GCC --gc-sections和查找符号依赖 [英] GCC --gc-sections and finding symbol dependencies

问题描述

我想减小我的elf可执行文件的大小。我正在编译 -ffunction-sections -fdata-sections 并链接到 -gc

I'm trying to reduce the size of my elf executable. I'm compiling with -ffunction-sections -fdata-sections and linking with -gc-sections, but it appears some of the symbols that I believe are unused are not being discarded.

在GNU工具链中有一些命令我可以运行以找出正在使用的符号和位置？

Is there some command in the GNU toolchain I can run to find out which symbols are being used and where?

• 工具链：GNU arm-none-eabi


• 平台：Cortex-M4


• 语言：C ++

典型构建标志：

编译： arm-none-eabi-g ++。exe -Wall -O3 -mthumb -std = c ++ 11 - mcpu = cortex-m4 -mfpu = fpv4-sp-d16 -mfloat-abi = softfp -fsingle-precision-constant -ffunction-sections -fdata-sections

链接： arm-none-eabi-g ++。exe -static -mthumb -mcpu = cortex-m4 -mfpu = fpv4-sp-d16 -mfloat-abi = softfp -Wl， gc-sections -Wl，-TLinkerScript.ld

感谢您的帮助。

推荐答案

我找不到一个显示符号依赖的命令。但是，我能够通过使用以下技术获取我需要的信息：

I wasn't able to find a command that showed the symbol dependencies. However, I was able to get the information I needed by using the following technique:

• 将问题中的符号添加到 / DISCARD / 部分。这将输出一个错误消息，显示哪个符号正在使用它。它看起来像这样：在< lib0.a文件路径>（< object0文件路径>）的'<符号1>部分中引用的< symbol0& ; （object1文件路径>）的符号0>'


• 继续将这些消息中的符号添加到调用堆栈 / DISCARD / 部分，直到找到问题的根源。

• Add the symbol in question to the /DISCARD/ section of the linker script. This will output an error message revealing which symbol is using it. It looks something like this: <symbol0>' referenced in section '<symbol1>' of <lib0.a file path>(<object0 file path>): defined in discarded section '<symbol0>' of <lib1.a file path>(object1 file path>)
• Continue to add symbols from these messages up the call stack to the /DISCARD/ section until you find the root of the problem.

根的问题对我来说是一个继承自另一个类。这创建了一个虚拟表，编译器不能删除虚拟表中引用的死代码。

The root of the problem for me was having a class that inherited from another. This created a virtual table, and the compiler cannot remove dead code that is referenced in a virtual table.

学到的经验教训：如果你想减少代码大小并仍然使用C ++，不要使用继承。 GNU工具链以前有一个 -fvtable-gc 切换来帮助这个，但它被删除了一段时间前。我将重构我的代码来解决我的具体问题。

Lesson learned: if you want to reduce code size and still use C++, don't use inheritance. The GNU toolchain used to have a -fvtable-gc switch to help with this, but it was removed some time ago. I will refactor my code to address my specific problem.

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