[aarch64-port-dev ] AArch64: Neoverse N1 atomic operations and -moutline-atomics

[Andrew Haley]

On 5/18/20 8:10 AM, Nick Gasson wrote:
> I've done some profiling of SPECjbb on N1. Building with
> -moutline-atomics gives around +4% on critical-jOPS and +0.2% on
> max-jOPS averaged over several runs. However enabling this globally will
> cause a slight regression on machines that don't support the LSE
> extension due to the overhead of the function call and branch (~1% for
> critical-jOPS on one machine).

Mmm. That's not a nice decision to have to make.

> Question is whether it's best to enable this by default in OpenJDK or
> leave it to downstream distributions to decide?

I don't really understand why there is a conditional branch there.
They could just do the test at startup and patch the code. Maybe it'd
hurt DSO startup time or something? Maybe they don't like the idea of
temporarily enabling exec privilege on a page of writable data.

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