RFR: 8287697: Limit auto vectorization to 32-byte vector on Cascade Lake [v2]

[Sandhya Viswanathan]

> We observe ~20% regression in SPECjvm2008 mpegaudio sub benchmark on Cascade Lake with Default vs -XX:UseAVX=2.
> The performance of all the other non-startup sub benchmarks of SPECjvm2008 is within +/- 5%. 
> The performance regression is due to auto-vectorization of small loops. 
> We don’t have AVX3Threshold consideration in auto-vectorization. 
> The performance regression in mpegaudio can be recovered by limiting auto-vectorization to 32-byte vectors.
> 
> This PR limits auto-vectorization to 32-byte vectors by default on Cascade Lake. Users can override this by either setting -XX:UseAVX=3 or -XX:SuperWordMaxVectorSize=64 on JVM command line.
> 
> Please review.
> 
> Best Regard,
> Sandhya

Sandhya Viswanathan 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 six additional commits since the last revision:

 - Merge branch 'master' into maxvector
 - x86 build fix
 - Fix 32-bit build
 - review comment resolution
 - Change option name and add checks
 - Limit auto vectorization to 32 byte vector on Cascade Lake

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8877/files
  - new: https://git.openjdk.java.net/jdk/pull/8877/files/d677fd9a..7f4c41e2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8877&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8877&range=00-01

  Stats: 74281 lines in 805 files changed: 25008 ins; 42847 del; 6426 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8877.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8877/head:pull/8877

PR: https://git.openjdk.java.net/jdk/pull/8877