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by tialaramex 20 days ago
You're correct that by an "as if" rule the JIT may in some cases be able to do this trick, but the specification says you're getting a heap allocation and so in most cases, including the example I gave that's exactly what happens.

It doesn't matter that you say this is "much closer to something like using an arena". It's definitely work that we needn't do at all and that's AFAICT that's how Casey ends up over-correcting so badly.

> This is pretty much the most efficient one can be after per-object type arenas and simply not allocating.

Sure. "A C is pretty much the highest grade one can get in this class, after grades A and B".
1 comments
gf000 20 days ago
What "work" do we do needlessly? It's a pointer bump. With reclamation being no work. Comparatively a malloc call will have to do extra work to defragment.

Also, a stack is not a separate hardware element of the RAM, it's not faster than a sufficiently hot part of the heap - it just so happens that the current stack frame is pretty likely to be in cache.

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tialaramex 20 days ago
> What "work" do we do needlessly? It's a pointer bump

Any work here was needless. There was no need to do work.

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gf000 19 days ago
On the allocation happy path there is literally less work in case of Java. Like at least try to get what I'm saying and argue with that - I'm not saying that Java is faster or whatever, but that it has chosen different tradeoffs, and "stack vs heap" is an oversimplified model that doesn't help us understand the real performance impacts.

A pointer bump is a pointer bump, vs a malloc call that will try to find place, do some housekeeping, etc. And your Vec will need at least a single allocation, the backing buffer is not on the stack.

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tialaramex 19 days ago
> On the allocation happy path there is literally less work in case of Java

Doing more allocations isn't "less work". No matter how many times you mumble "But it's just a pointer bump" the alternative was no work.

> it has chosen different tradeoffs

Sure, it's easier to implement this, the New Jersey style. But that's not a benefit to anybody else, so if you're not Sun Microsystems (which you aren't, Sun no longer exists) then that's not actually a benefit at all and this was a bad trade.

Which is why people have expended an eyewatering amount of effort on Project Valhalla to some day fix this stuff.

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