When writing programs with code that can be executed in parallel in C, we definitely use the O flags to optimize the code.
gcc -Olevel [options] [source files] [object files] [-o output file]
In large projects, we usually split the code into several files. My question, for which I've found no answer, is this:
Does the program's performance drop at all, due to the fact that we split the code into files and the O flags don't have enough information to optimize any further? Is there such a possibility?
When you break code into separate files, it could potentially split it into more than one translation unit, which the compiler generally can't optimize across.
Take for example a constant defined in one translation unit but referenced in a number of others. All of the calculations that reference the constant have to be performed at run-time since the constant can't be folded into them at compile time.
Link-time optimization (-flto) is one way around the limitation.