RFR: 8374349: [VectorAPI]: AArch64: Prefer merging mode SVE CPY instruction

[Andrew Haley]

On Fri, 23 Jan 2026 02:09:03 GMT, Eric Fang <erfang at openjdk.org> wrote:

> > Can we do without `PreferSVEMergingModeCPY`?
> 
> Thanks, @theRealAph. That’s a fair question – in general, fewer options are definitely preferable.
> 
> For this change, the main reason I introduced `PreferSVEMergingModeCPY` as an Arch-level flag is that the benefit and trade-offs of using the merging-mode sequence can be quite **microarchitecture-dependent**. 

The question is not whether something may possibly be better or worse, but is it significantly so? If you're going to add another flag, you have to provide evidence that the difference matters. The burden is on you to show that it does.

> From a user’s point of view, the default behaviour should still be sensible: the option is enabled by default on `Neoverse V1/V2`, where we’ve confirmed the improvement, and disabled elsewhere. If, over time, we gain enough confidence that the merging-mode sequence is strictly preferable across a wider range of hardware, I’m happy to follow up with a separate change to hard-wire the behaviour and drop the flag.

That's the wrong way to think about it. There are thousands of tiny decisions we've made in the AArch64 port, and the number of possible tweaks is almost infinite.  If we need a flag in the future we can add one, but then we'll do a better job once we understand what we really need.

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PR Comment: https://git.openjdk.org/jdk/pull/29359#issuecomment-3789258021