RFR: 8316694: Implement relocation of nmethod within CodeCache [v32]

[Chad Rakoczy]

On Thu, 26 Jun 2025 10:40:57 GMT, Evgeny Astigeevich <eastigeevich at openjdk.org> wrote:

>> Chad Rakoczy has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 90 commits:
>> 
>>  - Merge remote-tracking branch 'origin/master' into JDK-8316694-Final
>>  - Update how call sites are fixed
>>  - Merge remote-tracking branch 'origin/master' into JDK-8316694-Final
>>  - Fix pointer printing
>>  - Use set_destination_mt_safe
>>  - Print address as pointer
>>  - Use new _metadata_size instead of _jvmci_data_size
>>  - Merge remote-tracking branch 'origin/master' into JDK-8316694-Final
>>  - Only check branch distance for aarch64 and riscv
>>  - Move far branch fix to fix_relocation_after_move
>>  - ... and 80 more: https://git.openjdk.org/jdk/compare/f799cf18...70e4164e
>
> src/hotspot/cpu/aarch64/nativeInst_aarch64.cpp line 90:
> 
>> 88:   // Patch the constant in the call's trampoline stub.
>> 89:   address trampoline_stub_addr = get_trampoline();
>> 90:   if (trampoline_stub_addr != nullptr && dest != trampoline_stub_addr) {
> 
> I think you will not need the checks if you rewrite the code as follows:
> ```c++
> address addr_call = ...;
> assert();
> 
> if (!Assembler::reachable_from_branch_at(addr_call, dest)) {
>   address trampoline_stub_addr = get_trampoline();
>   assert (trampoline_stub_addr != nullptr, "we need a trampoline");
>   assert (! is_NativeCallTrampolineStub_at(dest), "chained trampolines");
>   nativeCallTrampolineStub_at(trampoline_stub_addr)->set_destination(dest);
>   dest = trampoline_stub_addr;
> }
> set_destination(dest);
> ICache::invalidate_range(addr_call, instruction_size);
> 
> 
> If `dest` is a trampoline in the current nmethod, it is always reachable. So you will not go into setting trampoline's target to itself. Also we will call `get_trampoline`, which involves `CodeCache::find_blob` and ` a traversal of relocations, only if we need a trampoline.

I would need to check the assumptions that other callers make about this function. In the current state it updates the trampoline regardless if the branch is reachable or not. With your change it would require the caller to also update the trampoline to make sure it is not stale.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r2169438056