Combining source code into a single file for optimization

Question

I was aiming at reducing the size of the executable for my C project and I have tried all compiler/linker options, which have helped to some extent. My code consists of a lot of separate files. My question was whether combining all source code into a single file will help with optimization that I desire? I read somewhere that a compiler will optimize better if it finds all code in a single file in place of separate multiple files. Is that true?

Answer 1

A compiler can indeed optimize better when it finds needed code in the same compilable (*.c) file. If your program is longer than 1000 lines or so, you'll probably regret putting all the code in one file, because doing so will make your program hard to maintain, but if shorter than 500 lines, you might try the one file, and see if it does not help.

The crucial consideration is how often code in one compilable file calls or otherwise uses objects (including functions) defined in another. If there are few transfers of control across this boundary, then erasing the boundary will not help performance appreciably. Therefore, when coding for performance, the key is to put tightly related code in the same file.

I like your question a great deal. It is the right kind of question to ask, in my view; and, though the complete answer is not simple enough to treat fully in a Stackexchange answer, your pursuit of the answer will teach you much. Though you may not yet realize it, your question really regards linking, a subject every advancing programmer eventually has to learn. Your question regards symbol tables, inlining, the in-place construction of return values and several, other, subtle factors.

At any rate, if your program is shorter than 500 lines or so, then you have little to lose by trying the single-file approach. If longer than 1000 lines, then a single file is not recommended.