How can I tell gcc not to inline a function

Question

Say I have this small function in a source file  static void foo        and I build an optimized version of my binary yet I don t want this function inlined  for optimization purposes   is there a macro I can add in a source code to prevent the inlining

User · Answer

A portable way to do this is to call the function through a pointer:

void (*foo_ptr)() = foo;
foo_ptr();

Though this produces different instructions to branch, which may not be your goal. Which brings up a good point: what is your goal here?

User · Answer

I know the question is about GCC  but I thought it might be useful to have some information about compilers other compilers as well   GCC s noinline function attribute is pretty popular with other compilers as well  It is supported by at least    Clang  check with   has attribute noinline   Intel C C   Compiler  their documentation is terrible  but I m certain it works on 16 0   Oracle Solaris Studio back to at least 12 2 ARM C C   Compiler back to at least 4 1 IBM XL C C   back to at least 10 1 TI 8 0   or 7 3  with --gcc  which will define   TI GNU ATTRIBUTE SUPPORT      Additionally  MSVC supports   declspec noinline  back to Visual Studio 7 1  Intel probably supports it too  they try to be compatible with both GCC and MSVC   but I haven t bothered to verify that  The syntax is basically the same     declspec noinline  static void foo void        PGI 10 2   and probably older  supports a noinline pragma which applies to the next function    pragma noinline static void foo void        TI 6 0  supports a FUNC CANNOT INLINE pragma which  annoyingly  works differently in C and C    In C    it s similar to PGI s    pragma FUNC CANNOT INLINE  static void foo void        In C  however  the function name is required    pragma FUNC CANNOT INLINE foo   static void foo void        Cray 6 4   and possibly earlier  takes a similar approach  requiring the function name    pragma  CRI inline never foo static void foo void        Oracle Developer Studio also supports a pragma which takes the function name  going back to at least Forte Developer 6  but note that it needs to come after the declaration  even in recent versions   static void foo void    pragma no inline foo    Depending on how dedicated you are  you could create a macro that would work everywhere  but you would need to have the function name as well as the declaration as arguments   If  OTOH  you re okay with something that just works for most people  you can get away with something which is a little more aesthetically pleasing and doesn t require repeating yourself  That s the approach I ve taken for Hedley  where the current version of HEDLEY NEVER INLINE looks like    if     HEDLEY GNUC HAS ATTRIBUTE noinline 4 0 0         HEDLEY INTEL VERSION CHECK 16 0 0         HEDLEY SUNPRO VERSION CHECK 5 11 0         HEDLEY ARM VERSION CHECK 4 1 0         HEDLEY IBM VERSION CHECK 10 1 0         HEDLEY TI VERSION CHECK 8 0 0          HEDLEY TI VERSION CHECK 7 3 0   amp  amp  defined   TI GNU ATTRIBUTE SUPPORT        define HEDLEY NEVER INLINE   attribute      noinline      elif HEDLEY MSVC VERSION CHECK 13 10 0     define HEDLEY NEVER INLINE   declspec noinline   elif HEDLEY PGI VERSION CHECK 10 2 0     define HEDLEY NEVER INLINE  Pragma  noinline    elif HEDLEY TI VERSION CHECK 6 0 0     define HEDLEY NEVER INLINE  Pragma  FUNC CANNOT INLINE     else    define HEDLEY NEVER INLINE HEDLEY INLINE  endif   If you don t want to use Hedley  it s a single public domain   CC0 header  you can convert the version checking macros without too much effort  but more than I m willing to put in

User · Answer

You want the gcc-specific noinline attribute      This function attribute prevents a   function from being considered for   inlining  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  To keep such calls from being   optimized away  put             asm         Use it like this   void   attribute     noinline   foo

User · Answer

GCC has a switch called  -fno-inline-small-functions  So use that when invoking gcc  But the side effect is that all other small functions are also non-inlined

User · Answer

In case you get a compiler error for   attribute    noinline    you can just try   noinline int func int arg

User · Answer

Use the noinline attribute   int func int arg    attribute    noinline         You should probably use it both when you declare the function for external use and when you write the function

User · Answer

I work with gcc 7 2   I specifically needed a function to be non-inlined  because it had to be instantiated in a library   I tried the   attribute    noinline   answer  as well as the asm     answer  Neither one solved the problem   Finally  I figured that defining a static variable inside the function will force the compiler to allocate space for it in the static variable block  and to issue an initialization for it when the function is first called   This is sort of a dirty trick  but it works

User · Answer

static   attribute     noinline    void foo          This is what worked for me

[c] How can I tell gcc not to inline a function?

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