RFR: 8359359: AArch64: share trampolines between static calls to the same method [v6]

[Mikhail Ablakatov]

On Wed, 8 Oct 2025 15:55:15 GMT, Evgeny Astigeevich <eastigeevich at openjdk.org> wrote:

>> Mikhail Ablakatov has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 10 commits:
>> 
>>  - Merge commit '41520998aa8808452ee384b213b2a77c7bad668d'
>>  - remove implementation-dependent logic from emit_shared_trampolines()
>>  - cleanup: update copyright headers
>>  - Make the value type of the dictionary a struct instead of Pair typedef
>>  - Remove share_rc_trampoline_for and share_sc_trampoline_for
>>  - Merge branch 'master'
>>  - Address review comments from @theRealAph and @eastig
>>    - use relocInfo::relocType instead of TrampolineCallKind
>>    - move rtype from keys to values; reduce the footprint of the patch
>>  - Lift the vm.opt.TieredCompilation == null requirement from the tests
>>  - Combine the two shared trampoline request hash tables
>>  - 8359359: AArch64: share trampolines between static calls to the same method
>>    
>>    Modify the C2 compiler to share trampoline stubs between static calls
>>    that resolve to the same callee method. Since the relocation target for
>>    all static calls is initially set to the static call resolver stub, the
>>    call's target alone cannot be used to distinguish between different
>>    static method calls. Instead, trampoline stubs should be shared based
>>    on the actual callee.
>>    
>>    The `SharedTrampolineTest.java` was designed to verify the sharing of
>>    trampolines among static calls. However, due to imprecise log analysis,
>>    the test currently passes even when trampolines are not shared.
>>    Additionally, comments within the test suggest ambiguity regarding
>>    whether it was intended to assess trampoline sharing for static calls
>>    or runtime calls. To address these issues and eliminate ambiguity, this
>>    patch renames and updates the existing test. Furthermore, a new test is
>>    introduced, using the existing one as a foundation, to accurately
>>    evaluate trampoline sharing for both static and runtime calls.
>
> src/hotspot/cpu/aarch64/codeBuffer_aarch64.cpp line 62:
> 
>> 60:     assert(cb->stubs()->remaining() >= MacroAssembler::max_trampoline_stub_size(), "pre-allocated trampolines");
>> 61:     address dest = value.dest;
>> 62:     LinkedListIterator<int> it(value.offsets.head());
> 
> It looks like we have missed an assert here when we were writing this code.
> It's worth to add an assert: `assert(!value.offsets.is_empty(), "must have offsets of call sites")`.

Done, see https://github.com/openjdk/jdk/pull/25954/commits/3c983dcf88997dfba85e164e547097453facb717

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PR Review Comment: https://git.openjdk.org/jdk/pull/25954#discussion_r2474405473