RFR: 8256073: Improve vector rematerialization support

[Tobias Hartmann]

On Mon, 9 Nov 2020 21:46:00 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

Looks reasonable to me.

src/hotspot/share/prims/vectorSupport.cpp line 96:

> 94:       case T_DOUBLE: arr->bool_at_put(index,  (*(jlong*)addr) != 0); break;
> 95: 
> 96:       default: assert(false, "unsupported: %s", type2name(elem_bt));

Why did you replace `fatal` by `assert`?

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Marked as reviewed by thartmann (Reviewer).

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