Question

Example:

if (almost_always_false_condition) {
       // do something
}

Is there a way to suggest compiler that in 99% condition will be false.
The condition calculation takes ~60 cycles to be checked, and it cannot be calculated at compile time by compiler itself.

(gcc 4.3)

Answer 1

If you want to suggest to GCC that a condition is likely to have a given value as a hint for arranging code flow, you should use __builtin_expect( ):

if (__builtin_expect(almost_always_false_condition,0)) {
   // do something
}

However, it sounds like you want to find a way to avoid evaluating the condition, which __builtin_expect( ) won’t do. Is there a way that you can quickly approximate the condition, and only do the full check when the approximation is true:

if (__builtin_expect(fastCheckThatIsTrueIfFullConditionIsTrue,0)) {
    // most of the time, we don't even get to here, so you don't need
    // to evaluate the condition
    if (almostAlwaysFalseCondition) {
        // do something
    }
}

Can you tell us more about what the condition is?