RFR: 8256073: Improve vector rematerialization support [v2]

[Vladimir Kozlov]

On Fri, 20 Nov 2020 16:29:21 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> Having #1131, #1132, and #1134 in place, the only missing piece left to have vector rematerialization fully working is support of non-contiguous vector values in vector rematerialization logic. This patch covers that.
>> 
>> Current version makes the assumption that vector values are contiguously laid in memory. It's the case for on-stack locations, but for in-register values it's not the case (at least, on x86). Rewritten version doesn't make such assumption for in-register case anymore and processes every vector element independently.
>> 
>> (Along the way, the refactoring fixes a bug when handling a corner case: the case when a vector instance is scalarized by EA and the primitive array field (VectorPayload.payload) has a constant value (NULL) is erroneously treated as requiring custom rematerialization and it hits an assert.)
>> 
>> Testing (with other relevant patches):
>> - [x] jdk/incubator/vector w/ -XX:+DeoptimizeALot and -XX:UseAVX={3,2,1,0} on AVX512-capable hardware
>> - [x] hs-precheckin-comp, hs-tier1, hs-tier2
>
> Vladimir Ivanov has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - Address review comments.
>  - Merge branch 'master' into 8256073.rematerialization
>  - Improve vector rematerialization support

Marked as reviewed by kvn (Reviewer).

src/hotspot/share/prims/vectorSupport.cpp line 94:

> 92:     // (In generated code, the conversion is performed by VectorStoreMask.)
> 93:     //
> 94:     // TODO: revisit when predicate registers are fully supported.

Good and thanks.

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PR: https://git.openjdk.java.net/jdk/pull/1136