O0 O1 O2 O3 optimization principle

In order to speed up the efficiency of code execution , quite a lot OJ The platform will start automatically O2 Optimize .

Here we talk about how to optimize .

O0 Optimize

#pragma GCC optimize(0)

1、 Assign variables to registers .

2、 Analyze the various situations that will occur in the cycle , Keep only one branch that exits the loop .

3、 Delete useless code .

4、 Simplify expressions and declarations .

5、 Will use inline The declared inline function becomes a function call .

O1 Optimize

#pragma GCC optimize(1)

contain O0 Various optimization functions of , And increased. ：
1、 In variable assignment , Assign the value directly to the variable instead of giving the address of the variable .

2、 Remove useless variables and expressions .

3、 Remove the general expression .

O2 Optimize

#pragma GCC optimize(2)

contain O1 Various optimization functions of , And increased. ：

1、 Remove the global sub expression .

2、 Remove the allocation variables and expressions that are not used globally .

3、 Resolve cycle .

When only -O Option, the optimizer automatically -O2 Optimize .

O3 Optimize

#pragma GCC optimize(3)

contain O2 Various optimization functions of , And increased. ：

1、 Remove the function not called .

2、 Simplify functions that return unused values .

3、 Make embedded calls to small functions .

4、 Reorder the called function declarations , So that the optimized caller can find the function .

5、 Complete file level optimization .