RFR: 8221554: aarch64 cross-modifying code [v3]

[Andrew Haley]

So, the good news and the bad news:

Moving to cross_modify_fence reduces the number of ISBs from
3,840,210 maybe_isb()s to 74,538 cross_modify_fence()s on my
poster child application, which is recompiling all of java.base.

However, this is a program that runs for 187,501,798,979 insns,
so we've reduced the proportion of ISBs from 0.002% to 0.00004%.
I guess that's worth having, but I doubt that the improvement
would ever have been above the noise level.

On the good side, this at least makes AArch64 more like other
targets.

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