RFR: 8256073: Improve vector rematerialization support

[Vladimir Kozlov]

On Thu, 19 Nov 2020 18:16:06 GMT, Vladimir Kozlov <kvn 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
>
> Changes requested by kvn (Reviewer).

After thinking more on this a separate method could complicate changes. At least add a comment.

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