RFR: 8294314: Minimize disabled warnings in hotspot [v2]

[Kim Barrett]

On Fri, 23 Sep 2022 21:49:55 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> After [JDK-8294281](https://bugs.openjdk.org/browse/JDK-8294281), it is now possible to disable warnings for individual files instead for whole libraries. I used this opportunity to go through all disabled warnings in hotspot.
>> 
>> Any warnings that were only triggered in a few files were removed from hotspot as a whole, and changed to be only disabled for those files.
>> 
>> Some warnings didn't trigger in any file anymore, and could just be removed.
>> 
>> Overall, this reduced the number of disabled warnings by roughly half for gcc, clang and visual studio. The remaining warnings are sorted in "frequency", that is, the first listed warnings are triggered in the most number of files, while the last in the fewest number of files. So if anyone were to try to address the remaining warnings, it would make sense to chop of this list from the back.
>> 
>> I believe the warnings that are disabled on a per-file basis can most likely be fixed relatively easily.
>> 
>> I have verified this by Oracle's internal CI system, and GitHub Actions. (But I have not yet gotten a fully green run due to instabilities in GHA, however this patch can't reasonably have anything to do with that.) As always, warnings tend to differ a bit between compilers, so if someone wants to take this on a spin with some other version, please go ahead. If I missed some warning, in worst case we'll just have to add it back again, and in the meanwhile `configure --disable-warnings-as-errors` is an okay workaround.
>> 
>> It also turned out that JDK-8294281 did not save the new per-file warnings in VarDeps, so I had to move $1_WARNINGS_FLAGS from $1_BASE_CFLAGS to $1_CFLAGS (and similar for C++).
>> 
>> Annoyingly, the assert macro triggers `tautological-constant-out-of-range-compare` on clang, so while this is a single problem in a single file, this erupts all over the place in debug builds. If this can be fixed, the ugly extra `DISABLED_WARNINGS_clang += tautological-constant-out-of-range-compare` for non-release builds can be removed.
>
> Magnus Ihse Bursie has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - ppc64 triggers shift-negative-value on gcc :-(
>  - s390 triggers shift-negative-value on gcc :-(

I like the general idea.  In particular, the file-specific disables provide a
nice list of bite-sized potential cleanup RFEs.

However, shift-negative-value should remain globally disabled for gcc/clang;
see https://bugs.openjdk.org/browse/JDK-8240259.

There might be other warnings that should remain globally disabled; I've not
gone through the list carefully yet.

There used to be more comments around the warning disables. The Windows ones
had comments that gave titles to the numeric codes. And I think there were
some comments that discussed why a warning was disabled rather than fixing
code. I don't know when that all disappeared, but it was quite a while ago,
and I still miss that commentary.

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

Changes requested by kbarrett (Reviewer).

PR: https://git.openjdk.org/jdk/pull/10414