Why is “while (i++ < n) {}” significantly slower than “while (++i < n) {}”

Question

Apparently on my Windows 8 laptop with HotSpot JDK 1.7.0_45 (with all compiler/VM options set to default), the below loop

final int n = Integer.MAX_VALUE;
in         


        

Answer 1

The difference between ++i and i++ is that ++i effectively increments the variable and 'returns' that new value. i++ on the other hand effectively creates a temp variable to hold the current value in i, then increments the variable 'returning' the temp variable's value. This is where the extra overhead is coming from.

// i++ evaluates to something like this
// Imagine though that somehow i was passed by reference
int temp = i;
i = i + 1;
return temp;

// ++i evaluates to
i = i + 1;
return i;

In your case it appears that the increment won't be optimized by the JVM because you are using the result in an expression. The JVM can on the other hand optimize a loop like this.

for( int i = 0; i < Integer.MAX_VALUE; i++ ) {}

This is because the result of i++ is never used. In a loop like this you should be able to use both ++i and i++ with the same performance as if you used ++i.