[foreign-memaccess+abi] RFR: Split foreign vector load and store by null or not null base [v2]

[Radoslaw Smogura]

> Split store / load operation by if checking if base is null
> or not null.
> 
> When this happens base in Unsafe is not perceived with mixed
> access by VM, and VM does not insert barriers.
> 
> Test results gives the expected values where the case of polluted access is 2x multiplication of normal access.
> 
> After
> 
> Benchmark                                    (size)  Mode  Cnt    Score     Error  Units
> MemorySegmentVectorAccess.arrayCopy            1024  avgt   10    7.437 ±   0.195  ns/op
> MemorySegmentVectorAccess.directSegments       1024  avgt   10   15.593 ±   0.371  ns/op
> MemorySegmentVectorAccess.heapSegments         1024  avgt   10   16.997 ±   0.118  ns/op
> MemorySegmentVectorAccess.pollutedSegments2    1024  avgt   10   58.673 ± 105.783  ns/op
> MemorySegmentVectorAccess.pollutedSegments3    1024  avgt   10   67.216 ±  16.157  ns/op
> MemorySegmentVectorAccess.pollutedSegments4    1024  avgt   10  122.567 ± 263.950  ns/op
> MemorySegmentVectorAccess.pollutedSegments5    1024  avgt   10  114.725 ± 209.183  ns/op
> 
> 
> Before
> 
> Benchmark                                    (size)  Mode  Cnt    Score   Error  Units
> MemorySegmentVectorAccess.arrayCopy            1024  avgt   10    8.547 ± 0.115  ns/op
> MemorySegmentVectorAccess.directSegments       1024  avgt   10   15.536 ± 0.082  ns/op
> MemorySegmentVectorAccess.heapSegments         1024  avgt   10   15.818 ± 0.101  ns/op
> MemorySegmentVectorAccess.pollutedSegments2    1024  avgt   10  146.380 ± 1.127  ns/op
> MemorySegmentVectorAccess.pollutedSegments3    1024  avgt   10  290.784 ± 7.274  ns/op
> MemorySegmentVectorAccess.pollutedSegments4    1024  avgt   10  297.187 ± 5.096  ns/op
> MemorySegmentVectorAccess.pollutedSegments5    1024  avgt   10  310.166 ± 9.310  ns/op
> 
> 
> Additonally with profiling `load` and `store` method arguments as
> described in [1]
> 
> Benchmark                                    (size)  Mode  Cnt    Score   Error  Units
> MemorySegmentVectorAccess.arrayCopy            1024  avgt   10    7.480 ± 0.169  ns/op
> MemorySegmentVectorAccess.directSegments       1024  avgt   10   15.497 ± 0.062  ns/op
> MemorySegmentVectorAccess.heapSegments         1024  avgt   10   16.829 ± 0.132  ns/op
> MemorySegmentVectorAccess.pollutedSegments2    1024  avgt   10  145.436 ± 1.081  ns/op
> MemorySegmentVectorAccess.pollutedSegments3    1024  avgt   10  291.081 ± 2.297  ns/op
> MemorySegmentVectorAccess.pollutedSegments4    1024  avgt   10  305.388 ± 7.518  ns/op
> MemorySegmentVectorAccess.pollutedSegments5    1024  avgt   10  303.931 ± 3.412  ns/op
> 
> 
> [1] https://github.com/openjdk/panama-foreign/pull/700

Radoslaw Smogura has updated the pull request incrementally with one additional commit since the last revision:

  Add unswitching to masked vector operations
  Add benchmark covering this.
  
  After
  ```
  Benchmark                                          (size)  Mode  Cnt    Score    Error  Units
  MemorySegmentMaskedVectorAccess.arrayCopy            1024  avgt   10   16.700 ±  0.612  ns/op
  MemorySegmentMaskedVectorAccess.directSegments       1024  avgt   10   80.429 ±  2.897  ns/op
  MemorySegmentMaskedVectorAccess.heapSegments         1024  avgt   10   25.528 ±  0.296  ns/op
  MemorySegmentMaskedVectorAccess.pollutedSegments2    1024  avgt   10  122.809 ±  0.894  ns/op
  MemorySegmentMaskedVectorAccess.pollutedSegments3    1024  avgt   10  252.930 ±  4.623  ns/op
  MemorySegmentMaskedVectorAccess.pollutedSegments4    1024  avgt   10  451.579 ±  6.429  ns/op
  MemorySegmentMaskedVectorAccess.pollutedSegments5    1024  avgt   10  446.500 ± 39.156  ns/op
  ```
  
  Before
  ```
  Benchmark                                          (size)  Mode  Cnt    Score     Error  Units
  MemorySegmentMaskedVectorAccess.arrayCopy            1024  avgt   10   21.089 ±   0.219  ns/op
  MemorySegmentMaskedVectorAccess.directSegments       1024  avgt   10   81.384 ±   1.008  ns/op
  MemorySegmentMaskedVectorAccess.heapSegments         1024  avgt   10   25.626 ±   0.522  ns/op
  MemorySegmentMaskedVectorAccess.pollutedSegments2    1024  avgt   10  217.733 ±   5.467  ns/op
  MemorySegmentMaskedVectorAccess.pollutedSegments3    1024  avgt   10  441.045 ±   9.749  ns/op
  MemorySegmentMaskedVectorAccess.pollutedSegments4    1024  avgt   10  522.613 ± 104.997  ns/op
  MemorySegmentMaskedVectorAccess.pollutedSegments5    1024  avgt   10  449.814 ±   8.203  ns/op
  ```

-------------

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/711/files
  - new: https://git.openjdk.org/panama-foreign/pull/711/files/a6b2e879..b4680af8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=711&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=711&range=00-01

  Stats: 201 lines in 2 files changed: 184 ins; 4 del; 13 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/711.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/711/head:pull/711

PR: https://git.openjdk.org/panama-foreign/pull/711