Re: a question about function inlining

[LLeweLLyn Reese <llewelly at lifesupport dot shutdown dot com>]

"Kollschen, Chester (Logica)" <extern.chester.kollschen@volkswagen.de> writes:

> Hello.
> 
> I know the GCC C++ compiler can inline often-called functions automatically
> for performance reasons. I have to set the option -O3 for that. OK, but does
> it do inlining even across libraries?

Not as far as I know. You need to make the function's definition
    availible in the translation unit of the call site.

> 
> Say, I have a class like that:
> 
> class Test {
> public:
> 	Test();
> 	~Test();
> 	void called_very_often();

        inline void called_very_often();

> };
> 
> Test::Test() {
> }
> 
> Test::~Test() {
> }
>
> void Test::called_very_often() {

inline void Test::called_very_often() {

and put into header file.

> 	// one or two statements are placed here
> }
> 
> This class is compiled into a static library called Test.lib or libtest.a.
> When I have a C++-Code like this:
> 
> int main(int argc, char** argv) {
> 	Test * t = new Test();
> 	[...]
> 	t->called_very_often();

Calling the function with a pointer to class type makes no difference
    unless the function is virtual (which may prevent inlining).

> 	[...]
> 	delete t;
> }
> 
> When compiled with -O3, are there chances that the GCC C++ compiler will
> inline the member method called_very_often() of library class Test in main()
> even though it's compiled externally and called via a pointer? If not, what
> could I do to force that?

I don't know, except to write the code inline directly. You can also
    use -Winline, which will generate a warning if a function is not
    inlined. -finline-limit=some_large_number will tell gcc to inline
    larger functions. See:

http://gcc.gnu.org/onlinedocs/gcc-3.3/gcc/Optimize-Options.html#Optimize%20Options
and search for -finline-limit

http://gcc.gnu.org/onlinedocs/gcc-3.3/gcc/Warning-Options.html#Warning%20Options
and search for -Winline