Comment on "Revisiting default values"

[Raffaello Giulietti]

Hello,

IMO the "zero means null" proposal in [1] for instances implementing 
NoGoodDefault is the most consistent with the usual way of thinking Java.

Here's a possible way to slightly lower the cost of the check against 
the default (the "zero").

Of course, a default-hostile class always has proper instances in which 
at least one field (not necessarily the same in all instances) is "nonzero".

Oftentimes, by design, a class mandates some specific field to be 
"nonzero" in every instance. For example, the month (or the day) in 
LocalDate should not be zero; name or type in DirectMethodHandleDescImpl 
must be non-null; e0 in List12 must not be null; etc.

In such cases, once migrated to NoGoodDefault primitive classes, it 
suffices to check any of these fields for "zero" to witness the default 
value. That is, if such a field is "zero" then the instance is 
necessarily the unacceptable default. There's no need to compare all 
bits against "zero".

If the user could annotate such fields with something like @NonDefault 
or @DefaultWitness, the vm could possibly perform better in some 
circumstances, particularly on "fat" instances.


Greetings
Raffaello

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[1] 
https://mail.openjdk.java.net/pipermail/valhalla-spec-experts/2021-March/001486.html