RFR: 8291600: [vectorapi] vector cast op check is not always needed for vector mask cast [v2]

[Xiaohong Gong]

> Recently we found the performance of "`FIRST_NONZERO`" for double type is largely worse than the other types on x86 when `UseAVX=2`. The main reason is the "`VectorCastL2X`" op is not supported by the backend when the dst element type is `T_DOUBLE`. This makes the check of `VectorCast` op fail before intrinsifying "`VectorMask.cast()`" which is used in the
> "`FIRST_NONZERO`" java implementation (see [1]). However, the compiler will not generate the `VectorCast `op for `VectorMask.cast()` if:
> 
>  1) the current platform supports the predicated feature
>  2) the element size (in bytes) of the src and dst type is the same
> 
> So the check of "`VectorCast`" op is needless for such cases. To fix it, this patch:
> 
>  1) limits the specified vector cast op check to vectors
>  2) adds the relative mask cast op check for VectorMask.cast()
>  3) cleans up the unnecessary codes
> 
> Here is the performance of "`FIRST_NONZERO`" benchmark [2] on a x86 machine with `UseAVX=2`:
> 
> Benchmark                          (size) Mode Cnt Before  After   Units
> DoubleMaxVector.FIRST_NONZERO       1024  thrpt 15 49.266 2460.886 ops/ms
> DoubleMaxVector.FIRST_NONZEROMasked 1024  thrpt 15 49.554 1892.223 ops/ms
> 
> [1] https://github.com/openjdk/jdk/blob/master/src/jdk.incubator.vector/share/classes/jdk/incubator/vector/DoubleVector.java#L770
> [2] https://github.com/openjdk/panama-vector/blob/vectorIntrinsics/test/micro/org/openjdk/bench/jdk/incubator/vector/operation/DoubleMaxVector.java#L246

Xiaohong Gong 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:

 - Unify VectorMaskCast for all platforms
 - Merge branch 'master' into JDK-8291600
 - 8291600: [vectorapi] vector cast op check is not always needed for vector mask cast

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9737/files
  - new: https://git.openjdk.org/jdk/pull/9737/files/3a06474a..349ef765

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9737&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9737&range=00-01

  Stats: 48466 lines in 727 files changed: 22066 ins; 23424 del; 2976 mod
  Patch: https://git.openjdk.org/jdk/pull/9737.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9737/head:pull/9737

PR: https://git.openjdk.org/jdk/pull/9737