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A leap year check in three instructions

(hueffner.de)
420 points gnabgib | 1 comments | 15 May 25 21:57 UTC | HN request time: 0.206s | source
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drewg123 ◴[15 May 25 22:47 UTC] No.44000124[source]▶
I tend to be of the opinion that for modern general purpose CPUs in this era, such micro-optimizations are totally unnecessary because modern CPUs are so fast that instructions are almost free.

But do you know what's not free? Memory accesses[1]. So when I'm optimizing things, I focus on making things more cache friendly.

[1] http://gec.di.uminho.pt/discip/minf/ac0102/1000gap_proc-mem_...

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1. kragen ◴[16 May 25 00:18 UTC] No.44000639[source]▶
It's true that this code was optimized from 2.6ns down to 0.9ns, a saving of 1.7ns, while an L2 cache miss might be 80ns. But 1.7ns is still about 2% of the 80ns, and it's about 70% of the 2.6ns. You don't want to start optimizing by reducing things that are 2% of your cost, but 2% isn't insignificant.

The bigger issue is that probably you don't need to do leap-year checks very often so probably your leap-year check isn't the place to focus unless it's, like, sending a SQL query across a data center or something.