[8u] RFR + RFA: 8145096: Undefined behaviour in HotSpot

[Roland Westrelin]

I'd like to backport 8145096 to 8u as we've seen it cause a failure with
newer version of gcc:

hotspot/test/compiler/5091921/Test6890943.java crashes with:

#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00007f314142d48c, pid=1380, tid=0x00007f31548e8700
#
# JRE version: OpenJDK Runtime Environment (8.0) (build 1.8.0-internal-fastdebug-roland_2019_02_01_10_38-b00)
# Java VM: OpenJDK 64-Bit Server VM (25.71-b00-fastdebug mixed mode linux-amd64 compressed oops)
# Problematic frame:
# J 143 C2 Test6890943.calcWalkingRange(IIII)I (205 bytes) @ 0x00007f314142d48c [0x00007f314142cfa0+0x4ec]
#
# Core dump written. Default location: /home/roland/tmp/JTwork/scratch/0/core or core.1380
#
# An error report file with more information is saved as:
# /home/roland/tmp/JTwork/scratch/0/hs_err_pid1380.log
#
# If you would like to submit a bug report, please visit:
#   http://bugreport.java.com/bugreport/crash.jsp
#
Current thread is 139849848751872
Dumping core ...

with gcc 8.2.1

The change doesn't apply cleanly so here is a webrev for 8u:

http://cr.openjdk.java.net/~roland/8145096.8u/webrev.00/

Initial change:

https://bugs.openjdk.java.net/browse/JDK-8145096
http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/867bdec7c8c5

Roland.