Inline function v. Macro in C -- What's the Overhead (Memory/Speed)?

Question

I searched Stack Overflow for the pros/cons of function-like macros v. inline functions.

I found the following discussion: Pros and Cons of Different macro function / inline methods in C

...but it didn't answer my primary burning question.

Namely, what is the overhead in c of using a macro function (with variables, possibly other function calls) v. an inline function, in terms of memory usage and execution speed?

Are there any compiler-dependent differences in overhead? I have both icc and gcc at my disposal.

My code snippet I'm modularizing is:

double AttractiveTerm = pow(SigmaSquared/RadialDistanceSquared,3); double RepulsiveTerm = AttractiveTerm * AttractiveTerm; EnergyContribution +=     4 * Epsilon * (RepulsiveTerm - AttractiveTerm); 

My reason for turning it into an inline function/macro is so I can drop it into a c file and then conditionally compile other similar, but slightly different functions/macros.

e.g.:

double AttractiveTerm = pow(SigmaSquared/RadialDistanceSquared,3); double RepulsiveTerm = pow(SigmaSquared/RadialDistanceSquared,9); EnergyContribution +=     4 * Epsilon * (RepulsiveTerm - AttractiveTerm); 

(note the difference in the second line...)

This function is a central one to my code and gets called thousands of times per step in my program and my program performs millions of steps. Thus I want to have the LEAST overhead possible, hence why I'm wasting time worrying about the overhead of inlining v. transforming the code into a macro.

Based on the prior discussion I already realize other pros/cons (type independence and resulting errors from that) of macros... but what I want to know most, and don't currently know is the PERFORMANCE.

I know some of you C veterans will have some great insight for me!!

Answer 1

Calling an inline function may or may not generate a function call, which typically incurs a very small amount of overhead. The exact situations under which an inline function actually gets inlined vary depending on the compiler; most make a good-faith effort to inline small functions (at least when optimization is enabled), but there is no requirement that they do so (C99, §6.7.4):

Making a function an inline function suggests that calls to the function be as fast as possible. The extent to which such suggestions are effective is implementation-defined.

A macro is less likely to incur such overhead (though again, there is little to prevent a compiler from somehow doing something; the standard doesn't define what machine code programs must expand to, only the observable behavior of a compiled program).

Use whatever is cleaner. Profile. If it matters, do something different.

Also, what fizzer said; calls to pow (and division) are both typically more expensive than function-call overhead. Minimizing those is a good start:

double ratio = SigmaSquared/RadialDistanceSquared; double AttractiveTerm = ratio*ratio*ratio; EnergyContribution += 4 * Epsilon * AttractiveTerm * (AttractiveTerm - 1.0); 

Is EnergyContribution made up only of terms that look like this? If so, pull the 4 * Epsilon out, and save two multiplies per iteration:

double ratio = SigmaSquared/RadialDistanceSquared; double AttractiveTerm = ratio*ratio*ratio; EnergyContribution += AttractiveTerm * (AttractiveTerm - 1.0); // later, once you've done all of those terms... EnergyContribution *= 4 * Epsilon;