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

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?

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.

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?

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.

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) && 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 ?.

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.

In case you get a compiler error for __attribute__((noinline)), you can just try:

noinline int func(int arg)
{
    ....
}

static __attribute__ ((noinline))  void foo()
{

}

This is what worked for me.