Gcc keep unused functions. {I try use lib to reduce size, it's still large} .

Gcc keep unused functions This is for functions usd in an inner loop. Is there a better way to remove the unused functions while keeping the ones which are required like compiler calling functions like __stack_chck_fail(). Link with “--gc-sections” to remove unused sections. // Assuming pageptr is unsigned char * already unsigned char *pageptr = ; ((unsigned char volatile *)pageptr)[0] = pageptr[0]; For example, if I compile an application with gcc and let's say none of the symbols/functions of library2 are used. Therefore if garbage collection is enabled and we don't use KEEP to make an exception, that section will not be put in the executable. I'm using arm-none-eabi-g++ with options -Os -ffunction-sections -fdata-sections -Wl,--gc-sections in order to achieve this. On Mon, Aug 19, 2019 at 07:51:38PM +0900, Masahiro Yamada wrote: > GCC and Clang have different policy for -Wunused-function; GCC does > not report unused-function warnings at all for the functions marked > as 'static inline'. cpp (test cpp file) in main function . In your example, the executable will end up containing the code from main. a aren't propagated to the third party binaries--probably because the linker discards them as they're unused. This attribute also has the added benefit that, depending on circumstances, the function might not be emitted at all (it won't use space in the compiled file) if it's never called. Do not warn if a caller of a function marked with attribute warn_unused_result (see Variable Attributes) does not use its return value. but bear in mind that the compiler/linker options are gcc/clang-specific and that they have some minor performance/code-size cost. This attribute, attached to a function, means that the function is meant to be possibly unused. -s: Strip the debug information, so as to make the code as small as possible. – I'd like to know what switch you pass to the gcc compiler to turn off unused variable warnings? I'm getting errors out of boost on windows and I do not want to touch the boost code: C:\boost_1_52_0/ -Wno-unused-function for unused function – Ngo Thanh Nhan. ) I use gcc v4. functions, and some of those will not use all of the parameters, resulting in some compilers emitting "unused parameter" warnings. 8. > > We could use Clang for detecting unused Does gcc 4. c contains two functions, b1 and b2. The IDE simply displays a yellow line underneath the declaration. cpp $ g++ -fmax-errors=N foo. Say I have two files: File a. cpp -fPIC g++34 -shared -o ltop. How i can force getme function exported in ltop. cpp prints an unused variable warning: warning: unused variable ‘x’ [-Wunused-variable] However, the compiler doesn't give an unused-function warning. However, due to the limitation of I'm using GNU GCC linker. One copy is needed and the others can/must be discarded. -fPIC getme is not getting exported in ltop. org. The cast to a void is the best approach because it shows that you didn't "accidentally" keep the variable in your code - ie: this function might be an instance where you have a table of function pointers that need the same parameter types and return types, but in this particular table entry you are not using the parameter. h: static void foo() {} when included in a test. What I wonder is if there is any rationale why some many standard library functions is not declared with this attribute. Just look at it! xunused is a tool to find unused C/C++ functions and methods across source files in the whole project. When a function is declared static, but no other functions from the same translation unit use it, you get the $ gcc main. My function definition has an unused parameter (a pointer to a car struct), which raises a warning. The keyword You could use __attribute__((unused)), which is supported by both gcc and clang. GCC will not produce a warning for this variable. 1, you can keep a function without actually calling it, by calling another function on its address, or branching on a pointer to its address. You can If your toolchain is decently operating, the linker will remove all the unused functions and your code footprint should be the startup() and the interrupt service routines if any. I'm trying to find unused functions in my codebase by using GCC's -Wunused-function flag. Keeps data in separate data sections, so they can be discarded if unused. Since you're compiling with -ffunction-sections, each function will be put into its own section. so do you have any workaround like by making linker fragment file or tweaking some of your build system to realize my requirement. cpp file as follows: #include "unused. Sth like: #if defined(__GNUC__) #define ATTRIBUTE_UNUSED Since this can cause object file-size bloat, you can use -fno-keep-inline-dllexport, which tells GCC to ignore the attribute for inlined functions unless the -fkeep-inline-functions flag is used instead. Output: 0000000000000000 T f_b1 000000000000000b T f_b3 Now run objdump -t file. I found this parameter name '-Wno-unused-function' when I checked the seqtk Makefile, but there is no any informatinon about this on GCC help page via 'man gcc', so what does '-Wno-unused-function' ? the right The compiler generates duplicates definitions and expects the linker to deduplicate them, e. Is there any way to get the info about which objects are not linked in? gcc library1. You could then remove all non mentioned public functions and the static functions referred by them, which most compilers would complain about anyway. For example, the following function in a file unused. The default is -Wunused-result. It is not relevant. The standard allows implementation defined attributes too like the __attribute__ and __declspec ones to be used with the new syntax. In the linking stage, it looks the ELF sections and the unused symbols from libMyPlatform. {I try use lib to reduce size, it's still large} . Here is a full example. -ffunction-sections: Keeps funcitons in separate data sections, so they can be discarded if unused. . unused. It doesn't complain if the same functions are defined static inline or simply inline. gcc warns about unused static functions, but not static inline: is -Wunused-function. My answer followed this rambling path: Arthur O’Dwyer. Compile with -ffunction-sections switch, then link with -gc-sections. In GNU linkers, the feature is called "garbage collection" (ld --gc-sections). Using compiler and linker options to create individual sections for each function and global variable and to throw away unused code would generate a map file with only the referenced symbols. This lets compiler see "whole" program as if it was single source and will probably let it remove unused functions. You can use -ffunction-sections and -fdata-sections on static libraries, which will increase the size of the static library, as each function and global data variable will be put in a separate section. cc, either foo. One reason to inline, is if you know values at compile time (consts are passed in) and you want to flatten the function. Unused functions with static linkage could be removed by the compiler. As I would expect, compiling the below code with gcc -Wall -Wunused-function main. As per the GCC documentation that you have quoted, attribute used is applicable to definitions of static variables. 35. c gcc -shared -Wl, --version-script v. This warning is enabled by -Wall. Use link time optimization (-flto, see gcc manual for details). 25. Since version 4. And then use -Wl,--gc-sections on the program linking with this static library, which will remove unused sections. If you are comfortable hard-coding function decorations for specific architectures, you can use the /INCLUDE directive. prevent gcc from removing an unused variable. ) Note: this will only find unused complete functions, it FreeRTOS has the function vTaskSwitchContext() which is used only in some inline assembler code. GCC does not produce a warning for this variable. A safer alternative is to make it illegal for the compiler to optimize out the store by using the volatile type qualifier. Do we optimize out an unused Clang GCC ICC MSVC; static function-O0-O1-O0 I'm expecting that gcc remove same functions and keep only one from code, but not possible with any type of optimization (-O1, -O2, -O3, -Os, -Ofast, . The compiler generates duplicates definitions and expects the linker to deduplicate them, e. Then with you linker script you can place it somewhere else. Compiling with -Wall -Wextra -pedantic should have triggered a warning if it can be activated but it doesn't, so it cannot. Also note that you can use __asm__ instead of asm. This flag allows coarse control of this limit. Lui. h file that exports the So we can make a header file which include all functions in that library and then make single c files containing single functions . Another type of dead code elimination that GCC can do the removal of entire unused symbols (functions or variables) which can be achieved with:-fdata-sections -ffunction-sections -Wl,--gc-sections as mentioned at: How to remove unused C/C++ symbols with GCC and ld? These flags are not enabled in the various GCC -O levels (-O1, -O2, etc) by default. +1 for pointing out this issue – No, in general the executable only contains the functions that are needed for execution. This pruning has several benefits: Edit: I am able to use the above options for gcc to remove the unused functions if I am linking a static library directly to executable. The problem with using cout in the instrumentation function is that the instrumentation function is being called by __libc_csu_init() which is a very early part of the runtime's initialization - before global C++ objects get a chance to be constructed (in fact, I think __libc_csu_init() is responsible for kicking off those constructors - at least indirectly). c:8:5: warning: ignoring return value of ‘foo’, declared with attribute warn_unused_result [-Wunused-result] 8 | foo(); | ^~~~~ This is as expected. 1 Common Function Attributes ¶. Issue preventing GCC from optimizing out global variable. -Wl,–gc-sections: Tell the linker to garbage collect and discard unused sections. The compiler will strip out anywhere from all to none of the unused functions, including the wno_unused_myheadername, since it can see all of the definitions and can probably determine that the If the compiler can detect no use of the global, it can omit them. No, you can only tell gcc to ignore all warn_unused_result flags with -Wno-unused-result but the respective -Wunused-result only sets the default (to warn only on flags). GCC does not produce a warning for this function. My IDE doesn't have a "Keep symbols" textbox (or "Input" tab, for that matter). GCC will warn about an unused internal function with the -Wunused-function option which is also enabled in one the groups: -Wall, Please keep Normally, the compiler warns if a variable is declared but is never referenced. A file-level declaration of a function or variable in a compilation unit means that some other compilation unit might reference it. EDIT2: you may not For the case of unused whole functions (and unused global variables), GCC can actually do most of the work for you provided that you're using GCC and GNU ld. The compiler always unused This attribute, attached to a function, means that the function is meant to be possibly unused. I can only find the function in the static library throught "nm", whose label is "T". 3. The problem is: When I use LTO, GCC does not consider the inline assembler code and thinks the function is not used, thus removes it. Obviously, the function unused() is never called. The --gc-sections option in GCC can trim unused functions and variables during linking. The problem is clear. 1 will remove unused static and unused static inline functions over at godbolt. a top. e. Windows only support PE format while Linux supports ELF format, allowing Linux to use Gold linker. I would like to remove all unused symbols from my compiled C++ binary. For instance if a function is used in two different places with different static parameters, inlining can reduce branches. If there is a specific function in the static library that is stripped by the linker as unused, but you really need it (one common example is JNI_OnLoad() function), you can force the linker to keep it (and naturally, all code that is called from this function). In any case, when the compiler finds an unreachable location, it can optimize Like in GCC, for example, you can declare the function with __attribute__((unused)) an the warning for this function will no longer be issued. I'm now 60% certain that this is some kind of optimizer bug. Both appear to work in GCC. a to be visible in the final binary that's produced? If you try to build bigger projects with -flto you have to make sure that you use a version of binutils that supports gcc's liblto_plugin. cpp Mar 26, 2014 · functions, and some of those will not use all of the parameters, resulting in some compilers emitting "unused parameter" warnings. ==&gt; http I used printme() function in test. c -o include. used as per above example I would like to retain the function someFunc, I do use __used attribute for the compiler to emit the object code but due to linker --gc-sections option the emitted code section . The format attribute allows you to identify your own functions which take format strings as arguments, so that GCC can check the calls to these functions for errors. c include. But when you keep the init section in the linker : "KEEP(*(. 9. Unfortunately, I'm not aware of a way to get those compilers to apply this to all functions globally, you have to do it explicitly on a case-by-case basis. 1 with LD 2. Also after using --keep=foo_*. To control this optimization use the --remove, --no_remove, --first, --last, and --keep linker options. GNU binutils on Linux) are managing entire object files, not individual functions. volatile stops the compiler from removing the empty assembly. o for fuller information about the symbols. keep. If a compiler doesn't recognize an attribute (a gcc attribute when compiling on MSVC as example) it'll simply be Linux gcc did successfully strip away iostream and functions without the need for -flto while Mingw-w64 gcc just can't do it properly. , -Wno-error=unused-variable) works as expected (reported as warning only). But it only works if I do not make a copy of any members of the struct. gcc supports a new way, that does this splitting automatically. Since gc-section option was removing the crtbegin functions such as __do_global_dtors_aux call__do_global_dtors_aux,frame_dummy, call_frame_dummy etc. c Run nm file. init))", then those function are kept while gc-sections option clears other unused This function attribute prevents a function from being considered for inlining. 04, Binutils 2. so -ltop -L. c; gcc; shared-libraries; static-libraries; dead-code; Share. But I can't find it in the application. o include. In practise, I don't think 6. It looks like this removes some (but not all) unused functions, especially some heavy one. However, I am concerned that when I do use car in the future, I will forget to delete this dummy statement, causing some very nasty Note that having this label might affect how GCC optimizes your function. They could remove unused virtual functions, but the compiler would have to add a lot of information about which virtual functions it was calling so that the linker could remove unused ones (and possibly compact the vtable). and #if 0 can be used to directly ignore a function from executing. cpp file (translation unit). cpp but I do not know enough about Assembly to interpret the result with certainty - the resulting file was tiny and I was unable to recognize the function calls, because the code of the static functions of all policies was inlined. In practise, I don't think linkers do this because the gain is probably small and the amount of development effort required rather large. So, to repeat the link, to remove the dead/unused functions: Compile with “-fdata-sections” to keep the data in separate data sections and “-ffunction-sections” to keep functions in separate sections, so they (data and functions) can be discarded if unused. I saw this, which gives an overview using gcc, which is the toolchain I'm using: How to remove unused C/C++ symbols with GCC a Removal of unused functions with external linkage is necessarily a function of the linker rather then the compiler. – Target-san. add a dummy mov keep, %eax; remove --gc-sections; The output goes back to 1. The linker then fails because the function call in the inline assembler code cannot be resolved. This puts each function into it's own section and then tells linker to remove unused sections. you can just ignore them using conditional preprocessor directives. so . During compilation and linking, the compiler and linker create a symbol table. This warning seems to trigger both when a function is never used and when a This time around, unused function foo() is still present, but bar() is not. This warning must have been turned on either explicitly by passing -Wunused-parameter to the compiler or implicitly by passing -Wall -Wextra (or possibly some other combination of flags). Then I link the two objects into a shared library. GCC 10 in august 2021) and accept to spend a lot more computer time to build your software you might compile and link your application It depends on the unused stuff. Hence that causes to crash of the program. cube, on the other hand, is static - so if it's used anywhere, it must be in this translation unit. Stuff mostly about C++. Particularly when moving packed data between functions compiled with GCC and the native Microsoft compiler (either via From what I know there is currently no tool that does all the things you have mentioned, however there is one that helps in cleaning up the unused include headers: include-what-you-use "Include what you use" means this: for every symbol (type, function variable, or macro) that you use in foo. In order to handle archives of these objects you have to use the gcc wrappers: gcc-ar, gcc-nm and gcc-ranlib. If a function is static it can only be used within the current file, so its definition must also be in Delving into the assembly listing, it seems that GCC compiles and calls the empty functions when the optimizations are disabled. Thus, the final binary will be smaller than without those flags. There is no warning; adding the attribute has no effect. so it is not showing getme symbol This function attribute prevents a function from being considered for inlining. Improve this answer. I tried examining the assembly code produced by gcc with the optimization turned on: g++ -S -O3 -std=c++11 main. $ g++ -O2 -march=native deadcode. Till now our company used GCC based compilers and we didn't need to include conditional compilation for this purpose. Commented Oct 16, The reputation requirement helps protect this question from I'm writing some code that's going to be used on an embedded device so I want to keep the code size down by having the linker discard some of the third-party library functions I'm not using. That way, I could simply keep a list of files (one per application I build) and pass the major versions of gcc from 4. The command used are like: gcc -c a. The one I thought is the solution -Wno-error=unused unfortunately doesn't seem to work (they are still reported as errors), Individually setting the flags (e. Enabled at levels -O2, -O3 and -Os. Anyway, unless there is an option to keep ALL the symbols in the output, Yes, use [[gnu::unused]]. c (main. Therefore, if there is no entry point specified for an image, use the --entry linker option to specify an entry point and permit unused section elimination to work, The linker can only eliminate a section if it is entirely unused. access (access-mode, ref-index) ¶access (access-mode, ref-index, size-index)The access attribute enables the detection of invalid or unsafe accesses by functions to which they apply or their callers, as well as write-only accesses to objects that are never read from. c contains two functions, a1 and a2; File b. 9 gcc produces slim object files that only contain the intermediate representation. If a direct copy is made and passed to a function, the compiler fails to optimize unused functions. o gcc -flto -Os test. If you try =r instead of +r, gcc will optimize it away but icc will keep it. For unused functions that I don't want to see a warning from, should I be specifying the attribute on . On macOS, ld64 calls it "dead stripping" (ld -dead_strip). I've tried using the -ffunction-sections and -fdata-sections options along with -Wl,--gc-sections but unused functions remain. There is no reference to the symbol keep, and therefore its containing section . Since it isn't used here, the compiler (at least gcc with -Wunused-function) warns about this Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Visit the blog In the days before C89, most C compilers would ignore any unused arguments, and would allow callers to omit arguments associated with unused parameters. If you use a recent GCC (e. vector_size (bytes) This attribute specifies the vector size for the variable, measured in bytes. I want to keep this parameter for future use, so to suppress the warning, I added the dummy statement (void) car; to the very beginning of the function. When compiling the source, you can also skip the --print-gc-sections part and let the linker remove the functions silently, but keep them in the source. Here: The fact that the MSP430 Linker is clever enough to remove individual unused functions from an object file is an enhancement to other linkers. The following attributes are supported on most targets. 0 and later $ g++ -Wfatal-errors foo. So the warning should only be given for a static function, interesting. And as an answer that is now deleted by the author pointed out, your ApplicationID is not static, so attribute used has no effect. a to be visible in the final binary that's produced? will not only suppress the warning about the unused function, but the two warnings about the arguments and the warning about the variable as well. First off, GCC only supports warn_unused_result since 3. +r means the datum is both read from and write to by the assembly, so compiler can not optimized it out. However, when in doubt the compiler errs on the safe side, this means there is a good chance your executable carries a lot of unnecessary code, this is mostly when either the compiler/linker isn't sure if some code is needed, or when it's unable to split off the unneeded I have an application linked with a few static libraries. 3 and here is very minimal code: How to discard unused function after optimization in GCC. Now, -Wunused-function is useful to detect static functions which are no longer invoked, meaning they are useless, and should therefore will not only suppress the warning about the unused function, but the two warnings about the arguments and the warning about the variable as well. unused This attribute, attached to a function, means that the function is meant to be possibly unused. J. Specifying each, of these to keep is cumbersome. Tell the linker to garbage collect and discard unused sections. Improve this question. 4) of gcc warns about unused static functions, even in a header file when -Wall is used. Share. 31. map -Wl, --strip-all -o mydll By default, linkers handle object files as a whole. Turning off optimization fixes the problem, but it is unnecessary. However, the volatile keyword will probably help prevent it from doing anything that would cause problems. How can I force the symbols from libMyPlatform. - static unused functions does not get into the objectfile, so they won't appear in the programm => Best way in this case is to split files to be as small as possible (often one function per file). 1 all the way to 11. In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully. When the --gc-sections option is enabled, the linker will analyze the symbol table to determine which code and data are not used, and then remove them from the final output. cpp file). someFunc is removed. Follow edited Jun 15, 2018 at 19:57. cc or foo. Like already said unused isn't part of the standard attributes specified by the standard. cpp -o deadcode I will focus on “standard” tools running on Linux, mostly tools from GCC and binutils. How different compilers deal with provably unused entities. #if and #ifdef can be used to ignore a function. The compiler doesn't know about these other places, so it cannot flag the function as unused. So cout gcc -c -o file. And move or omit the section with a linker script. SDCC can then eliminate the unused functions as they are in separate modules [enables] garbage collection of unused input sections. Linkers are quite likely to remove non-virtual member functions (it's easy and saves space). I'd be surprised if it can find dead code across multiple compilation units. o gcc -c -Os -flto test. It will not warn about unused functions with external linkage, though, since that would make it impossible to write a library. No matter what I do, GCC will always keep void Foo::bar() and hello around. c main. Anybody can write an example for a several minutes, and check that GCC with -Os, -fdata-sections -ffunction-sections, and -Wl,--gc-sections remove unused free functions and methods of non-virtual classes, but can't remove methods (even non-virtual) from virtual classes (= classes that have at least one virtual method). Makes sense. o -o test You never declare static functions in a header file intended for use in other modules, because the purpose behind making a function static is "hiding" it from users outside your modules. code after a return from function), rather than unused functions Quoting from gcc warning options page:-Wno-unused-result. Warn whenever a static function is declared but not defined or a non-inline static function is unused. GCC (G++ in this context) can and does omit some variables - for example, the old 'embed a version in the object file' tricks do not work with modern GCC - the file static value is detected as unused and optimized out of the object file. virtual void myFunc(int& a, int& b) { a; // unused variable b; } However, to support other compilers and to make the intent clear, you can use a macro or function unused_argument to make it clear. (This requires, for The +r modifier and the volatile modifier in the assembly is essential. You can verify this by using: readelf --syms --dyn-syms <your -fvisibility=hidden makes it so that all functions are hidden by default, and I manually mark public functions with __attribute__((visibility("default"))). Hot Network Questions Gather on first list, apply to second list This the global solution to a local problem, since you are now preventing the discard of any unused function, even though all you care about is one specific function. I'm looking a way to not raise "unused function" warning only for functions with specific name and not for the rest of file completely. Unused section elimination requires an entry point. Disable GCC simplification of expressions known at compile time. GCC also supports attributes on variable declarations (see Specifying Attributes of Variables), labels (see Label Attributes), enumerators (see Enumerator Attributes), statements (see Statement Attributes), types (see Specifying Attributes of Types), and on field you do not have any pragma directives for ignoring a function from executing. The main function is empty. To keep such calls from being optimized away, put asm (""); Since std::string is a template class (std::string is only a typedef to std::basic_string<char>), only the used methods will be instantiated, so no unused methods will ever be compiled and so they can't be removed from your executable. It is built upon clang to parse the source code (in parallel). c: In function ‘main’: main. But the trick I'm using is, which I found more readable is: #define UNUSED(x) (void)x; It works however only for the variables, and arguments of the methods, @inetknght: From a behaviour point of view there are only two ways of not retruning: loop forever or abort execution. 3-x, by default only So, to remove unused functions, you need: Compile with “-fdata-sections” to keep the data in separate data sections and “-ffunction-sections” to keep functions in separate sections, so they (data and functions) can be discarded if unused. o files with the -ffunction-sections -fdata-sections options enabled, and link the final program with the --gc-sections option. Link with “–gc-sections” to remove unused sections. ver -Wl, -Map=out. In short, we keep wanted unused functions and removed unwanted functions into ELF image. It just returns the return code of 0, nothing more. Commented Apr 28, 2015 at 10:32. o library2. gcc doesn't flag these warnings by default. I did inlining and it improved performance 30% in benchmark. a (which is an archive of object files) required to Because our need functions are complicated, when compile as position independent, the binary code size will be very large. If the optimizations are enabled, also with -O2 and -O3, it compiles only the necessary stack handling code, but it optimizes out the call instructions. Unused parameter warnings can simply be suppressed by passing -Wno-unused-parameter to the compiler, but note that this disabling The +r modifier and the volatile modifier in the assembly is essential. There is no better explanation then in GCC documentation about Variable Attributes: unused. o to list the symbols. 7. Keeps funcitons in separate data sections, so they can be discarded if unused. GCC In the linking stage, it looks the ELF sections and the unused symbols from libMyPlatform. c // GCC 4. excl section. -Wunused-function Warn whenever a static function is declared but not defined or a non-inline static function is unused. o main. c -o test. If it has internal linkage and is thus not visible outside the current translation unit (. These attributes are not currently implemented for Objective-C. Validation of proper calling conventions is generally useful, but some existing APIs may require that functions be passed varying numbers and types of arguments. Explanation. Several compilers support the GCC-style function attribute, and may or may not define __GNUC__ and friends: Clang (check with __has_attribute(warn_unused_result) How can we keep each pair of contours and removing others? Grounding a 50 AMP circuit for Induction Stove Top At least GCC should be able to "devirtualize" those function calls, since the dynamic type of the object is known, but without something like -fwhole-program it's unlikely to discard unused virtual functions, because another translation unit might need their definitions. h" If you use link time optimization, compiler should be able to remove unused functions, even if you put all of them into common section. c gcc -c b. h test. Recent Date Tag About. Anything at global scope (and that includes most functions) can only be removed by the linker, and the granularity of most linkers is the object file, so as soon as the object file is incorporated into your program, you get everything that is in it. Follow Analysing the ELF file in run-time, since not every function will be called in every run such as gprof (some functions take days until they are called, but in the code flow you can see that they are eventually called) A utility that discovers dead-code inside functions (i. And the dynamic library will call a function which came from one of the static library. #ifdef checks whether a MACRO is defined or not. Regarding non-template classes: virtual functions will always end up in the executable, regardless whether they are C++17 introduces the attribute [[maybe_unused]]. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Visit the blog How can I have gcc remove the unused functions? EDIT: I was able to sort of make it work as you see above. if a C function declared 'static inline' ends > up inlined everywhere), but the dependency analysis is very > conservative and AFAIK setting -Os does not change anything about it. – it makes sense that taking the address of a function does not allow unused parameters to get optimized away, because the function pointer is typed to the function signature, (even when you cast it to void*) and the compiler can't in general be sure if you use it again by some strange magic. In linker script I created additional section . This won't function with assembler files, but in c-files it seems to be ok. My goal is to fill unused Flash memory space so that I will always get a binary output in the same size for CRC calculation. EDIT 2: Let me try this one more time. use the gcc compiler flags -Wunused and -Wunreachable-code, and then use a tool like lcov to find the unused methods. Is The other day, I was asked whether we should worry about the space taken up by unused functions in our codebase. This batch is pretty large, and includes the specific warning -Wunused-function. so When i do nm ltop. I assume this a standardized version of GCC and Clang's: __attribute__((unused)). Here are all of the methods I have found so far for suppressing these: This first set of Functions with these attributes are useful for initializing data that will be used implicitly during the execution of the program. Static C functions are visible only inside the translation unit * where they are defined. excl"))); int f3(void) { } These tell GCC to compile f3 into . So the linker can only remove this entire section if all functions in this section are unused. In theory compiler could automatically generate KEEP statements based on used attributes but this isn't done for historical reasons. 4. I'm not 100% sure if this works with multi-file projects, but you could always give it a shot! yes there will be They could remove unused virtual functions, but the compiler would have to add a lot of information about which virtual functions it was calling so that the linker could remove unused ones (and possibly compact the vtable). Here are all of the methods I have found so far for suppressing these: This first set of Perform interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters passed by reference by parameters passed by value. Both compiler and linker may remove functions which they consider to be unused (which usually means not reachable from main) so to preserve a function you need to inform both of the tools. o -o main. The compiler can detect some unused code and data for point 1 (e. The same restrictions and effects apply to function attributes used with ordinary functions or C++ member functions. In the Linux kernel source code, some unused functions and variables are marked __atrribute__((unused)). The linker will discard all the function never More optimization might enable it to > eliminate more code (e. g++34 -c top. Then compile all individual C files. Follow The __root attribute can be used on either a function or a variable to ensure that, when the module containing the function or variable is linked, the function or variable is also included, whether or not it is referenced by the rest of the program. h should #include a . If you declare a global variable or function that is unused, gcc will optimized it out (with warning), but if you declared the global variable or the function with '__attribute__ To prevent this, you may mark your function with __attribute__((used)) in your coverage builds. All in all it keeps the function. Note that since foo isn't being referenced in the main path, g++ correctly ignores the function entirely but gcc does not. The compiler can't in general check for unused functions, as functions may be used outside the current . The function foo, and the subsequent call to pthread_setaffinity_np, is unused. In gcc and clang, you can mark specific functions with the warn_unused_result attribute and it will let you know if you ignore the result (assuming you're not using the -Wno-unused-result flag, of course). Visual Studio equivelent of gcc __attribute__((unused)) in c++11 (or lower)? 8 Why do most SAS troops keep wearing their new red berets even after being given permission to use their old FreeRTOS has the function vTaskSwitchContext() which is used only in some inline assembler code. text. When using gcc or clang, it's generally a good idea to enable a number of warnings, and a first batch of warnings is generally provided by -Wall. However a linker is not required to support that and any support is toolchain dependent and may require specific link-time optimisation switches to be applied. You can use __attribute__((used)) gcc (also works in clang) specific (I see that the question is tagged gcc) attributes for this: This attribute, attached to a function, means that code must be emitted for the function even if it appears that the function is not referenced. o it is showing so many 8 bytes removed, though they are being called from caller functions. GCC will not produce a warning for this function. 1 How to prevent vector operation optimization over this function with arm-none-eabi-gcc compiler? 3 unused This attribute, attached to a variable, means that the variable is meant to be possibly unused. This does have an impact, notably by preventing certain categories of Compile with “ -fdata-sections ” to keep the data in separate data sections and “ -ffunction-sections ” to keep functions in separate sections, so they (data and functions) can be In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully. The keyword I need some help for compiling with GCC under MinGW. Why the linker does not remove unused symbols when linking a static library before shared? Because most linkers (e. asked The program does not do much. o g++34 -c test. Remove unused functions from a library: The major issue with removing unused functions in libraries is that the library object files are already compiled, and usually the compiler will pack all functions of one module into a common section. Besides that, I wonder why you would want this, it is not that uncommon to Just one small thing, better using __attribute__((__unused__)) as __attribute__((unused)), because unused could be somewhere defined as macro, personally I had a few issues with this situation. cpp -fPIC ar rcs libtop. Output: The functions with "hidden" attribute are removed only from "symtab", but they can still be visible in the "dynsym" table. – GCC will help you find dead code within a compilation. It then shows all functions that had a definition but no use. function declarations? void maybe_used_function() [[maybe_unused]]; or function definitions? Can anyone explain the working of unused and used variable attributes in gcc. And they all warn about unused for static functions, but none warn for unused static inline. To keep such calls from being optimized away, put asm (""); I'm compiling a code for an embedded ARM microcontroller and I'm trying to optimize the binary size. If that does not works with your compiler, I think that doing sizeof(a); would works. gcc -Wall will warn you about unused static functions and some other types of unreachable code. Also, unspecified storage class functions will remain if unused, without warning (even with optimizations). Clang does report unused-function warnings if they > are defined in source files instead of headers. -finline-limit=n ¶ By default, GCC limits the size of functions that can be inlined. elf I know that gcc has the compiler and linker flags to remove unused symbols: My version (5. > I don’t actually know if GCC is capable of dropping unused static > variables at all, by the way. o file. builtin_unreachable does one of those things (I think which one is undefined) and tells it to the compiler in case it's not smart enough to find out from static analysis. Adding -Wl,--gc-sections to your final gcc invocation will remove the unused baz section. However, hidden functions are not removed unless marked as static (which I can't do to C++ methods, obviously). With --split_sections I can remove the functions but not the variables. informs you about unused code, such as classes, functions, variables, etc. Shouldn't the "used" tag alone be enough? It is not sufficient, and it is not necessary. And this application will load a dynamic library through dlopen. inline functions, implicit template instantiations, vtables, type_info objects. Jan 7, 2021 · 有时候我们编译一个大的项目的时候，会出现很多错误使得屏幕堆满了很多无用的信息。一般情况下我们需要找到首次出现错误的地方，在gcc中添加编译选项可以使编译停止在第一次出现错误的地方：$ gcc-Wfatal-errors foo. So keep modules (let's say include. If the function does not have side-effects, there are optimizations other than inlining that causes function calls to be optimized away, although the function call is live. How to discard unused function after optimization in GCC. g. Let’s compile it with optimizations turned on. One possible solution would be to move your "library" functions out of the "Executable" (main) CCS project and into a "Static Library" CCS sub-project. Schengen Visa - Purpose vs Length of Stay Los Angeles Airport Domestic to International Transfer in 90mins @user694733 ARM platform has better support in terms of tools. int f3(void) __attribute__((section(". On Windows, the documentation says With GCC, build your . So, the solution is to apply the warn_unused_result attribute on the function. Hide it behind a macro. Strip the debug information, so as to make the For example, in GCC 7. 0. My current solution Visual Studio equivelent of gcc __attribute__((unused)) in c++11 (or lower)? Why do most SAS troops keep wearing their new red berets even after being given permission to use their old beige ones? -Wno-unused of course doesn't display any warning, so it is not what I need. -Wunused). However, due to the limitation of My project uses small amount of the interface functions and I want to remove all the unused functions including the unused static variables. Header file must be included in each C file so that we can call other functions of same library. But it doesn't remove the unused functions if I link the static library to a shared library. This attribute, attached to a variable, means that the variable is meant to be possibly unused. c), but build like this: gcc -c -Os -flto include. lots of unused functions(we don't need) included. Tested on Ubuntu 14. o), and any object files from libmine. If you use GCC, you can put your function into a seperate section. gxsbh dwjvkb gzgyv mjakmy ridzyn pmha tpnk jlpbahec ali ukrzs