Null restriction on interface typed fields

[Olexandr Rotan]

Sorry if I am missing out something, but if null-restricted type contains
null value (as vm initializes it), isn't the null value itself is a marker
of the fact that value hasn't been set yet?

As I understand, if you assert non-nullity of null able type or type of
unknown nullability with some sort of syntax (like ! in many languages),
when in fact the value is null, exception should be thrown, so vm already
obligated to perform null-check on this assignments. Same goes for
reflective set, which should be treated as unknown nullity in any condition
I guess unless vm could specialize instructions for the same method call
but different nullity of args (either way vm must check nullity of value in
at least part of the situations). This leads to a fact that if person even
somehow manages to sneak through compiler analysis and try to set null to
non-nullable field, it will immediately fail. Therefore, if value is null -
it is unset.

That said, does it in fact matter that VM initializes this fields with
null, if null bit (I guess that is how it stored), in fact is a marker of
whether field has been initialized or not? VM could just check if value of
field is null, and if so, throw an exception. Moreover, one way or another,
as you said, checks on value access must be performed. So is this analysis
really converges to "check if field is null", or there is more to it and I
miss it?

On Mon, Jun 17, 2024, 22:16 Chen Liang <chen.l.liang at oracle.com> wrote:

> Indeed, another feature about the strict fields is their strong
> encapsulation against reflective setters; another safeguard against
> representable invalid values and their friendliness toward constant
> folding. John Rose's chart here seems up-to-date: objects-reloaded
> (openjdk.org) <https://cr.openjdk.org/~jrose/values/objects-reloaded.pdf> (This
> strictness can also benefit the upcoming Stable Values JEP)
>
> Also for readers' convenience, I think this is the earliest and most
> comprehensive document about the strict field designs, seems still accurate
> for the most part: Cleanup on aisle zero: Constructive classes
> (openjdk.org)
> <https://cr.openjdk.org/~jrose/jls/constructive-classes.html>
>
> - Chen
> ------------------------------
> *From:* Brian Goetz <brian.goetz at oracle.com>
> *Sent:* Monday, June 17, 2024 1:00 PM
> *To:* Chen Liang <chen.l.liang at oracle.com>; valhalla-dev at openjdk.org <
> valhalla-dev at openjdk.org>
> *Subject:* Re: Null restriction on interface typed fields
>
> Yes, the eventual plan is that all references can be null-restricted.
> We've been struggling with a specific problem, though, which is that for a
> reference type like `String!` or `Runnable!`, the VM is going to initialize
> such variables with .. null.  This is obviously a safety problem, because
> we've put a value in a variable that is not provably within the value set
> of the variable's type.  It was for this reason that earlier discussion
> focused on nullity control for (implicitly constructible) values first, and
> other types later.
>
> Since then, we've figured out that we can solve this problem with better
> flow analysis.  Just like the DA analysis done by the compiler, the VM can
> do a similar analysis during verification time of fields that the compiler
> marks as "must be written before they are read" (where any this-escape
> might lead to reading those fields.)  This goes under the name of "strict
> fields", and we should be writing more about this soon.
>
> Once we have this tool in our kit, the limitations on what types can be
> null-restricted -- and the safety with which we can enforce this -- will be
> greatly broadened.
>
> On 6/9/2024 5:04 AM, Chen Liang wrote:
>
> Hello valhalla community,
> In our current iteration of valhalla, we only support null restriction on
> value fields, as for references, null is a valid default value. Meanwhile,
> interfaces are not restricted to be value or identity, yet some value
> classes will only be represented via interfaces, most notably Map.entry().
>
> In addition, consider this scenario in the ClassFile API: we have
> ClassBuilder.withMethod(xxx, Consumer<MethodBuilder>) and
> withMethodBody(xxx, Consumer<CodeBuilder>). A straightforward
> implementation of withMethodBody would be withMethod(xxx, new
> WithCode(cbConsumer)), where WithCode is (value) record
> WithCode(Consumer<CodeBuilder> build) implements Consumer<MethodBuilder>...
> In this WithCode record, we can see that we are interested in declaring
> "build" null-restricted; if build is a value lambda that simply captures
> context variables, then WithCode can be inlined to the same captures should
> "build" be NR, without concerns about representing a null build value in
> the inlined form.
>
> Another example is in Algebraic types:
> sealed interface Operation permits O1, O2, ...
> Record O1(Interface1 xxx)
> Record O2(Interface2 xxx)
> For value-based Interface1 and Interface2 values, we might seek to NR the
> fields so the record representations can be simpler.
>
> Thus, I recommend considering support for NR on interface fields in
> addition to on explicit value type fields. Though this might lead down to
> the rabbit hole of "heap pollution of null on reference pointers", I still
> believe its benefit outweighs its cost, especially in cases where these
> values can be restricted on the stack in escape analysis, as in most
> functional APIs (function only invoked conditionally, function objects not
> stored when method exits).
>
> Chen Liang
>
>
>
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