RFR: 8316694: Implement relocation of nmethod within CodeCache [v18]

[Chad Rakoczy]

On Thu, 29 May 2025 11:24:46 GMT, Evgeny Astigeevich <eastigeevich at openjdk.org> wrote:

>> If we want to guarantee that a trampoline exists if `Assembler::reachable_from_branch_at` fails we would need to update Graal to use the check as well
>
>> The null trampoline check is needed because on debug builds branches of distance >2M will fall into the if (!Assembler::reachable_from_branch_at(addr(), x)) block but Graal would not have generated a trampoline for that call because it is still <128M. It is still safe to use that distance but it is just different than what HotSpot expects
> 
> This logic looks strange to me.
> You are saying that a trampoline is only null in case of Graal but dest is always valid in this case.
> This is a bug in Graal: it always uses 128M branch range despite Hotspot can change the range to smaller values in debug builds.
> When Graal fixes the bug you will have undefined behaviour in this place.
> We must handle the situation where no trampoline is available.
> Options:
> 1. This is a bug in code generation. If the bug can be easy to reproduce with debug builds, use assert. If no, use guarantee.
> 2. This is an expected case. We need to generate a trampoline. This can be complicated.
> 
> I think it's a bug situation.

Updated the code to have a guarantee check here instead. Filed the following which will fix Graal to use the same range check
https://github.com/oracle/graal/issues/11291
https://bugs.openjdk.org/browse/JDK-8358096

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PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r2121784842