[vectorIntrinsics] RFR: 8283413: Add C2 mid-end and x86 back-end implementation for bit REVERSE and REVERSE_BYTES operation [v4]

[Sandhya Viswanathan]

On Mon, 4 Apr 2022 19:05:00 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> src/hotspot/share/opto/compile.cpp line 3417:
>> 
>>> 3415:             is_user_blend = true;
>>> 3416:             break;
>>> 3417:           }
>> 
>> What if one of the out for n is a phi? And blend is an out of phi?
>
> Here we are trying to optimize following two scenarios by looking point graph patterns. So if there is a match then transformation will go though else graph will not be modified.  
> 
> [Non-predicated pattern] 
>         ReverseV (ReverseV X) =>  X  
> [Predicated pattern created for targets supporting masked instructions ] 
>         ReverseV (Reverse X , Mask) Mask => X

But the transformation is not conservative. This issue is still open.

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