RFR: 8210498: nmethod entry barriers

[Andrew Haley]

On 10/8/18 1:54 PM, Erik Österlund wrote:
> Also note that the implementation space of the barrier itself has some 
> flexibility. Rickard's first prototype involved having an unconditional 
> branch patched in over a nop. Since the nop is removed in the frontend, 
> it seemed like the most conservative starting point. But since there was 
> no measurable difference to the conditional branch, that was more 
> favourable in the end, since it hade the additional advantage of not 
> requiring a code cache walk in the safepoint. But if you have a platform 
> where the trade off is not as obvious, both mechanisms could easily be 
> supported.

Can you describe this mechanism a little more? I don't really understand
how that would work, even on x86.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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