Is cutting if statements by using function pointers going to be more efficient?

Question

So, there's this rule to try to pull if statements out of high repetition loops:

for( int i = 0 ; i < 10000 ; i++ )
{
    if( someModeSettingOn )  doThis( data[i] ) ;
    else  doThat( data[i] ) ;
}

They say, it's better to break it up, to put the if statement outside:

if( someModeSettingOn )
  for( int i = 0 ; i < 10000 ; i++ )
    doThis( data[i] ) ;
else
  for( int i = 0 ; i < 10000 ; i++ )
    doThat( data[i] ) ;      

(In case you're saying "Ho! Don't optimize that yourself! The compiler will do it!") Sure the optimizer might do this for you. But in Typical C++ Bullshit (which I don't agree with all his points, eg his attitude towards virtual functions) Mike Acton says "Why make the compiler guess at something you know? Pretty much best point of those stickies, for me.

So why not use a function pointer instead?

FunctionPointer *fp ;
if( someModeSettingOn )  fp = func1 ;
else fp = func2 ;

for( int i = 0 ; i < 10000 ; i++ )
{
    fp( data[i] ) ;
}

Is there some kind of hidden overhead to function pointers? Is it is efficient as calling a straight function?

Answer 1

In this example it's impossible to say which case will be faster. You need to profile this code on target platform/compiler to estimate it.

And in general, in 99% case such code need not to be optimized. It's example of evil premature optimization. Write human-readable code and optimize it only if need after profiling.