Force if statement to execute only once

user_14024743 0
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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.

c
4 Answers

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)) {
user_16871190

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

user_29019520

Make foo and fooFirst, and then call like this

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

foo and fooFirst can share code.

user_39370

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();
}
user_90727530
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