RFR: 8291555: Implement alternative fast-locking scheme [v28]

[Thomas Stuefe]

On Thu, 16 Mar 2023 20:47:59 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> In my last changes I made a stupid mistake and don't set the condition flags correctly to force the slow-path, on aarch64. This is only relevant when we exceed the lock-stack capacity, that is why it's failing so rarely. I don't see a similar problem on x86_64 - have we observed any failures on x86_64? I pushed a fix for aarch64.

I noticed this too for arm; I used cmp to clear EQ but using tst seems better. I also do it inside fast_lock, to give it a defined exit state wrt EQ|NE, since it saves me from having to think about this on every call site. But at least the fail case may be fiddly without conditional execution.

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PR: https://git.openjdk.org/jdk/pull/10907