vec); 会引发拷贝开销。
验证data.emplace_back(std::move(vec)); 不会引发拷贝开销，具体调试过程如下：
(gdb) b 59
Breakpoint 4 at 0x4010ba: file p.cpp, line 59.
(gdb) b 63
Breakpoint 5 at 0x401147: file p.cpp, line 63.
(gdb) c
Continuing.
poor_function elapsed_time: 59091892 us
Breakpoint 4, better (data=empty std::__cxx11::list) at p.cpp:59
59 vec.emplace_back(i, i+1, i+2);
2: g_ele->data_._M_impl = {> = {<__gnu_cxx::new_allocator> = {}, }, _M_start = 0x7ffff42f6010,
_M_finish = 0x7ffff5a2a010, _M_end_of_storage = 0x7ffff5a2a010}
(gdb) display vec._M_impl
3: vec._M_impl = {> = {<__gnu_cxx::new_allocator> = {}, }, _M_start = 0x616060, _M_finish = 0x616060,
_M_end_of_storage = 0x1d4a060}
(gdb) d 4
(gdb) c
Continuing.
Breakpoint 5, better (data=std::__cxx11::list = {...}) at p.cpp:63
63 g_ele = &data.front();
2: g_ele->data_._M_impl = {> = {<__gnu_cxx::new_allocator> = {}, }, _M_start = 0x7ffff42f6010,
_M_finish = 0x7ffff5a2a010, _M_end_of_storage = 0x7ffff5a2a010}
3: vec._M_impl = {> = {<__gnu_cxx::new_allocator> = {}, }, _M_start = 0x0, _M_finish = 0x0,
_M_end_of_storage = 0x0}
(gdb) n
55 std::vector vec;
2: g_ele->data_._M_impl = {> = {<__gnu_cxx::new_allocator> = {}, }, _M_start = 0x616060, _M_finish = 0x1d4a060,
_M_end_of_storage = 0x1d4a060}
3: vec._M_impl = {> = {<__gnu_cxx::new_allocator> = {}, }, _M_start = 0x0, _M_finish = 0x0,
_M_end_of_storage = 0x0}
可以看出，better函数中的临时对象vec的内存地址范围为0x616060~0x1d4a060（vec._M_impl._M_start 的值为 0x616060，vec._M_impl.M_end_of_storage 的值为 0x1d4a060）。全局变量test_data_2首元素的数据成员data_的内存地址范围为0x616060~0x1d4a060（g_ele->data._M_impl.M_start 的值为 0x616060，g_ele->data._M_impl._M_end_of_storage 的值为 0x1d4a060）。两者的内存地址范围相同。因此，全局变量test_data_2首元素的数据成员data_是直接将临时对象vec移动而来的，从而验证了data.emplace_back(std::move(vec)); 不会引发拷贝开销。另外，需要注意的是，临时对象vec在被移动之后将不再有效。
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