剥离未使用的运行时功能，其可膨胀可执行（GCC） [英] Strip unused runtime functions which bloat executable (GCC)

问题描述

我已经为ARM（cortex-m3）构建了GCC4.7.1交叉工具链。现在我链接一个C / C ++代码的可执行文件，肯定不会使用某些STL类（例如 std :: string ）。此外，例外和RTTI关闭。

虽然我正在寻找目标ELF（例如使用nm），但是有很多符号（来自libstdc ++）链接在我不会期望找到那里（例如 std :: exception ， std :: ios_base 等）

为什么会这样，如何摆脱这些东西来减少 .text 我的目标的部分大小？

一个同事给了我一个提示来覆盖一些GCC特定的存根功能：

 命名空间__gnu_cxx 
 {
 void __verbose_terminate_handler（）
 {
 for（;;）
; 
} 
} 
  

这个单独减少了大约20KB的代码大小。 >
我可以覆盖更多这样的存根吗？

---

更新：

好​​的，我发现一个非常愚蠢的错误，删除了我想知道的大部分东西，当修复它：

有一个<$ c $其中一个源文件中的c> #include< iostream> 语句（尽管没有从中调用）。这当然会链接到静态的 std :: cin ， std :: cout 和 std :: cerr 实例以及与这些相关的所有内容。

删除 #include< iostream>

---

然而：

还有 std :: exception 和 std :: basic_string 我想知道的东西：

 命名空间摘要：
 ======= ================================================== ===================== 
类型大小命名空间
 T 774'std'
 W 184'std :: string :: _Rep'
 W 268'std'
 W 472'std :: string'
类摘要：
 ============== ================================================== ============ 
类型大小类
 T 50'std :: error_category' 
 T 52'std :: type_info'
 T 54'std :: bad_exception'
 T 54'std :: exception'
 T 68'std :: bad_alloc'
 T 98'std :: length_error'
 T 214'std :: logic_error'
 W 268'std :: basic_string< char，std :: char_traits< char>，std :: allocator< char> ; >'
  

使用的代码大小没有太多，只有几百个字节，所以我可能会忽略它，但会感激，如果我也可以摆脱这一点。

由于我明确使用没有例外，我想知道为什么这些仍然被实例化链接。在运行时无法真正确定使用异常？！？

现在剩下的 __ gnu_cxx 命名空间中唯一剩下的是

 类型大小类
 T 58'__gnu_cxx :: recursive_init_error'
  / pre> 
 
 
这是另一个例外类。
 
 
 
 
 
 
  FINALLY： 
 
我使用了一些额外的标志来配置GCC4.7交叉版本：
   -  enable-gold = yes 
 --enable-lto 
 --enable-cxx-flags =' -  fno-exceptions -function-sections -fno-omit-frame-pointer'
  
后一个标志用于编译libstdc ++，并且与用于构建目标代码基本相同是合理的行动）最后的事情是，我发现一个意想不到的例外情况（包括 __ gnu_cxx :: recursive_init_error ）
 
 
 <在我们的代码库中使用 std :: string 。修正后，引用 std :: basic_string< char，std :: char_traits< char>，std :: allocator< char> < / code>也消失了。
 
 
 
所以我很高兴现在的结果，没有更多的不必要的，意外的开销从libstdc ++，没有理由不使用C ++优先于C。
解决方案
 
 
 
 为什么会这样，我摆脱这个东西来减少我的目标的 .text 部分大小？ 
 
 
从 libstdc ++ 链接的东西可能有（静态）引用，这将被实例化，无论它们是否在实际中被调用或引用执行的codepathes。
 
仔细搜索包含这种引用声明的 #include 语句的使用（例如 #include< iostream>  
 
 
当使用不同的C ++编译标志安装GCC时， libstdc ++ 正在构建如用于最终目标的目标，可能有不想要的东西链接或被实例化（例如 __ gnu_cxx :: recursive_init_error 异常类，尽管使用了 -fno-exceptions 标志）。在构建工具链时，使用  -  enable-cxx-flags 配置选项与预期的目标编译标志同步。
 
 
 
还要仔细查看代码库中某些STL类的不必要/意外使用（例如 std :: string ），尽管它们可能会编译并链接而不会出现错误或警告，尽管某些功能（例如 new（））在您的裸机平台上并不真正支持。
 
 
 是否有更多这样的存根可以覆盖？ 
 
 
根据'newlib'构建工具链（请参阅  -  en / disable-newlib-provided-syscalls 配置选项），可能需要覆盖此处提供的某些或所有存根。
 
I have build the GCC4.7.1 cross-toolchain for ARM (cortex-m3). Now I'm linking an executable from C/C++ code that surely doesn't use some certain STL classes (e.g. std::string). Furthermore exceptions and RTTI are turned off.

Though when I'm looking to the target ELF (e.g. using nm), there's a lot of symbols (apparantly from the libstdc++) linked in I wouldn't expect to find there (e.g. std::exception, std::ios_base, etc.).

Why is this there, and how can I get rid of this stuff to reduce the .text section size of my target?

A coworker gave me a tip to override some GCC specific stub function:
namespace __gnu_cxx
{
    void __verbose_terminate_handler()
    {
        for (;;)
            ;
    }
}
This alone reduced the code size about 20KB.

Are there more such stubs I can override?



UPDATE:

OK, I found one really stupid error that removed most of the stuff I was wondering about, when fixing it:

There was an #include <iostream> statement left (though nothing called from there) in one of the source files. This will of course link in the static std::cin, std::cout and std::cerr instances and all the stuff that comes along with these.

Removing the #include <iostream> statement reduced the .text segment about another > 100KB portion.



Nevertheless:

There's still the std::exception and std::basic_string stuff I'm wondering about:
Namespace summaries:
==============================================================================
Type         Size Namespace 
T             774 'std'
W             184 'std::string::_Rep'
W             268 'std'
W             472 'std::string'
Class summaries:
==============================================================================
Type         Size Class 
T              50 'std::error_category'
T              52 'std::type_info'
T              54 'std::bad_exception'
T              54 'std::exception'
T              68 'std::bad_alloc'
T              98 'std::length_error'
T             214 'std::logic_error'
W             268 'std::basic_string<char, std::char_traits<char>, std::allocator<char> >'
There isn't really much code size used, just about several 100 bytes, so I could live with neglecting it, but would appreciate, if I can get rid of this also.

Since I'm explicitly using no exceptions I wonder why these are still instantiated when linking. Usage of exceptions or not can't be really determined at runtime?!?

The only remaining thing from the __gnu_cxx namespace I have left now is 
Type         Size Class 
T              58 '__gnu_cxx::recursive_init_error'
It's another exception class.



FINALLY:

I used some additional flags to configure the GCC4.7 cross build:
--enable-gold=yes 
--enable-lto 
--enable-cxx-flags='-fno-exceptions -ffunction-sections -fno-omit-frame-pointer'
The latter flags are used to compile the libstdc++ and are essentially the same as used for building the target code (which is a reasonable action anyway). Exception references where gone afterwards (including the __gnu_cxx::recursive_init_error).

Last thing was, I found an unexpected use of std::string in our codebase. After fixing that, the reference to std::basic_string<char, std::char_traits<char>, std::allocator<char> > also disappeared.

So I'm happy with the result now, no more unnecessary, unexpected overhead from libstdc++, no reason not to use C++ in preference over C.
解决方案

  
Why is this there, and how can I get rid of this stuff to reduce the .text section size of my target?
There might be (static) references for stuff linked in from the libstdc++, that will be instantiated no matter if they are called or referenced in the actually executed codepathes.

Carefully search for usage of #include statements that contain such reference declarations (e.g. #include <iostream>).

When libstdc++ is build during installation of GCC with different C++ compile flags as used for the finally intended target, there might be unwanted stuff linked in or being instantiated (e.g. the __gnu_cxx::recursive_init_error exception class despite usage of the -fno-exceptions flag). Use the --enable-cxx-flags configuration option to synchronize with the intended target compile flags when building the toolchain.

Also look carefully for unwanted/unexpected usage of certain STL classes in your codebase (e.g. std::string), they might compile and being linked without errors or warnings, despite certain features (e.g. new()) aren't really supported on your bare metal platform.

  
Are there more such stubs I can override?
Depending on how 'newlib' was build for the toolchain (see --en/disable-newlib-supplied-syscalls configuration option), it might be necessary to override certain or all stubs provided there.

                        
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