RFR: 8316694: Implement relocation of nmethod within CodeCache

[Evgeny Astigeevich]

On Wed, 26 Feb 2025 00:12:49 GMT, Chad Rakoczy <duke at openjdk.org> wrote:

> Shouldn't the check be `src->blob() == nullptr` so the assert passes if relocating an `nmethod`?

Yes, you are right.

>> src/hotspot/cpu/aarch64/sharedRuntime_aarch64.cpp line 1382:
>> 
>>> 1380: 
>>> 1381:     // First instruction must be a nop as it may need to be patched after relocation
>>> 1382:     __ nop();
>> 
>> Could you please explain what a problem it fixes?
>
> When an `nmethod` gets marked as not entrant the [first instruction is updated](https://github.com/openjdk/jdk/blob/11a37c829c12d064874416a7b242596cf23972e5/src/hotspot/cpu/aarch64/nativeInst_aarch64.cpp#L368) to verify that the code is no longer called. The first instruction must be a jump or nop for this

This means they are never made not entrant. I don't think we will relocate them. Let's have a function: `bool CodeCache::is_relocatable(nmethod*)`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r1971314537
PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r1984158432