[jdk21u-dev] RFR: 8328938: C2 SuperWord: disable vectorization for large stride and scale [v2]

[Aleksey Shipilev]

> Unclean backport to prevent accidents in C2 loop optimizations. The patch is unclean, because JDK 21u misses major SuperWord refactorings. I applied the hunk by hand in the similar place, and also used `slp->iv_stride()` in one place to get this thing to work.
> 
> @eme64, if you want to take a look at this?
> 
> Additional testing:
>  - [x] New regression test fails without the patch, passes with it
>  - [x] Linux x86_64 server fastdebug, `all`
>  - [x] Linux x86_64 server fastdebug, 100K Fuzzer tests
>  - [ ] Linux x86_64 server fastdebug, Maven CTW

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Check if stride is actually con before touching iv_stride

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

Changes:
  - all: https://git.openjdk.org/jdk21u-dev/pull/495/files
  - new: https://git.openjdk.org/jdk21u-dev/pull/495/files/e8def153..1ee99f65

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=495&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=495&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/495.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/495/head:pull/495

PR: https://git.openjdk.org/jdk21u-dev/pull/495