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
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- 原文
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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.
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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 -Otimeaims to produce fastercode than-O2 -Otime, at the risk of increasingyour image size-O3 -Ospaceaims 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：
