RFR: 8286823: Default to UseAVX=2 on all Skylake/Cascade Lake CPUs

[olivergillespie]

The current code already does this for 'older' Skylake processors,
namely those with _stepping < 5. My testing indicates this is a
problem for later processors in this family too, so I have removed the
max stepping condition.

The original exclusion was added in https://bugs.openjdk.java.net/browse/JDK-8221092.

A general description of the overall issue is given at
https://en.wikipedia.org/wiki/Advanced_Vector_Extensions#Downclocking.

According to https://en.wikichip.org/wiki/intel/microarchitectures/cascade_lake#CPUID,
stepping values 5..7 indicate Cascade Lake. I have tested on a CPU with stepping=7,
and I see CPU frequency reduction from 3.1GHz down to 2.7GHz (~23%) when using
-XX:UseAVX=3, along with a corresponding performance reduction.

I first saw this issue in a real production workload, where the main AVX3 instructions
being executed were those generated for various flavours of disjoint_arraycopy.

I can reproduce a similar effect using SPECjvm2008's xml.transform benchmark.


java --add-opens=java.xml/com.sun.org.apache.xerces.internal.parsers=ALL-UNNAMED \
--add-opens=java.xml/com.sun.org.apache.xerces.internal.util=ALL-UNNAMED \
-jar SPECjvm2008.jar -ikv -ict xml.transform


Before the change, or with -XX:UseAVX=3:


Valid run!
Score on xml.transform: 776.00 ops/m


After the change, or with -XX:UseAVX=2:


Valid run!
Score on xml.transform: 894.07 ops/m


So, a 15% improvement in this benchmark. It's possible some benchmarks will be negatively
affected by this change, but I contend that this is still the right move given the stark
difference in this benchmark combined with the fact that use of AVX3 instructions can
affect *all* processes/code on the host due to the downclocking, and the fact that this
effect is very hard to root-cause, for example CPU profiles look very similar before and
after since all code is equally slowed.

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

Commit messages:
 - 8286823: Default to UseAVX=2 on all Skylake/Cascade Lake CPUs

Changes: https://git.openjdk.java.net/jdk/pull/8731/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8731&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286823
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8731.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8731/head:pull/8731

PR: https://git.openjdk.java.net/jdk/pull/8731