RFR: 8274632: Possible pointer overflow in PretouchTask chunk claiming [v2]

[Kim Barrett]

> Please review this change to PretouchTask, fixing the mechanism used to
> claim chunks for parallel touching.  The old mechanism unconditionally added
> a potentially large value to the current pointer and then checked the result
> for overflow.  But since pointer arithmetic overflow is UB, that's not a
> reliable approach.
> 
> Testing:
> mach5 tier1

Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:

  remove unused _start_addr

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5791/files
  - new: https://git.openjdk.java.net/jdk/pull/5791/files/8b57b3eb..b3e5e12a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5791&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5791&range=00-01

  Stats: 3 lines in 2 files changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5791.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5791/head:pull/5791

PR: https://git.openjdk.java.net/jdk/pull/5791