RFR: 8355769: Optimize nmethod dependency recording [v2]

[Andrew Dinn]

On Tue, 29 Apr 2025 12:47:25 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> During nmethod installation, we record the dependencies between InstanceKlass/CallSite and newly coming `nmethod`. In `DependencyContext::add_dependent_nmethod`, we are linearly scanning to see if the `nmethod` is already in the dependencies list. This costs quite a bit, especially with lots of compiled methods per IK.
>> 
>> This is not a significant issue for normal JIT compilations, where the JIT costs dominate. But for Leyden, this kind of scan is a significant part of AOT code installation. For example in well-trained javac runs, there are chains of 500+ `nmethods` for some IKs that take 10+ us to scan. This is easily half of the entire AOT method installation cost.
>> 
>> Fortunately, the way we do the nmethod dependency recording, it allows us to shortcut the scan. Since dependency recording holds the `CodeCache_lock` while adding new `nmethod` all over the various dependency lists, those dependency lists are ever in two states: no `nmethod` in the chain (no need to scan!), or `nmethod` is at the head of the chain (no need to scan!). 
>> 
>> Additional testing:
>>  - [x] Ad-hoc benchmarks
>>  - [x] Linux x86_64 server fastdebug, `all`
>>  - [x] Linux AArch64 server fastdebug, `all`
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fiddle with locks

Looks good.

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Marked as reviewed by adinn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24933#pullrequestreview-2803881954