RFR: 8288445: AArch64: C2 compilation fails with guarantee(!true || (true && (shift != 0))) failed: impossible encoding

[Dean Long]

On Wed, 22 Jun 2022 06:13:21 GMT, Hao Sun <haosun at openjdk.org> wrote:

>>> Instead of introducing immI_positive, I wonder if we can generate orr dst src for zero shift count, just as the SVE part does
>> 
>> I considered using left-shift by 0, but I don't think it's worth it to optimize this case in the back-end.  If we really want to optimize shift by 0, I think it should be done in the front-end.
>
>> > Instead of introducing immI_positive, I wonder if we can generate orr dst src for zero shift count, just as the SVE part does
>> 
>> I considered using left-shift by 0, but I don't think it's worth it to optimize this case in the back-end. If we really want to optimize shift by 0, I think it should be done in the front-end.
> 
> Okay.
> 
> I think we may want to make the same update to the corresponding SVE part, e.g. https://urldefense.com/v3/__https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/aarch64_sve.ad*L3662__;Iw!!ACWV5N9M2RV99hQ!IhsRjR9bgw_S9FerpoGgB7xx4uxR8awLunoOhseIUPtJ-yMlca1Ilz5T8OeLOn24rdzKzFpnbInbeQNtzHeE2b4wDy8j$ , as it's better to align with the NEON and SVE implementations.
> 
> It may deserve one separate patch, not in this bugfix one.

@shqking Yes, that's a good idea for a separate RFE targeted for jdk20.

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PR: https://git.openjdk.org/jdk19/pull/40