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

[Stefan Karlsson]

On Tue, 24 Sep 2024 23:26:08 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.

FWIW, I think these asserts adds extra noise to these functions and I don't think we will be much more happy about having to read them over and over again when we read this functions / debug code through these functions. I would have preferred if this was one of those things that we require from our platforms and place a check in globalDefinitions, or some other prominent place that checks HotSpot's assumptions of the compilers / platforms.

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PR Comment: https://git.openjdk.org/jdk/pull/21172#issuecomment-2376415052