RFR: 8322535: Change default AArch64 SpinPause instruction

[Erik Österlund]

On Mon, 29 Jan 2024 17:24:50 GMT, Andrew Haley <aph at openjdk.org> wrote:

> > 8324221.
> 
> 
> 
> Arm tell us that 'yield' is basically implemented as a nop.

It is not doing much in their current designs, indeed. That's their (questionable?) implementation choice. New AmpereOne chips do however implement yield.

Since the ISA interface gives us a yield instruction dedicated for this and at least one new chip implements it, it makes sense to me that it is the *default* instruction, rather than none, going forward.

Then we can continue to recognize chips that didn't implement yield and try to figure out how to deal with that awkwardness separately, and hope that vendors (indeed including ARM), start implementing the ISA, instead of having us doubling down on horrible hacks that try to quack like a yield instruction.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/17430#issuecomment-1916168950