Analysis on efficiency of pre-increment and post-increment-白红宇的个人博客

Analysis on efficiency of pre-increment and post-increment

发布日期：2022-02-07 06:39:39 浏览次数：10 分类：技术文章

本文共 8176 字，大约阅读时间需要 27 分钟。

Let's have a look from the fundamental types to complex type.

int size = 0;

int x = 0;

size++;

++size;

1.The assembly code is as follows

size++;004015E3 8B 45 C4 mov eax,dword ptr [ebp-3Ch] 004015E6 83 C0 01 add eax,1 004015E9 89 45 C4 mov dword ptr [ebp-3Ch],eax ++size;004015EC 8B 4D C4 mov ecx,dword ptr [ebp-3Ch] 004015EF 83 C1 01 add ecx,1 004015F2 89 4D C4 mov dword ptr [ebp-3Ch],ecx

2. Now, we analysis the case when assign the increase value to variables

x = size++; x = ++size;

The assembly code is as follows

x = size++;00405B83 8B 45 C4 mov eax,dword ptr [ebp-3Ch] 00405B86 89 45 C8 mov dword ptr [ebp-38h],eax 00405B89 8B 4D C4 mov ecx,dword ptr [ebp-3Ch] 00405B8C 83 C1 01 add ecx,1 00405B8F 89 4D C4 mov dword ptr [ebp-3Ch],ecx x = ++size;00405B92 8B 55 C4 mov edx,dword ptr [ebp-3Ch] 00405B95 83 C2 01 add edx,1 00405B98 89 55 C4 mov dword ptr [ebp-3Ch],edx 00405B9B 8B 45 C4 mov eax,dword ptr [ebp-3Ch] 00405B9E 89 45 C8 mov dword ptr [ebp-38h],eax

Based on about analysis, we can conclude that the performance of pre-increment and post-increment is essentially identical for fundamental types.

However, things change for class types. Let's take the commonly-used STL iterator as example

vector<int > items;

vector<int >::iterator itr = items.begin();

itr++;

The code of "itr++" is

_Myt operator ++(int )

{

// postincrement

_Myt _Tmp = *this ;

++*this ;

return (_Tmp);

}

while for ++itr,

_Myt& operator ++()

{

// preincrement

++(*(_Mybase *)this );

return (*this );

}

Obviously, post-increment is based on pre-increment, surplus with constructing a temporary instance to store the original value of instance. Therefore, the performance of post-increment is doutlessly worse than that of pre-increment in this case.

In conclusion, we suggest developer get into a habit of using pre-increment. Certainly, there are exceptions where post-increment is the only choice. But it is out of topic of this article.

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