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I habitually write code with lots of functions, I find it makes it clearer. But now I'm writing some code in Fortran which needs to be very efficient, and I'm wondering whether over-using functions will slow it down, or whether the compiler will work out what's going on and optimise?
I know in Java/Python etc each function is an object, and so creating lots of functions would require them to be created in memory. I also know that in Haskell the functions are reduced into each other, so it makes little difference there.
Does anyone know about the case with Fortran? Is there a difference with using intent/pure functions/declaring fewer local variables/anything else?
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In a nutshell: function calls may or may not impact performance. The only way to tell is to profile your code. Don't try to guess where the slow code spots are, because the compiler and hardware have some incredible tricks up their sleeves.
On one hand, functions are great as a concept that makes software more readable a easier to maintain. On the other hand, too much calls to tiny functions doing little work can definitely slow things down.
This can become overhead if the execution time of function is less than the switching time from the caller function to called function (callee). For functions that are large and/or perform complex tasks, the overhead of the function call is usually insignificant compared to the amount of time the function takes to run.
According to the documentation and this MATLAB answer, functions are generally faster than scripts.
Function calls carry a performance cost for stack based languages, like Fortran. They have to add on to the stack and such.
Because of this, most compilers will try to inline function calls aggressively, if it is possible. Most of the time the compiler will make the right choice on whether or not to inline certain functions in your program.
This automatic inlining process will mean that there is no extra cost for writing your function (at all).
This means that you should write your code as cleanly and organized as possible, and it is likely that the compiler will do these optimizations for you. It is more important that your overall strategy for solving the problem is the most efficient than worry about performance of function calls.
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Just write the code in the simplest and most well-structured way you can, then when you have it written and tested you can profile it to see if there are any hotspots which require optimisation. Only at that point should you concern yourself with micro-optimisations, and this may not even be necessary if your compiler is doing its job.
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