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Comment by chrisseaton | Hacker News Reader

What do you mean by ‘devirtualizer’ and why would you want that triggered? Sounds like something you wouldn’t want to trigger?

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The devirtualizer (maybe this is the wrong JVM terminology, it’s basically what clang/GC call it) is part of the optimizer which sees that you have a virtual function call (like most in Java) where there is a unique caller, so you can replace the virtual call with a direct one and possibly inline it.

In the JVM I think this can only be done speculatively (you have to double check the type), but it still matters.

Ah right yes conflict of terminology.

In the JVM devirtualising means making a virtual object a full object again, so the opposite of what you want to be happening.

I don't think the JVM really names the optimisation you're talking about, but it does do it, through either a global assumption based on the class hierarchy, or an inline cache with a local guard.

the jvm names the optimization exactly like he said

IIRC it calls it monomorphisation.

What do you mean by “virtual object”, an object which has been broken up on the stack instead of a “real” object living as a single entity on the heap?

Remember back in cs101 where you had class animal, with subclasses dog and cat. You can call speak() on animal and get either ruff or meow depending on what animal is. Animal is a virtual object. The system needs to do some work to figure out which speak to call every time, the work isn't too bad, but it turns out that cpus basically never have the right thing in the cache, so it ends up being very slow compared to most of what you are doing.

When we say devirtualize we mean that we know animal is always a cat so we don't have to look up which speak to use.

Please don't take others for complete morons.

And that makes literally no sense in-context, you're mapping those words directly to the concept of virtual methods but that's the opposite of the way chrisseaton uses them (and they're clearly familiar with the concept of virtual calls and it's not what they're using "virtual" for), hence asking them what they mean specifically by this terminology.

> What do you mean by “virtual object”, an object which has been broken up on the stack instead of a “real” object living as a single entity on the heap?

Yes... but let's not say 'broken up on the stack' - the object's fields become dataflow edges. The object doesn't exist reifed on the stack - fields may exist on the stack, or in registers, or not at all.

LLVM calls the process of breaking a struct/object into dataflow variables "Scalar Replacement of Aggregates".

https://llvm.org/doxygen/classllvm_1_1SROA.html#details

Yes that's what the JVM calls it - and then the node the ties together the state is the virtual object.

See Aleksey Shipilёv on JVM method dispatch: https://shipilev.net/blog/2015/black-magic-method-dispatch/

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