RFR: 8340620: Fix -Wzero-as-null-pointer-constant warnings for CompressedOops [v2]

[Aleksey Shipilev]

On Fri, 27 Sep 2024 09:03:15 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Please review this change that fixes -Wzero-as-null-pointer-constant warnings
>> in CompressedOops code.  These all relate to CompressedOops::base().
>> 
>> I also added a couple of asserts to verify our assumptions about null pointer
>> constants being representationally zero. That isn't a Standard-conforming
>> assumption, but holds for all platforms we currently support. I considered,
>> and even explored, a couple of different options.
>> 
>> (1) Continue to have CompressedOops::base() be a pointer, but avoid that
>> assumption, being more careful about how zero-valued pointers are treated. But
>> that adds significant complexity that we can't test, since we don't support
>> any platforms needing that extra work.
>> 
>> (2) Change CompressedOops::base() to an integral adjustment.  This is probably
>> the correct approach, but is much more intrusive and wide ranging in the
>> changes required.  Maybe something for the future.
>> 
>> Testing: mach5 tier1-5
>> GHA testing, verifying builds on some platforms not supported by Oracle.
>> 
>> There are some simple changes to s390 and ppc code that I haven't tested,
>> beyond verifying compilation.
>
> Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:
> 
>   remove nullptr representation asserts

Still good, ship it.

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Marked as reviewed by shade (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21172#pullrequestreview-2333271086