RFR: 8284960: Integration of JEP 426: Vector API (Fourth Incubator) [v3]

[Vladimir Ivanov]

On Fri, 13 May 2022 08:24:24 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> src/hotspot/share/opto/c2compiler.cpp line 521:
>> 
>>> 519:     if (!Matcher::match_rule_supported(Op_SignumF)) return false;
>>> 520:     break;
>>> 521:   case vmIntrinsics::_VectorComExp:
>> 
>> Why `_VectorComExp` intrinsic is special? Other vector intrinsics are handled later and in a different manner.
>> 
>> What about `ExpandV` case?
>
> It was an attempt to facilitate in-lining of these APIs over targets which do not intrinsify them. I agree its not a generic fix since three APIs are piggybacking on same entry point and without the knowledge of opcode it will be inappropriate to take any call at this place, lazy intrinsification gives opportunity for some of the predications to concertize as compilation happens under closed world assumptions.

Still not clear why the code is shaped the way it is.

`Matcher::match_rule_supported_vector()` already checks that there are relevant matching rules.

The checks require both `CompressM` and `CompressV` to be present to enable the intrinsic. Is it important?

Also, it doesn't take `EnableVectorSupport` into account while all other vector intrinsics respect it.

>> src/hotspot/share/opto/compile.cpp line 3416:
>> 
>>> 3414: 
>>> 3415:   case Op_ReverseBytesV:
>>> 3416:   case Op_ReverseV: {
>> 
>> Can you elaborate, please, why it is performed so late in the optimization phase (at the very end during graph reshaping) and not during GVN?
>
> Its more of a chicken-egg problem here, for masked reverse operation, Reverse IR node is followed by a Blend Node, thus in such a case doing an eager Identity transform in Reverse::Identity will not work, also deferring this to blend may also not work since it could be a non-masked reverse operation, we could have handled it as a special case in inline_vector_nary_operation, but handling such special case in final graph reshaping looked more appropriate.
> 
> https://github.com/openjdk/panama-vector/pull/182#discussion_r845678080

Do you mean it's important to apply the transformation at the right node (pick the right node as the root) and it is hard to make a decision during GVN?

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PR: https://git.openjdk.java.net/jdk/pull/8425