what is missing

Posted Jan 28, 2006 21:39 UTC (Sat) by ringerc (subscriber, #3071) [Link]

It's not that simple unfortunately. If one was to try to give each function a "score" for inlining, where larger numbers mean better to inline, one might use at least the following rules:

• Function is called very frequently (+, large potential benefit)
• Function is is very small and simple (+, low code bloat)
• Function is called from many different locations (-, high code bloat)

... and I don't doubt there are more factors I've missed, especially when considering cache and register use issues. I'm about as far from an expert as you might find - "not totally clueless" perhaps.

As far as I know the ideal inline function is something like:

int one() { return 1; }

or

int addone(int in) { return in+1; }

or perhaps something slightly more complex that's called from relatively few places. Forcing the inlining of:

void runEventLoop()
{
   // 100 lines of branch-heavy event dispatch code including
   // infrequently used cases, error handling, etc
}

... could to very nasty things to your program's binary size and performance if it's called from many places at all.

Whether to inline a function is not simple to evaluate from static analysis of source code. The compiler does a somewhat better job, and can be given directives to control its inlining behaviour. Given this, it seems sensible to only force inlining where you know the compiler gets it wrong, or where you know it's absolutely critical to inline a function.