RFR: 8221554: aarch64 cross-modifying code

[Alan Hayward]

On Thu, 1 Oct 2020 06:04:03 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

> I am about to integrate concurrent stack scanning for ZGC. This patch changes things a bit so that disarming of the
> poll word is always done only by the thread itself. The consequence is that if a thread is in native or blocked, and
> the safepoint finishes, then the thread will when waking up, still take one slow path to figure out all is done, disarm
> the poll and run the cross modifying fence. The consequence is that we trivially need the cross modifying fence only in
> the poll slow path and nowhere else. So maybe if you wait for that one, we can delete a few more ISBs, and IMO also
> delete the verification code, as you can't really mess it up any more.

I'd need to see what the code looks like - but I'm thinking it just reduces the number of cross_modify_fence calls,
instead of reducing explicit isb calls in the backend (of which only 3 remain with this patch). Either way, it's good
news. I'm going to try rebasing locally on the top of your patch (I'm assuming it's this one
https://github.com/openjdk/jdk/pull/296 ) and see where that gets me.

I'd be cautious about removing the verification code - it should be useful for helping to catch issues if we do start
getting  1 run in a million crash dumps. But agreed, doesn't need to be there if the whole isb process has become
trivial.

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

PR: https://git.openjdk.java.net/jdk/pull/428