RFR: 8249893: AARCH64: optimize the construction of the value from the bits of the other two [v6]

[Tobias Hartmann]

On Thu, 5 Nov 2020 10:21:07 GMT, Boris Ulasevich <bulasevich at openjdk.org> wrote:

>> Thanks for clarifying the code. I still have one thing I'd like to establish and, perhaps, document in the code or maybe just in this thread.
>> 
>> I may have missed something in the preceding email thread but at present I am not aware of why you need to delay application of this transform to a new post-loops optimization stage (you said it was needed but I didn't see any reason given). Since that adds more complexity to the overall optimization process I think it needs carefully justifying. Of course, it might be that this requirement also applies for other Ideal transforms, so I am not suggesting adding this extra step is necessarily a bad idea. Could you explain why it is needed in this case?
>
>>  why you need to delay application of this transform to a new post-loops optimization stage
> 
> Unfortunately, BitfieldInsert transformation conflicts with vectorization:
> - if or/and/shift was converted to BFI it is no longer vectorized
> - vectorized or/and/shift operations are faster than BFI
> 
> I delayed my transformation to be sure loop and vectorization transformations is already done at the moment.

@bulasevich any plans to re-open and fix this?

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PR: https://git.openjdk.java.net/jdk/pull/511