For loop performance difference, and compiler optimization

Question

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I chose David\'s answer because he was the only one to present a solution to the difference in the for-loops with no optimization flags. The other answers demonstrate what

Answer 1

Processors are complicated. Execution time depends on many things, many of which are outside your control. Just a few possibilities:

a. Your computer probably doesn't have a constant clock speed. It could be that the clock speed is usually set rather low to avoid wasting energy / battery life / producing excessive heat. When your program starts running, the OS figures out that power is needed and increases the clock speed. To verify, change the order of the calls - if the second loop executed is always faster than the first one, that may be the reason.

b. The exact execution speed, especially for a tight loop like yours, depends on how instructions are aligned in memory. Some processors may run a loop faster if it is completely contained within one cache line instead of two, or in two cache lines instead of three. Some compilers will add nop instructions to align loops on cache lines to optimise for this, most don't. Quite possible that one of the loops was aligned better by pure luck and therefore runs faster.

c. The exact execution speed may depend on the exact order in which instructions are dispatched. Slightly different code may run at different speeds due to subtle differences in the code which may be processor dependent, and anyway may be hard for the compiler to consider.

d. There is some evidence that Intel processors may have problems with artificially short loops which may happen only with artificial benchmarks. Your code is quite close to "artificial". There have been cases discussed in other threads where very short loops ran unexpectedly slow, and adding instructions made them run faster.