[vectorIntrinsics] RFR: 8283413: Add C2 mid-end and x86 back-end implementation for bit REVERSE and REVERSE_BYTES operation [v6]

[Xiaohong Gong]

On Tue, 19 Jul 2022 17:27:22 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> FTR, Reverse[bytes] related transforms are special since they cannot be done locally in associated Identity routines since corresponding IR  nodes may be succeeded by blends which are generated after creation of Reverse[bytes] IR, thus GVN related routines for both blend and Reverse[bytes] IR may not be able to handle this transform,  strict pattern match based transform during final graph re-shaping looks promising here.
>
>> Hi @jatin-bhateja ，currently I'm looking at https://bugs.openjdk.org/browse/JDK-8290485, and noticed that you said the transformations for Reverse[bytes] are special that we cannot do the transformations when it is used by a VectorBlend. Could you please explain why? Per my understanding, the masked Reverse[bytes] with VectorBlend should also work, no matter what changes are applied to the Reverse[bytes], since we only need to make sure the non-masked lanes come from the first input, and the masked lanes from the second (Reverse[bytes]). Please correct me if I misunderstood. Thanks a lot!
> 
> Hi @XiaohongGong , I re-implemented this before JEP integration by introducing a new [Node flag: Flag_is_predicated_using_blend](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/node.hpp#L788), this allowed us to move these transforms back to Idealization routines. Idea here was to prevent removing cascaded ReverseBytes if they are predicated, unless both the predicates comply. Thanks!

Do you mean the `Flag_is_predicated_using_blend` used in the Idea is used to prevent optimizations like `ReverseBytesV (ReverseBytesV X , MASK) , MASK =>  X` when `ReverseBytesV` is predicated but implemented with `VectorBlend` ? So if it is,  then I think the input of ReverseBytesV should be `VectorBlend X (ReverseBytesV X) MASK` while not `ReverseBytesV X`, right?  Thanks!

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PR: https://git.openjdk.org/panama-vector/pull/182