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.
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.