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by iscoelho 891 days ago
Sure, you're not wrong, but my focus was that the benchmark is indicative of the raw performance impact of bounds checks (when they are unable to elided) in a real world algorithm (as opposed to a micro-benchmark).

With that said, convincing a compiler to elide bounds checks (especially Java's JIT compiler) is a hugely frustrating (and for some algorithms futile) task.

It could be an argument that bounds checks make up a small percentage of total application performance. However, I've profiled production Java servers where >50% of the CPU was encryption/compression. JDK implementations of those algorithms are heavily impacted by (and commonly fail to elide) bounds checks.

Performance matters!
2 comments
xxs 891 days ago
>With that said, convincing a compiler to elide bounds checks (especially Java's JIT compiler) is a hugely frustrating (and for some algorithms futile) task.

Adding explicit checks does work to a certain degree but it can change with the compiler, and it requires to keep checking the generated assembly - not fun (no unsafe, either but still)

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iscoelho 891 days ago
These explicit checks are basically what I mean by "convincing the compiler" (and sometimes, it isn't quite convinced!) - Yeah. Not fun at all.

Especially in Java, because "the assembly" can change as the JIT evolves. What is optimized today may not be tomorrow.

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xxs 891 days ago
Some 12-13y back (time does fly) Cliff Click (hotspot architect) had a series of blogs on optimizations, incl. the lattice checks (i.e. within bounds). Was quite insightful. It was called: "Too Much Theory" [0]

>What is optimized today may not be tomorrow.

Exactly. (Also most developers will have exceptionally hard time maintaining such code)

[0]: https://web.archive.org/web/20120328222841/http://cliffc.org...

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sitkack 890 days ago
I understand where you are coming from, but without comparing the generated assembly, we are comparing an implementation of bounds checks. I think we should have a bounds check instruction and operates concurrently.

I should play around this with this using a couple RISC-V cores.

> Performance matters!

https://www.youtube.com/watch?v=r-TLSBdHe1A by Emery Berger

Another, yesbut, encryption and compression are and will handled by on die accelerators.

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