RFR: 8371046: Segfault in compiler/whitebox/StressNMethodRelocation.java with -XX:+UseZGC

[Vladimir Kozlov]

On Sat, 22 Nov 2025 00:43:59 GMT, Chad Rakoczy <duke at openjdk.org> wrote:

>> I also assume that trampoline's code instructions can construct far distance address.
>
>> My question is how you handle 3rd case? And how you distinguish 2 and 3 cases?
> 
> We should never run into the 3rd case. If a trampoline _may_ be needed it will be there.
> 
> A trampoline will not be generated only if the destination is known to always be reachable. Here are some situations where this could happen:
> - no far branches (code cache size <= branch range)
> - runtime call is reachable from anywhere in code cache 
>    - (code cache begin - runtime call <= branch range) && (code cache end - runtime call <= branch range)
> 
> Whether or not a trampoline is generated is dependent on the callee destination not the caller address. So we can't have the case where a trampoline is needed for a given call but it doesn't exist.

May be we should change the assert to guarantee in `Relocation::pd_set_call_destination()` to make sure we catch incorrect patching it product VM.

Looking on `NativeCall::set_destination_mt_safe` and `reachable` is calculated based on distance between address of call instruction and destination. Which could be different for cloned nmethod.

On x86 were have guarantee in `NativeCall::set_destination()`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28241#discussion_r2553244181