一、优化掉没有调用的函数，减少程序空间

我做了下测试，在开一级优化下，我写了一个函数，但是没有调用，查看map文件发现

它仍然占用程序空间，所以我怀疑不是优化级别的事情。于是百度了下，发现要勾选C/C++的

One ELF Section per Function。什么是意思呢？就是每个文件的函数单独出一个连接文件，

这样连接的时候，那些没有调用的函数就被移除掉了。

GD32F130FXP6学习笔记十三：mdk优化掉不调用的函数和防止没有显式调用的函数被优化掉

查看编译后的map文件：

GD32F130FXP6学习笔记十三：mdk优化掉不调用的函数和防止没有显式调用的函数被优化掉

看到了没有？移除了。

二、mdk怎么实现IAR的__root效果

__root是指被修饰的变量或者函数在用户程序里面没有显式调用情况下，也不会被优化掉。

像一些系统配置和函数指针调用，就需要__root属性。

那么mdk怎么实现呢？查看了下编译器手册，并没有__root关键字，看了下连接器手册，有

--keep关键字。

官方说明：

--keep=section_id

ARM Compiler toolchain v5.02 for µVision Linker Reference

`--keep=section_id`

This option specifies input sections that must not be removed by unused section elimination.

--keep=section_id

Where section_id is one of the following:

symbol: Specifies that an input section defining symbol is to be retained during unused section elimination. If multiple

For example, you might use --keep=int_handler.

To keep all sections that define a symbol ending in _handler, use--keep=*_handler.; definitions of symbol exist,armlink generates an error message.
object(section): Specifies that section fromobject is to be retained during unused section elimination. If a single instance ofsection is

For example, to keep the vect section from the vectors.o object use: --keep=vectors.o(vect)

To keep all sections from the vectors.o object where the first three characters of the name of the sections arevec,; use:--keep=vectors.o(vec*); generated, you can omit section, for example,file.o(). Otherwise, you must specify section.
object: Specifies that the single input section from object is to be retained during unused section elimination. If you use this

For example, you might use --keep=dspdata.o.

To keep the single input section from each of the objects that has a name starting withdsp, use --keep=dsp*.o.; short form and there is more than one input section in object, the linker generates an error message.

All forms of the section_id argument can contain the* and ? wild characters. Matching is case-insensitive, even on hosts

with case-sensitive file naming. For example:

--keep foo.o(Premier*) causes the entire match forPremier* to be case-insensitive
--keep foo.o(Premier) causes a case-sensitive match for the string Premier.

Use *.o to match all object files. Use * to match all object files and libraries.

You can specify multiple --keep options on the command line.

If you name a symbol with the same name as an object, then --keep=symbol_id searches for a symbol that matchessymbol_id:

You can force --keep to match an object with --keep=symbol_id(). Therefore, to keep both the symbol and the object,

specify --keep foo.o --keep foo.o().