8361836: RISC-V: Relax min vector length to 32-bit for short vectors

[DingliZhang]

Follow up JDK-8359419, RVV supports all vector type conversion APIs in the Vector API.
So we only need to relax the length limit of the short type to achieve a significant improvement in JMH performance for converting between short and wider types.

Test

qemu-system UseRVV:

• Run jdk_vector (fastdebug)
• Run compiler/vectorapi (fastdebug)

Performance

Following shows the performance improvement of relative VectorAPI JMHs on k1 (256-bit RVV):

Benchmark                                             (SIZE)   Mode   Units   Before     After    Gain
VectorFPtoIntCastOperations.microDouble128ToShort128     512  thrpt  ops/ms   52.280   840.112   16.07
VectorFPtoIntCastOperations.microDouble128ToShort128    1024  thrpt  ops/ms   28.156   429.322   15.25
VectorFPtoIntCastOperations.microFloat64ToShort64        512  thrpt  ops/ms   14.242   479.509   33.67
VectorFPtoIntCastOperations.microFloat64ToShort64       1024  thrpt  ops/ms    6.906   242.690   35.14

PS: VectorFPtoIntCastOperations.microFloat64ToShort64 is added by JDK-8359419.

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8361836: RISC-V: Relax min vector length to 32-bit for short vectors (Enhancement - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/26239/head:pull/26239
$ git checkout pull/26239

Update a local copy of the PR:
$ git checkout pull/26239
$ git pull https://git.openjdk.org/jdk.git pull/26239/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 26239

View PR using the GUI difftool:
$ git pr show -t 26239

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/26239.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back dzhang! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

@DingliZhang The following label will be automatically applied to this pull request:

• hotspot-compiler

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

[mlbridge]

Webrevs

• 01: Full - Incremental (0773a366)
• 00: Full (06598543)

[feilongjiang]

Should we add some assert or guarantee for uncovered types?