RFR: 8291555: Replace stack-locking with fast-locking

[Robbin Ehn]

On Mon, 12 Sep 2022 07:54:48 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> Nice! From your snippets above I am gleaning that your implementation has the actual lock-stack in Java. Is that correct? Is there a particular reason why you need this? Is this for Loom? Would the implementation that I am proposing here also work for your use-case(s)?
> 

Yes, the entire implementation is in Java.

    void push(Object lockee, long fid) {
        if (this != Thread.currentThread()) Monitor.abort("invariant");
        if (lockStackPos == lockStack.length) {
            grow();
        }
        frameId[lockStackPos] = fid;
        lockStack[lockStackPos++] = lockee;
    }


We are starting from the point of let's do everything be in Java.
I want smart people to being able to change the implementation.
So I really don't like the hardcoded assembly in remove_activation which do this check on frame id on the lock stack.
If we can make the changes to e.g. popframe and take a bit different approach to JVMS we may have a total flexible Java implementation.
But a flexible Java implementation means compiler can't have intrinsics, so what will the performance be....
We have more loose-ends than we can handle at the moment.

Your code may be useable for JOM if we lock the implementation to using a lock-stack and we are going to write intrinsics to it.
There is no point of it being in Java if so IMHO.

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PR: https://git.openjdk.org/jdk/pull/9680