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

[Kim Barrett]

On Tue, 28 Nov 2023 02:21:26 GMT, Hao Sun <haosun at openjdk.org> wrote:

>> 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

Oracle CI is using vanilla gcc13.2.1 (so no distribution-specific patches),
and not seeing these warnings. -Wnonnull warnings are enabled by -Wall, unless
otherwise suppressed. -Wnonnull warnings are suppressed for one file
(ad_$(HOTSPOT_TARGET_CPU_ARCH).cpp), but that's not the file involved here. So
I don't understand why Oracle CI is not seeing these warnings too.  This needs
to be investigated.

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PR Comment: https://git.openjdk.org/jdk/pull/16819#issuecomment-1829042601