RFR: 8366424: Missing type profiling in generated Record Object methods

[Chen Liang]

On Fri, 26 Sep 2025 22:13:00 GMT, Chen Liang <liach at openjdk.org> wrote:

> Hotspot profiles by bytecode; as a result, some shared methods become polluted and suffer in type profiling, as described in depth in [this essay](https://cr.openjdk.org/~jrose/jvm/equals-profile.html) by John Rose. The record methods generated by `ObjectMethods::bootstrap` just proved itself another victim in this RFE.
> 
> To bypass this issue, I naively generated distinct bytecode to allow distinct profiles for now. If hotspot adds any kind of split profiles exposed via internal APIs, we can migrate to such split profile and throw away these extra copies of bytecode.
> 
> In particular, in a method handle tree, each leaf method handle seems not separately profiled - for example, all DMH to Object.hashCode share the same profile regardless of their position in a MH tree, making MH trees less useful than explicitly rolled bytecode, unfortunately.
> 
> The attached benchmark should be a good demonstration of the effect of type profiling.

Initial benchmark results:

Benchmark                                  Mode  Cnt     Score    Error   Units
RecordMethodsBenchmark.equalsDistinct     thrpt   15   413.727 ±  5.317  ops/us
RecordMethodsBenchmark.equalsGenerated    thrpt   15   410.474 ±  6.298  ops/us
RecordMethodsBenchmark.equalsPolluted     thrpt   15   185.471 ±  3.800  ops/us
RecordMethodsBenchmark.hashCodeDistinct   thrpt   15  1190.923 ± 21.937  ops/us
RecordMethodsBenchmark.hashCodeGenerated  thrpt   15  1201.802 ± 20.521  ops/us
RecordMethodsBenchmark.hashCodePolluted   thrpt   15   239.675 ±  3.195  ops/us


Shows the generated method bodies benefit from type profiling.

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PR Comment: https://git.openjdk.org/jdk/pull/27533#issuecomment-3340658520