RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning [v3]

[Patricio Chilano Mateo]

On Tue, 22 Oct 2024 15:56:21 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> Note also that `inc_held_monitor_count` clobbers `rscratch2`.  That might be worth a comment at the call site.
>> I guess `inc_held_monitor_count` is so hot that we can't push and pop scratch registers, in which case it'd clobber nothing.
>
>> Historically, silently using `rscratch1` and `rscratch2` in these macros has sometimes turned out to be a mistake. Please consider making `rscratch2` an additional argument to `fast_lock`, so that it's explicit in the caller. It won't make any difference to the generated code, but it might help readbility.
> 
> Hmm, forget that. It's rather tricky code, that's true, but I think we're OK.

I see we are already using rscratch1 in these locking macros so I could change it to use that instead. But looking at all other macros in this file we are already using rscratch1 and rscratch2 too, so I think we would be fine either way.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1813513144