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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: 0.322s | 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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PaulKeeble ◴[15 May 25 23:27 UTC] No.44000378[source]▶
The thing is about these optimisations (assuming they test as higher performance) is that they can get applied in a library and then everyone benefits from the speedup that took some hard graft to work out. Very few people bake their own date API nowadays if they can avoid it since it already exists and techniques like this just speed up every programme whether its on the critical path or not.
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codexb ◴[15 May 25 23:30 UTC] No.44000391[source]▶
That's basically compilers these days. It used to be that you could try and optimize your code, inline things here and there, but these days, you're not going to beat the compiler optimization.
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1. godelski ◴[16 May 25 01:38 UTC] No.44001055[source]▶
Definitely not true.

You aren't going to beat the compiler if you have to consider a wide range of inputs and outputs but that isn't a typical setting and you can actually beat them. Even in general settings this can be true because it's still a really hard problem for the compiler to infer things you might know. That's why C++ has all those compiler hints and why people optimize with gcc flags other than -O.

It's often easy to beat Blas in matrix multiplication if you know some conditions on that matrix. Because Blas will check to find the best algo first but might not know (you could call directly of course and there you likely won't win, but you're competing against a person not the compiler).

Never over estimate the compiler. The work the PL people do is unquestionably useful but they'll also be the first to say you can beat it.

You should always do what Knuth suggested (the often misunderstood "premature optimization" quote) and get the profile.