RFR: 8326385: [aarch64] C2: lightweight locking nodes kill the box register without specifying this effect

[Roberto Castañeda Lozano]

On Mon, 11 Mar 2024 14:01:10 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> This changeset introduces a third `TEMP` register for the intermediate computations in the `cmpFastLockLightweight` and `cmpFastUnlockLightweight` aarch64 ADL instructions, instead of using the legacy `box` register. This prevents potential overwrites, and consequent erroneous uses, of `box`.
>> 
>> Introducing a new `TEMP` seems conceptually simpler (and not necessarily worse from a performance perspective) than pre-assigning `box` an arbitrary register and marking it as `USE_KILL`, an alternative also suggested in the [JBS issue description](https://bugs.openjdk.org/browse/JDK-8326385). Compared to mainline, the changeset does not lead to any statistically significant regression in a set of locking-intensive benchmarks from DaCapo, Renaissance, SPECjvm2008, and SPECjbb2015.
>> 
>> #### Testing
>> 
>>  - tier1-7 (linux-aarch64 and macosx-aarch64) with `-XX:LockingMode=2`.
>
> src/hotspot/cpu/aarch64/aarch64.ad line 16022:
> 
>> 16020: %}
>> 16021: 
>> 16022: instruct cmpFastLockLightweight(rFlagsReg cr, iRegP object, iRegP box, iRegPNoSp tmp, iRegPNoSp tmp2, iRegPNoSp tmp3)
> 
> Do we need to specify the box register at all, if we never use it? It means that the register allocator assigns an actual register to it, right? This could be a problem in workloads that are both locking-intensive *and* with high register pressure. You may just not see it with dacapo, etc, because aarch64 has so many registers to begin with.

There is, unfortunately, no ADL construction to specify that an operand such as `box` is not used at all. Yes, the register allocator will assign a register to `box`. Avoiding this would require fairly intrusive changes in C2 (add lightweight locking-specific, single-input versions of the `FastLock` and `FastUnlock` nodes and adapt all the C2 logic that deals with them), which I think would be best addressed in a separate RFE (assuming the additional register pressure is a problem in practice). Furthermore, to my understanding, the box operand is likely to be needed again in the context of Lilliput's [OMWorld](https://bugs.openjdk.org/browse/JDK-8326750) sub-project.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/18183#discussion_r1519942947