CODEWITHSUNDEEP
cperformanceoptimization
MaiaVictor
MaiaVictor

Reputation: 52957

Is there, as in JavaScript, a performance penalty for creating functions in C?

In JavaScript, there are, often, huge performance penalties for writing functions. For example, if you use this function:

function double(x){ return x*2; }

inside an inner loop, you are probably hitting your performance considerably, so it is really profitable to inline that kind of function for intensive applications. Does this, in general, hold for C? Am I free to create those kind of functions for everything, and rest assured the compiler will do the job, or is hand inlining still important?

Upvotes: 0

Views: 132

Answers (3)

Jeremy West
Jeremy West

Reputation: 12685

The answer is: it depends.

I'm currently using MSVC compiler and GCC for a project at work and my experience is that they both do a pretty good job. Furthermore, the cost of a function call in native code can be pretty small, especially in functions that do not need to be accessible outside the executable (like functions not exported in a shared library). For these functions, there is more flexibility with how the call is actually implemented.

A few things to note: it's much easier for a compiler to optimize calls to static functions. Functions with external linkage often require link time optimization since one must know how and where the function is actually called, as well as the implementation, to do much optimization or inlining. This requires examining more than one compilation unit at a time.

I would say that you should use functions where it makes sense and makes the code easier to read and maintain. In general, it is safe to assume that the cost is smaller than it would be in JavaScript. But in the end, you'd have to profile the code to say anything more precise.

UPDATE: I want to emphasize that functions can be inlined across compilation units, but this requires link-time optimization (or whole program optimization). This is supported in both GCC (https://gcc.gnu.org/wiki/LinkTimeOptimization) and MSVC (http://msdn.microsoft.com/en-us/library/0zza0de8.aspx).

These days, if you can beat the compiler by copying the body of a function and pasting it everywhere you call that function, you probably need a different compiler.

Upvotes: 3

c-smile
c-smile

Reputation: 27460

There are no inner functions in C. Dot. So the rest of your question is kind of irrelevant.

Anyway, as of "normal" functions in C compiler may or may not inline them ( replace function invocation by its body ). If you compile your code with "optimize for size" it may decide to do not do inlining for obvious reason.

Upvotes: 1

merlin2011
merlin2011

Reputation: 75545

In general, with optimizations turned on, gcc will tend to inline short functions provided that they are defined in the same compilation unit that they are called in.

Moreover, if the calling function and called function are in different compilation units, the compiler does not have a chance to inline them regardless of what you request.

So, if you want to maximize the chance of the compiler optimizing away a function call (without manually inlining), you should define the function call in .h file or in the same c file that it is called in.

Upvotes: 2

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