null checks vs. class resolution, and translation strategy for casts

[John Rose]

On Apr 9, 2020, at 1:16 PM, John Rose <john.r.rose at oracle.com> wrote:
> 
> On Apr 9, 2020, at 1:03 PM, Brian Goetz <brian.goetz at oracle.com> wrote:
>> 
>> 
>>> I have a proposal for a translation strategy:
>> 
>> Casts to inline classes from their reference projections will be frequent.  Because the reference projection is sealed to permit only the value projection, a cast is morally equivalent to a null check. We want to preserve this performance model, because otherwise we're reinventing boxing.
>> 
>> Going through `ldc X.class / invokevirtual Class.cast` will surely be slow in the interpreter, but also risks being slow elsewhere (as do many of the other options.)
>> 
>> So let me add to your list: is it time for a `checknonnull` bytecode, which throws NPE if null, or some other more flexible checking bytecode?  (Alternatively, if we're saving bytecodes: `invokevirtual Object.<nullcheck>`), where <nullcheck> is a fake method that always links to a no-op, but invokevirtual NPEs on a null receiver.)
> 
> Um, this feels a lot like a premature optimization.  Let’s not add
> `checknonnull` intrinsics to the interpreter (the very most
> expensive way to do it) until we have tried the other alternatives
> (Objects.requireNonNull, etc.) and have proven that the costs
> are noticeable.  And a spec EG is not the place to evaluate such
> questions; it has to be demonstrated in a prototype.
> 
> I see now why you are angling for verifier rules that know about
> sealing relations.  I think that also is premature optimizations.
> Actually, verifier rules (not interpreter bytecodes) are the most
> costly way to get anything done.
> 
> Sorry to be a party pooper here, but that’s how it looks right now.
> 
> — John

P.S. The Object.<checknotnull> idea is clever, and we have done
things like that in the past; the interpreter has special fast entry
points for certain math functions.  These were added due to certain
benchmarks being slow >20 years ago; who knows if they are still
relevant.  We could do the same for Objects.requireNonNull; that
would be a less intrusive (more sneaky) version of Object.<checknotnull>.
No specs were harmed in making this proposal.