kei5 怎么优化，可以使代码运行最快效率最高

只看该作者 · 2018-8-31 14:11

最近做个项目使用到优化的时候，一直有一些疑问：
1，使用的是STM32F1，刚开始的时候使用O1级别优化，调试完成之后，改成了O2，做斜率的时候，明显感觉到了程序速度提升近2倍。而且其它的感觉也比以前块，以eeprom为例，本来O1的时候完全没问题，改成O2 后，就只能读不能写，找了半天才发现时序变了。
2，C++ Code Generation 里面那么多选项，那些选项除了常用几个，其他具体都是做什么用的，感觉搞得不
是很明白，希望高手解答？？
3，怎么样选择能使程序跑的最快，怎么样选择能使代码量最小了，感觉搞得也不是特别明白
说明：我程序在选择O1优化时候为81k左右，选择O2时降低到了73K左右，选择O3时候跟O1差不多，但是速度方面，O2和O3差的不多，跟O1都有明显提升

只看该作者 · 2018-8-31 14:51

1、空间优化
2、时间优化
3、1和2折中

只看该作者 · 2018-8-31 17:12

本帖最后由 hclwj181 于 2018-8-31 17:22 编辑

iDiy 发表于 2018-8-31 14:51
1、空间优化
2、时间优化
3、1和2折中

感觉O2程序量最小，不知道选择哪个最快，试了下除了O0感觉都差不多，代码量O2明显最小

只看该作者 · 2018-8-31 17:49

本帖最后由 iDiy 于 2018-9-1 11:46 编辑

hclwj181 发表于 2018-8-31 17:12
感觉O2程序量最小，不知道选择哪个最快，试了下除了O0感觉都差不多，代码量O2明显最小 ...

Arm® Compiler performs several optimizations to reduce the code size and improve the performance of your application. However, optimization techniques can result in the loss of debug information, increased build time, or increase in image size. Optimization levels are always a trade-off between these three parameters.

Arm Compiler provides optimization options for the different optimization trade-offs. Primarily, you can optimize for performance or for image size. However, there are several options for finer control of the optimizations techniques. The optimization options are:

-O0
This is the default optimization setting. It turns off most optimizations, and gives the best correlation between the built image and your application source code.
Use the following options to optimize performance:

-O1
This results in more optimizations for performance, when compared to -O0. It also reduces the information available for debugging, and might result in an increased image size. Arm recommends this option for debugging.
-O2
This results in more optimizations for performance, when compared to -O1. It also reduces the information available for debugging, and might result in an increased image size.
-O3
This results in more optimizations for performance, when compared to -O2. It also reduces the information available for debugging, and might result in an increased image size.
-Ofast
This results in more optimizations for performance, when compared to -O3. It also reduces the information available for debugging, and might result in an increased image size. At this optimization level, Arm Compiler might violate certain language standards.
-Omax
This results in more optimizations for performance, when compared to -Ofast. It also reduces the information available for debugging, and might result in an increased image size. At this optimization level, Arm Compiler might violate certain language standards. Arm recommends this option for best performance.
Use the following options to optimize for size:

-Os
This results in reduced code size, and also reduces the information available for debugging. Using this option might make your code slower.
-Oz
This results in more reduced image size, when compared to -Os, and also reduces the information available for debugging. Using this option is likely to make your code slower than -Os. Arm recommends this option for best code size.