Force if statement to execute only once

Question

I have a piece of code that will be executed many times (5,000+), and an if statement that will be true only the first time. I've thought of using a "FIRST" variable and compare it each time, but it just seems like a waste to check it every single time, even if I know it's not needed.

bool FIRST = true;

void foo(){
if(FIRST){
   /* do something only once */
   FIRST = false;
}
   /* something something... */
}

I also don't know if there is some compiler optimization that does this automatically, or another way to do it; if there is, please let me know.

And yes, I know that a mere if statement isn't a big thing, but it just annoys me.

Maybe you can refactor the code so the first case is not in the loop? — Barmar, Jul 31 '20 at 00:01

score 6 · Answer 1 · answered Jul 30 '20 at 23:47

6

If you're using gcc, there are macros called unlikely and likely that can allow it to make optimizations based on a condition.

In your case the condition will only be true the first time, so you would use unlikely:

if (unlikely(FIRST)) {

answered Jul 30 '20 at 23:47

dbush

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in this case we still need set FIRST to the false? Can't get it how this works ... – Nick S Jul 30 '20 at 23:55
@NickS The rest of the code doesn't change. All this does is tell the compiler the condition will be false most of the time and to optimize for that case. – dbush Jul 30 '20 at 23:58

score 4 · Answer 2 · answered Jul 30 '20 at 23:50

From a compiler optimization point of view, I think an if statement is your best bet, as it will probably compile down to something like a single JNZ, so as long as FIRST stays true, it will be pretty optimized.

You might also want to take a look at this thread

score 3 · Answer 3 · answered Jul 30 '20 at 23:50

3

Make foo and fooFirst, and then call like this

fooFirst();
for (...) {
   foo();
}

foo and fooFirst can share code.

answered Jul 30 '20 at 23:50

Lou Franco

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KamilCuk · Accepted Answer · 2020-07-31T00:04:27.930

3

You can call the function via a function pointer and remove the if check on the second call by changing the function pointer to a function without the if:

void foo_init(void);
void foo_real(void);    
void (*foo)(void) = foo_init;

void foo_init(void) {
   foo = foo_real;
   /* do something only once */
   foo_real();
}

void foo_real(void) {
   /* something something... */
}

int main() { 
    foo();
    foo();
}

edited Jul 31 '20 at 00:04

answered Jul 30 '20 at 23:59

KamilCuk

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Why do you need the test? The function only gets called when `foo == foo_init` – Barmar Jul 31 '20 at 00:03
Och right, so it could be just removed. I think I was thinking to make the function thread-safe - then a mutex in the function body + check `if (foo == foo_init)` is needed. – KamilCuk Jul 31 '20 at 00:03

Force if statement to execute only once

4 Answers4