Java Math.min/max performance

Question

EDIT: maaartinus gave the answer I was looking for and tmyklebu\'s data on the problem helped a lot, so thanks both! :)

I\'ve read a bit about how HotSpot has some \

Answer 1

It's hard to tell why Math.max is slower than a Ops.max, but it's easy to tell why this benchmark strongly favors branching to conditional moves: On the n-th iteration, the probability of

Math.max( array[i], max );

being not equal to max is the probability that array[n-1] is bigger than all previous elements. Obviously, this probability gets lower and lower with growing n and given

final int[] array = new int[(8*1024*1024)/4];

it's rather negligible most of the time. The conditional move instruction is insensitive to the branching probability, it always take the same amount of time to execute. The conditional move instruction is faster than branch prediction if the branch is very hard to predict. On the other hand, branch prediction is faster if the branch can be predicted well with high probability. Currently, I'm unsure about the speed of conditional move compared to best and worst case of branching.¹

In your case all but first few branches are fairly predictable. From about n == 10 onward, there's no point in using conditional moves as the branch is rather guaranteed to be predicted correctly and can execute in parallel with other instructions (I guess you need exactly one cycle per iteration).

This seems to happen for algorithms computing minimum/maximum or doing some inefficient sorting (good branch predictability means low entropy per step).

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¹ Both conditional move and predicted branch take one cycle. The problem with the former is that it needs its two operands and this takes additional instruction. In the end the critical path may get longer and/or the ALUs saturated while the branching unit is idle. Often, but not always, branches can be predicted well in practical applications; that's why branch prediction was invented in the first place.

As for the gory details of timing conditional move vs. branch prediction best and worst case, see the discussion below in comments. My my own benchmark shows that conditional move is significantly faster than branch prediction when branch prediction encounters its worst case, but I can't ignore contradictory results. We need some explanation for what exactly makes the difference. Some more benchmarks and/or analysis could help.