RFR: 8286941: Add mask IR for partial vector operations for ARM SVE [v3]

[Xiaohong Gong]

On Thu, 16 Jun 2022 09:54:17 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> Xiaohong Gong has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:
>> 
>>  - Address review comments, revert changes for gatherL/scatterL rules
>>  - Merge branch 'jdk:master' into JDK-8286941
>>  - Revert transformation from MaskAll to VectorMaskGen, address review comments
>>  - 8286941: Add mask IR for partial vector operations for ARM SVE
>
> src/hotspot/share/opto/vectornode.cpp line 1669:
> 
>> 1667:   if (Matcher::vector_needs_partial_operations(this, vt)) {
>> 1668:     return VectorNode::try_to_gen_masked_vector(phase, this, vt);
>> 1669:   }
> 
> This is a parent node of TrueCount/FirstTrue/LastTrue and MaskToLong which perform mask querying operation on concrete predicate operands, a transformation here looks redundant to me.

The main reason to add the transformation here is: the FirstTrue needs the reference to the real vector length for SVE, that we need to generate a predicate when the vector length is smaller than the max vector size.  Please check the changes of `partial_op_sve_needed` in aarch64_sve.ad.

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PR: https://git.openjdk.org/jdk/pull/9037