RFC: linux-aarch64 and LSE support

[Kim Barrett]

On Sep 7, 2022, at 5:30 AM, Dmitry Chuyko <dmitry.chuyko at bell-sw.com> wrote:
> 
> Hi Kim,
> 
> The implementation of the main JDK-8261027 change doesn't just try to provide the functionality when built with older GCC (or not GCC). What it does is dynamically switch to a more advanced implementation if the appropriate hardware capabilities were detected during the VM start. The selected code is picked by the compiler. So there are 2 implementations and non-LSE one is the default.  This allows us to provide a single binary for all supported ARM devices and get better performance where possible.

That's close to what the __atomic intrinsics with -moutline-atomics do. Except
the compiler intrinsic approach has various benefits accrued from being built
into the compiler.  (And the code that we need to maintain is much simpler.)

> From another hand, a less advanced implementation is always used initially in the default configuration. As you noticed, JDK-8282322 just makes it possible to create a build that uses the LSE variant from the start.

JDK-8282322 requires an appropriate -march or similar option. The intrinsic
approach gets the same desired effect (use only LSE) when such an option is
provided, but inline.  That's one of the benefits.