RFR: 8343800: Cleanup definition of NULL_WORD

[Kim Barrett]

On Mon, 2 Dec 2024 09:04:59 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Please review this change to how NULL_WORD is defined.  Instead of being a
>> macro whose definition is in compiler-specific files with a conditionally
>> defined expansion, it is now a simple integral constant with a single shared
>> definition.
>> 
>> Testing: mach5 tier 1-5, GHA sanity tests.
>
> src/hotspot/share/utilities/globalDefinitions.hpp line 563:
> 
>> 561: const jfloat max_jfloat = jfloat_cast(max_jintFloat);
>> 562: 
>> 563: // A named constant for the integral representation of a Java null.
> 
> Some commentary on when/where/why to use this would be good. I see it being used in a lot of places related to oops (exceptions, VMResult) but other uses are not obviously (to me) Java object related.

I _think_ this change could have been made a long time ago.  But there was messy stuff related to 32bit, and
also maybe Solaris?  It was also hard to tell, because of the conflation with NULL and the various complexities
there.

I think some of the current uses could be classed as abuse, but again, it's hard to know without a little more
clarity in its definition.  I made a couple of attempts at cleaning up usage a while ago, but ended up abandoning
them because of that lack of clarity.  What came out of that was the realization that something like this might
be possible, and would be a useful precursor to usage changes.  That comment is my stake in the ground for
helping to distinguish good uses vs abuse.  I'm planning to revisit uses with that in mind.

The vast majority of uses are in x86/x64 assembly code, where I think it is usually about encoding a Java null
in an instruction.  I've looked at changing how that's done, but the messiness of the definition, crossed with
the 32/64bit issues there, again led me to balk.  Nuking 32bit x86 might reduce the complexity to something
more manageable.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22468#discussion_r1865710829