Null restriction on interface typed fields

[Chen Liang]

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


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/valhalla-dev/attachments/20240617/e953184b/attachment-0001.htm>