How to remove unused C C symbols with GCC and ld

Question

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     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

User · Accepted Answer

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

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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

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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

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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

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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

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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  lt stdio h gt  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

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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

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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

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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

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You ll want to check your docs for your version of gcc  amp  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

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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

[c++] How to remove unused C/C++ symbols with GCC and ld?

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