编译器_keil的优化选项问题

最近发现在keil编译的时候，出现莫名的问题，貌似代码有被优化掉的问题，后来查了下相关的资料，貌似懂了点。
我选择的是默认的default优化方式，上网看了下，默认的是level2级别优化，最后选择level0就没有问题了
下面是网上找的资料，介绍了优化功能介绍

Getting the Best Optimized Code for your Embedded Application 

ARM Compilation Tools 
The ARM Compilation Tools are the only compilation tool s co -developed with the ARM processors, and specifically 
designed to optimally support the ARM architecture. They are a result of 20 years of development, and are recognized as the 
industry -leading C and C++ compilation tools for the ARM, Thumb, and Thumb -2 instructions sets. 
The ARM Compilation tools consist of:  
•  The ARM Compiler, which enables you to compile C and C++ code. It is an optimizing compiler, and features 
command - line options to enable you to control the level of optimization  
•  Linker and Utilities, which assign addresses and lay out sections of code to form a final image  
•  A selection of libraries, including the ISO standard C libraries, and the MicroLIB C library which is optimized for 
embedded applications 
•  Assembler, which generates machine code instructions from  ARM, Thumb or Thumb-2 assembly- level   source code  
 
Compiler Options for Embedded Applications   
The ARM Compilation Tools include a number of compiler optimizations to help you best target your code for your chosen 
microcontroller device and   application area.   
They can be accessed from within µVision by clicking on Project –   Options for Target. 
T he options described this document  can be found on the Target   an d  C/C++   tabs  of   the  Options for Targets  dialog.  
 

MDK Compiler Optimizations   

•  Cross- Module  
Optimization  takes information from a prior build and uses it to place UNUSED functions into their 

own ELF section in the corresponding object file. This option is also known as Linker Feedback, and requires you to 
build your application twice to take adv antage of it for reduced code size.   
Cross-Module Optimization has been shown to reduce code size, by removing unused functions from your application. It 
can also improve the performance   of your application, by allowing modules to share inline code.  
 

•  The  M icroLIB   C library
has been optimized to reduce the size of embedded applications. It is a subset of the ISO 

standard C runtime library, and offers a tradeoff between functionality and code size. Some of the standard C library 
functions such as memcpy()   are  slower, while some features of the