RFR: 8316234: AArch64: fastdebug build failure with GCC 12 and 13 due to -Wnonnull warning in vm_version_linux_aarch64.cpp [v2]

[Hao Sun]

> I tested {release, fastdebug, slowdebug} JDK builds with GCC 11/12/13 on AArch64, and fastdebug build with GCC 12 and 13 failed due to -Wnonnull warning.
> 
> The warning is raised because GCC 12 and 13 still think that `buf` can be null even if we have the assertion `assert(buf != nullptr, ...);`. Note that GCC 12 and 13 correctly determine the value range of `buflen`, i.e. `1 <= buflen <= 0xffff ffff ffff ffff`.
> 
> Note-1: Since it involves the analysis of `assert` statements, that's why release build can pass.
> 
> Note-2: I suppose that the incorrect optimization result of `nonnull/nullability` for variable `buf` only occurs at high GCC optimization levels. That's why slowdebug build can pass.
> 
> Note-3: To be honest, I didn't fully understand why this warning is raised by GCC 12 and 13. Recently we struggled with GCC 12 false positive warnings many times. See [1][2].
> 
> To suppress this warning, we try to fool GCC compiler via minor code change in this patch.
> 
> [1] https://bugs.openjdk.org/browse/JDK-8299580
> [2] https://bugs.openjdk.org/browse/JDK-8294031

Hao Sun has updated the pull request incrementally with one additional commit since the last revision:

  print out the value for better debugging

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16819/files
  - new: https://git.openjdk.org/jdk/pull/16819/files/d80bebbd..a0b0ac2a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16819&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16819&range=00-01

  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16819.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16819/head:pull/16819

PR: https://git.openjdk.org/jdk/pull/16819