Question

This is probably going to language dependent, but in general, what is the performance difference between accessing and writing to an array?

For example, if I am trying to write a prime sieve and am representing the primes as a boolean array.

Upon finding a prime, I can say

for(int i = 2; n * i < end; i++)
{
    prime[n * i] = false;
}

or

for(int i = 2; n * i < end; i++)
{
    if(prime[n * i])
    {
        prime[n * i] = false;
    }
}

The intent in the latter case is to check the value before writing it to avoid having to rewrite many values that have already been checked. Is there any realistic gain in performance here, or are access and write mostly equivalent in speed?

Answer 1

In general this depends much more on the machine than the programing language. The writes often will take a few more clock cycles because, depending on the machine, more cache values need to be updated in memory.

However, your second segment of code will be WAY slower, and it’s not just because there’s “more code”. The big reason is that anytime you use an if-statement on most machines the CPU uses a branch predictor. The CPU literally predicts which way the if-statement will run ahead of time, and if it’s wrong it has to backtrack. See http://en.wikipedia.org/wiki/Pipeline_%28computing%29 and http://en.wikipedia.org/wiki/Branch_predictor to understand why.

If you want to do some optimization, I would recommend the following:

• Profile! See what’s really taking up time.
• Multiplication is much harder than addition. Try rewriting the loop so that i += n, and use this for your array index.
• The loop condition “should” be totally reevaluated at every iteration unless the compiler optimizes it away. So try avoiding multiplication in there.
• Use -O2 or -O3 as a compiler option
• You might find that some values of n are faster than others because of cache locality. You might think of some clever ways to rewrite your code to take advantage of this.
• Disassemble the code and look at what it’s actually doing on your processor