Can I selectively (force) inline a function?

Question

In the book Clean Code (and a couple of others I have come across and read) it is suggested to keep the functions small and break them up if they become large. It also suggests that functions should do one thing and one thing only.

In Optimizing software in C++ Agner Fog states that he does not like the rule of breaking up a function just because it crosses a certain threshold of a number of lines. He states that this results in unnecessary jumps which degrade performance.

First off, I understand that it will not matter if the code I am working on is not in a tight loop and that the functions are heavy so that the time it takes to call them is dwarfed by the time the code in the function takes to execute. But let's assume that I am working with functions that are, most of the time, used by other objects/functions and are performing relatively trivial tasks. These functions follow the suggestions listed in the first paragraph (that is, perform one single function and are small/comprehensible). Then I start programming a performance critical function that utilizes these other functions in a tight loop and is essentially a frame function. Lastly, assume that in-lining them has a benefit for the performance critical function but no benefit whatsoever to any other function (yes, I have profiled this, albeit with a lot of copying and pasting which I want to avoid).

Immediately, one can say that tag the function inline and let the compiler choose. But what if I don't want all those functions to be in a `.inl file or exposed in the header? In my current situation, the performance critical functions and the other functions it uses are all in the same source file.

To sum it up, can I selectively (force) inline a function(s) for a single function so that the end code behaves like it is one big function instead of several calls to other functions.

Answer 1

There is nothing that prevents you to put inline in a static function in a .cpp file.

Some compilers have the option to force an inline function, see e.g. the GCC attribute((always_inline)) and a ton of options to fine tune the inlining optimizations (see -minline-* parameters).

My recommendation is to use inline or even better static inline wherever you see fit, and let the compiler decide. They usually do it pretty well.

Answer 2

You cannot force the inline. Also, function calls are pretty cheap on modern CPUs, compared to the cost of the work done. If your functions are large enough to need to be broken down, the additional time taken to do the call will be essentially nothing.

Failing that, you could ... try ... to use a macro.

Answer 3

No, inline is a recommendation to the compiler ; it does not force it to do anything. Also, if you're working with MSVC++, note that __forceinline is a misnomer as well ; it's just a stronger recommendation than inline.

Answer 4

This is as much about good old fashioned straight C as it is about C++. I was pondering this the other day, because in an embedded world, where both speed and space need to be carefully managed, this can really matter (as opposed to the all too oft "don't worry about it, your compiler is smart and memory is cheap prevalent in desktop/server development).

A possible solution that I have yet to vet is to basically use two names for the different variants, something like

inline int _max(int a, int b) {
    return a > b ? a : b;
}

and then

int max(int a, int b) {
    return _max(a, b);
}

This would give one the ability to selectively call either _max() or max() and yet still having the algorithm defined once-and-only-once.

Answer 5

Inlining – For example, if there exists a function A that frequently calls function B, and function B is relatively small, then profile-guided optimizations will inline function B in function A.

VS Profile-Guided Optimizations

You can use the automated Profile Guided Optimization for Visual C++ plug-in in the Performance and Diagnostics Hub to simplify and streamline the optimization process within Visual Studio, or you can perform the optimization steps manually in Visual Studio or on the command line. We recommend the plug-in because it is easier to use. For information on how to get the plug-in and use it to optimize your app, see Profile Guided Optimization Plug-In.

Answer 6

If you have a known-hot function an want the compiler inline more aggressively than usual the flatten attribute offered by gcc/clang might be something to look into. In contrast to the inline keyword and attributes it applies to inlining decisions regarding the functions called in the marked function.

__attribute__((flatten)) void hot_code() {
    // functions called here will be inlined if possible
}

See https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html and https://clang.llvm.org/docs/AttributeReference.html#flatten for official documentation.

Answer 7

Compilers are actually really really good at generating optimized code.

I would suggest just organizing your code into logical groupings (using additional functions if that enhanced readability), marking them inline if appropriate, and letting the compiler decide what code to optimally generate.

Answer 8

Quite surprised this hasn't been mention yet but as of now you can tell the compiler (I believe it may only work with GCC/G++) to force inline a function and ignore a couple restrictions associated with it.

You can do so via __attribute__((always_inline)).

Example of it in use:

inline __attribute__((always_inline)) int pleaseInlineThis() {
   return 5;
}

Normally you should avoid forcing an inline as the compiler knows what's best better than you; however there are several use cases such as in OS/MicroController development where you need to inline calls where if it is instead called, would break the functionality.

C++ compilers usually aren't very friendly to controlled environments such as those without some hacks.