RFR: 8352042: [leyden] Parallel precompilation

[Vladimir Ivanov]

On Fri, 14 Mar 2025 12:22:00 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> For simplicity reasons, our current precompilation code uses effectively single-threaded blocking compiles. Even for simple cases, this makes assembly phase remarkably long. This does not scale well. We need to do precompilations in parallel. 
> 
> Unfortunately, I haven't found a good way to check that all async compilations are completed, so I introduced a new one.
> 
> Additional testing:
>  - [x] Ad-hoc precompilation tests
>  - [x] Linux x86_64 server fastdebug, `runtime/cds`

Thanks for taking care of it, Aleksey. 

Overall, looks good. `CompileTask::_active_tasks` feels a bit ad-hoc, but IMO it's fine for leyden-premain.

Also, since AOT code preparation is a synchronous task during assembly phase, it makes sense to revise how many compiler threads are used.

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Marked as reviewed by vlivanov (Committer).

PR Review: https://git.openjdk.org/leyden/pull/49#pullrequestreview-2687756004