[aarch64-port-dev ] RFR: 8242029: AArch64: skip G1 array copy pre-barrier if marking not active

[Andrew Haley]

On 4/7/20 8:33 AM, Erik Österlund wrote:

> Note that you only need ISB when returning from call_VM, not
> call_VM_leaf. Leaf calls can’t safepoint, and hence the ISB is
> redundant. Arraycopy uses leaf calls. So while this optimization is
> great for this case, maybe removing ISB fronleaf calls has a wider
> effect.
>
> It also appears to me that with Stuart’s new nmethod entry barriers
> enabled, ISB is never required on returns, as oop are no longer
> embedded in the instruction stream then (which is what the ISB
> protects against).

That's probably true. In order to get it right for sure we'd need to
insert a bunch of assertions in debug mode. I'll have a look.

A (fairly) recent change to the ARM ARM (DDI 0487E, B2.3.5, Ordering
of instruction fetches, first para.) means that we no longer have to
be quite so paranoid about issuing ISBs. When call_VM was written the
specification of what might happen was so loose that it was almost
impossible to comply with.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671