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

[Roman Kennke]

On Fri, 17 Mar 2023 06:15:28 GMT, Thomas Stuefe <stuefe 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.
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> 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.

Cmp(r,r) would not clear EQ, but set it. Unless you do cmp(r,0) on a non-null register. Tst is better at least on x86 because it encodes smaller. *shrugs*

You can do it in the shared fast_lock() but it's really only needed in C2, that's why I'm doing it there. Maybe I'm too perfectionist when it comes to assembly code?

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