How to remove unused C/C++ symbols with GCC and ld

问题:

I need to optimize the size of my executable severely (ARM development) and I noticed that in my current build scheme (gcc + ld) unused symbols are not getting stripped.

The usage of the arm-strip --strip-unneeded for the resulting executables / libraries doesn\'t change the output size of the executable (I have no idea why, maybe it simply can\'t).

What would be the way (if it exists) to modify my building pipeline, so that the unused symbols are stripped from the resulting file?

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I wouldn\'t even think of this, but my current embedded environment isn\'t very \"powerful\" and saving even 500K out of 2M results in a very nice loading performance boost.

Update:

Unfortunately the current gcc version I use doesn\'t have the -dead-strip option and the -ffunction-sections... + --gc-sections for ld doesn\'t give any significant difference for the resulting output.

I\'m shocked that this even became a problem, because I was sure that gcc + ld should automatically strip unused symbols (why do they even have to keep them?).

回答1:

For GCC, this is accomplished in two stages:

First compile the data but tell the compiler to separate the code into separate sections within the translation unit. This will be done for functions, classes, and external variables by using the following two compiler flags:

-fdata-sections -ffunction-sections

Link the translation units together using the linker optimization flag (this causes the linker to discard unreferenced sections):

-Wl,--gc-sections

So if you had one file called test.cpp that had two functions declared in it, but one of them was unused, you could omit the unused one with the following command to gcc(g++):

gcc -Os -fdata-sections -ffunction-sections test.cpp -o test -Wl,--gc-sections

(Note that -Os is an additional compiler flag that tells GCC to optimize for size)

回答2:

If this thread is to be believed, you need to supply the -ffunction-sections and -fdata-sections to gcc, which will put each function and data object in its own section. Then you give and --gc-sections to GNU ld to remove the unused sections.

回答3:

You\'ll want to check your docs for your version of gcc & ld:

However for me (OS X gcc 4.0.1) I find these for ld

-dead_strip

Remove functions and data that are unreachable by the entry point or exported symbols.

-dead_strip_dylibs

Remove dylibs that are unreachable by the entry point or exported symbols. That is, suppresses the generation of load command commands for dylibs which supplied no symbols during the link. This option should not be used when linking against a dylib which is required at runtime for some indirect reason such as the dylib has an important initializer.

And this helpful option

-why_live symbol_name

Logs a chain of references to symbol_name. Only applicable with -dead_strip. It can help debug why something that you think should be dead strip removed is not removed.

There\'s also a note in the gcc/g++ man that certain kinds of dead code elimination are only performed if optimization is enabled when compiling.

While these options/conditions may not hold for your compiler, I suggest you look for something similar in your docs.

回答4:

Programming habits could help too; e.g. add static to functions that are not accessed outside a specific file; use shorter names for symbols (can help a bit, likely not too much); use const char x[] where possible; ... this paper, though it talks about dynamic shared objects, can contain suggestions that, if followed, can help to make your final binary output size smaller (if your target is ELF).

回答5:

The answer is -flto. You have to pass it to both your compilation and link steps, otherwise it doesn\'t do anything.

It actually works very well - reduced the size of a microcontroller program I wrote to less than 50% of its previous size!

Unfortunately it did seem a bit buggy - I had instances of things not being built correctly. It may have been due to the build system I\'m using (QBS; it\'s very new), but in any case I\'d recommend you only enable it for your final build if possible, and test that build thoroughly.

回答6:

While not strictly about symbols, if going for size - always compile with -Os and -s flags. -Os optimizes the resulting code for minimum executable size and -s removes the symbol table and relocation information from the executable.

Sometimes - if small size is desired - playing around with different optimization flags may - or may not - have significance. For example toggling -ffast-math and/or -fomit-frame-pointer may at times save you even dozens of bytes.

回答7:

It seems to me that the answer provided by Nemo is the correct one. If those instructions do not work, the issue may be related to the version of gcc/ld you\'re using, as an exercise I compiled an example program using instructions detailed here

#include <stdio.h>
void deadcode() { printf(\"This is d dead codez\\n\"); }
int main(void) { printf(\"This is main\\n\"); return 0 ; }

Then I compiled the code using progressively more aggressive dead-code removal switches:

gcc -Os test.c -o test.elf
gcc -Os -fdata-sections -ffunction-sections test.c -o test.elf -Wl,--gc-sections
gcc -Os -fdata-sections -ffunction-sections test.c -o test.elf -Wl,--gc-sections -Wl,--strip-all

These compilation and linking parameters produced executables of size 8457, 8164 and 6160 bytes, respectively, the most substantial contribution coming from the \'strip-all\' declaration. If you cannot produce similar reductions on your platform,then maybe your version of gcc does not support this functionality. I\'m using gcc(4.5.2-8ubuntu4), ld(2.21.0.20110327) on Linux Mint 2.6.38-8-generic x86_64

回答8:

strip --strip-unneeded only operates on the symbol table of your executable. It doesn\'t actually remove any executable code.

The standard libraries achieve the result you\'re after by splitting all of their functions into seperate object files, which are combined using ar. If you then link the resultant archive as a library (ie. give the option -l your_library to ld) then ld will only include the object files, and therefore the symbols, that are actually used.

You may also find some of the responses to this similar question of use.

回答9:

I don\'t know if this will help with your current predicament as this is a recent feature, but you can specify the visibility of symbols in a global manner. Passing -fvisibility=hidden -fvisibility-inlines-hidden at compilation can help the linker to later get rid of unneeded symbols. If you\'re producing an executable (as opposed to a shared library) there\'s nothing more to do.

More information (and a fine-grained approach for e.g. libraries) is available on the GCC wiki.

回答10:

From the GCC 4.2.1 manual, section -fwhole-program:

Assume that the current compilation unit represents whole program being compiled. All public functions and variables with the exception of main and those merged by attribute externally_visible become static functions and in a affect gets more aggressively optimized by interprocedural optimizers. While this option is equivalent to proper use of static keyword for programs consisting of single file, in combination with option --combine this flag can be used to compile most of smaller scale C programs since the functions and variables become local for the whole combined compilation unit, not for the single source file itself.

回答11:

You can use strip binary on object file(eg. executable) to strip all symbols from it.

Note: it changes file itself and don\'t create copy.