KEIL/MDK编译优化选项注意事项

来源：https://blog.csdn.net/chengdong1314/article/details/53463183

KEIL编译器C语言编译选项优化等级说明

原文

0     Minimum optimization. Turns off most optimizations.It gives the best possible debug view and the lowest level of optimization.             近乎不优化，用于调试代码。出现代码行不能设置断点可如此设置试试。
1           Restrictedoptimization. Removes unused inline functions and unused static functions.Turns off optimizations that seriously degrade the debug view. Ifused with --debug, this option gives a satisfactorydebug view with good code density. 部分优化。移除未调用的内联函数和静态函数，关闭debug窗口优化，此状态也能用于调试
2           Highoptimization. If used with--debug, the debug viewmight be less satisfactory because the mapping of object code tosource code is not always clear.

This is the default optimization level.

            默认优化等级。如果处于debug状态，部分代码行将不能被调试，具体做了什么优化好像没说
3     Maximumoptimization.-O3 performs the same optimizationsas-O2 however the balance between space and timeoptimizations in the generated code is more heavily weighted towardsspace or time compared with-O2. That is:

• -O3 -Otime aims to produce fastercode than-O2 -Otime, at the risk of increasingyour image size

• -O3 -Ospace aims to produce smallercode than-O2 -Ospace, but performance might bedegraded.

In addition, -O3 performs extra optimizationsthat are more aggressive, such as:

• High-levelscalar optimizations, including loop unrolling, for-O3-Otime. Thiscan give significant performance benefits at a small code size cost,but at the risk of a longer build time.

• More aggressive inlining and automatic inliningfor-O3 -Otime.

KEIL编译器C语言编译选项优化等级设置不一样造成代码运行的效果完全不一样，下面是SYD8801上运行的实例！

主函数如下：

int main()

{

         uint8_tkey_num=0;

         gpio_config();

         led_close();

         timer_0_enable(0x20,timer0_callback); // 32 * 31.25 us = 1ms

         __enable_irq();

         while(1)

         {

                   *(uint32_t*)GPO_CTRL &=0xffffff00;

                   *(uint32_t*)GPO_CTRL |=0x000000ff;

         }

}

现在的编译器C环境设置如下：

如果编译设置这样改：

编译、下载、复位现象是这样的：

所以一定要注意C编译器的优化选项，还有一点就是C编译器的设置选项也要关注的，虽然这里并没有出错，也就是如下图：

支持C99(变量声明在执行语句之后)

    C语言目前已经支持变量声明在执行语句之后的操作了，不过这是C99的内容，KELL默认是不支持C99的，也就是说下面的写法KELL将会报错：

    KELL要支持C99可以按照如下操作：

    在低版本的KELL中也可以按照如下操作：