Matcher method name mangling Was: Deconstruction patterns

[forax at univ-mlv.fr]

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> From: "Dan Heidinga" <heidinga at redhat.com>
> To: "Remi Forax" <forax at univ-mlv.fr>
> Cc: "Brian Goetz" <brian.goetz at oracle.com>, "amber-spec-experts"
> <amber-spec-experts at openjdk.java.net>
> Sent: Tuesday, March 7, 2023 2:21:47 PM
> Subject: Re: Matcher method name mangling Was: Deconstruction patterns

> On Tue, Mar 7, 2023 at 4:21 AM Remi Forax < [ mailto:forax at univ-mlv.fr |
> forax at univ-mlv.fr ] > wrote:

>> ----- Original Message -----
>>> From: "Brian Goetz" < [ mailto:brian.goetz at oracle.com | brian.goetz at oracle.com ]
>> > >
>>> To: "amber-spec-experts" < [ mailto:amber-spec-experts at openjdk.java.net |
>> > amber-spec-experts at openjdk.java.net ] >
>> > Sent: Monday, March 6, 2023 7:24:54 PM
>> > Subject: Deconstruction patterns

>> > Time to look ahead to the next installment of pattern matching:
>> > deconstruction patterns, which generalize record patterns. This
>> > document does an end-to-end walkthrough (at a sketchy level of detail)
>> > through declaration, overloading, use, translation, and reflection of
>> > deconstruction patterns.

>> > I would like to *not* discuss syntax at this time. There's a lengthy
>> > discussion to be had about syntax, and we'll have that, but let's nail
>> > down model, semantics, and translation first.

>> > As usual, I would prefer that people either (a) post a single reply
>> > addressing the totality of this sketch or (b) start _new threads_ if you
>> > want to discuss a specific aspect. A quick "I'll just reply to this
>> > minor detail" seems to often derail the conversation in such a way that
>> > it never comes back. If this all looks fine to you, a quick "no
>> > surprises here" will keep us from suspensefully waiting for feedback.


>> > # Deconstruction patterns -- translation, use, and reflection

>> > As we are wrapping up record patterns, it's time to look ahead to the
>> > next major
>> > part of the pattern matching story -- extending the capabilities of record
>> > patterns to all classes that want to support destructuring. Record
>> > patterns are
>> > simply a special case of _deconstruction patterns_ or _deconstructors_,
>> > where we
>> > derive the deconstructor API, implementation, and use from the state
>> > description
>> > of the record. For an arbitrary class, a deconstruction patterns will
>> > require
>> > an explicit member declaration, with a header identifying the names and
>> > types of
>> > the bindings and a body that extracts the bindings from the representation.

>> > ## Deconstructors

>> > Just as constructors are special cases of methods, deconstruction
>> > patterns are
>> > special cases of a more general notion of declared pattern, which also
>> > includes
>> > static matchers (the dual of static methods) and instance matchers (the
>> > dual of
>> > instance methods.) Specifically, unlike the more general notion of
>> > matcher, a
>> > deconstructor must be _total_; it must always match. This document will
>> > focus
>> > exclusively on deconstructors, and we'll come back to static and instance
>> > matchers in due time. (But note that some of the design choices in the
>> > simple
>> > case of deconstructors may be constrained by the more general case.)

>> > There are a number of choices for how we might syntactically represent a
>> > deconstructor (or more generally, a declared pattern.) For purposes of
>> > illustration, this document picks one possible syntactic expression of
>> > deconstructors, but it is premature to devolve into a syntax discussion
>> > at this
>> > time.

>> > ```
>> > class Point {
>> > final double x, y;

>> > public Point(double x, double y) {
>> > this.x = x;
>> > this.y = y;
>> > }

>> > public matcher Point(double x, double y) {
>> > x = this.x;
>> > y = this.y;
>> > }
>> > }
>> > ```

>> > This example illustrates two aspects of the duality between constructors and
>> > their corresponding deconstructors. Their APIs are duals: a constructor
>> > takes N
>> > parameters containing the desired description of the object state and
>> > produces a
>> > constructed object; a deconstructor starts from the constructed object
>> > and has N
>> > bindings (outputs) that receive the desired state components. Similarly,
>> > their
>> > implementations are duals: the body of the constructor initializes the
>> > object
>> > representation from the description, and the body of the deconstructor
>> > extracts
>> > the description from the representation. A deconstructor is best
>> > understood as
>> > a _co-constructor_.

>> > The `Point` example above is special in two ways. First, the internal
>> > representation of a `Point`, and the API of the constructor and
>> > deconstructor,
>> > are the same: `(double x, double y)`. We can call the API implied by the
>> > constructor and deconstructor the _external representation_, and for
>> > `Point`,
>> > both the internal and external representations are the same. (This is one of
>> > the requirements for being a candidate to be a record.) And second, the
>> > constructor is _total_; it does not reject any combinations of arguments.

>> > Here's another version of `Point` which does not have these special
>> > aspects; it
>> > uses the same internal representation as before, but chooses a pair of
>> > strings
>> > as the external representation:

>> > ```
>> > class Point2 {
>> > final double x, y;

>> > public Point2(String x, String y) {
>> > this.x = Double.parseDouble(x);
>> > this.y = Double.parseDouble(y);
>> > }

>> > public matcher Point2(String x, String y) {
>> > x = Double.toString(this.x);
>> > y = Double.toSTring(this.y);
>> > }
>> > }
>> > ```

>> > The method `Double::parseDouble` will throw `NumberFormatException` if its
>> > argument does not describe a suitable value, so unlike the `Point`
>> > constructor,
>> > the `Point2` constructor is partial: it will reject `new Double("foo",
>> > "bar")`.
>> > And the internal representation is no longer the same as the external
>> > representation. Less obviously, there are valid string values that we can
>> > provide to the constructor, but which cannot be represented exactly as
>> > `double`,
>> > and which will be approximated; the string value
>> > `"3.22222222222222222222222222222222222222"` will be approximated with the
>> > double value `3.2222222222222223`.

>> > This example highlights more clearly how the constructor and
>> > deconstructor form
>> > an _embedding-projection pair_ between the internal and external
>> > representations. While some external representations might be invalid,
>> > and some
>> > might result in approximation, deconstruct-then-construct is always an
>> > identity
>> > transformation. Indeed, the specification of `java.lang.Record`
>> > requires that
>> > if we deconstruct a record with its accessors, and pass the resulting values
>> > back to the constructor, we should get a new record that is `equals` to the
>> > original.

>> > The fact that constructor and deconstructor (and eventually, factory and
>> > static
>> > matcher) form an embedding-projection pair is why we are able to derive
>> > higher-level language features, such as [safer
>>> serialization]( [
>>> https://openjdk.org/projects/amber/design-notes/towards-better-serialization |
>>> https://openjdk.org/projects/amber/design-notes/towards-better-serialization ]
>> > )
>> > and [functional transformation of immutable
>>> objects]( [
>>> https://github.com/openjdk/amber-docs/blob/master/eg-drafts/reconstruction-records-and-classes.md
>>> |
>>> https://github.com/openjdk/amber-docs/blob/master/eg-drafts/reconstruction-records-and-classes.md
>> > ] ),
>> > from a matched set of constructor and deconstructor.

>> > Of course, users are free to implement constructors without
>> > deconstructors, or
>> > constructors and deconstructors whose external representations don't
>> > match up,
>> > or even matching constructors and deconstructors that are not
>> > behaviorally dual.
>> > But providing a matched set (or several) of constructors and deconstructors
>> > enables reliably reversible aggregation, and allows us to mechanically
>> > derive
>> > useful higher-level features such as withers.

>> > #### Overloading

>> > Just as constructors can be overloaded, deconstructors can be overloaded
>> > for the
>> > same reason: multiple constructors can expose multiple external
>> > representations
>> > for aggregation, and corresponding deconstructors can recover those multiple
>> > external representations. Any matching pair of
>> > constructor-deconstructor (and
>> > eventually, factory-deconstructor) is a candidate for use in higher-level
>> > features based on the embedding-projection nature of the
>> > constructor-deconstructor pair.

>> > Just as deconstruction is dual to construction, overloading of
>> > deconstructors is
>> > dual to that of constructors: rather than restricting which sets of
>> > parameters
>> > can be overloaded against each other, we do so with the bindings
>> > instead. For
>> > constructors of a given arity, we require that their signatures not be
>> > override-equivalent; for deconstructors of a given arity, we require the
>> > same of
>> > their bindings.

>> > For a deconstructor (and declared patterns in general), we derive a _binding
>> > signature_ (and an erased _binding descriptor_) which treats the binding
>> > list as
>> > a parameter list. The overload rule outlined above requires that binding
>> > signatures for two deconstructors of the same arity not be
>> > override-equivalent.
>> > (We will find it useful later to derive a `MethodType` for the binding
>> > descriptor; this is a `MethodType` whose return type is `V` and whose
>> > parameter
>> > types are the erased types of the bindings.)

>> > #### Digression: embedding-projection pairs

>> > Given two sets _A_ and _B_, a pair of functions `e : A -> B` and `p : B
>> > -> A`,
>> > forms an _embedding-projection pair_ if `p . e` (embed then project) is an
>> > identity function, and `e . p` (project then embed) _approximates_ the input
>> > according to a domain-specific approximation metric (which is a complete
>> > partial
>> > ordering on `B`.)

>> > When applied to constructor-deconstructor pairs, this says that
>> > deconstructing
>> > an object and then reconstructing it with the resulting bindings should
>> > result
>> > in an equivalent object, and constructing an object from an external
>> > representation and then deconstructing it back into that external
>> > representation
>> > should result in an approximation of the original external
>> > representation. (A
>> > complete partial ordering models constructor failure as the non-terminating
>> > bottom value, which is considered an infinitely bad approximation to
>> > everything.)

>> > Embedding-projection pairs have a number of desirable properties, such
>> > as the
>> > composition of two e-p pairs is an e-p pair; this property is at the
>> > heart of
>> > using constructor-deconstructor pairs for improved serialization and
>> > functional
>> > transformation.

>> > ## Invoking deconstructors

>> > We've already seen how to "invoke" deconstructors: through pattern matching.
>> > What we've been calling "record patterns" are merely deconstruction patterns
>> > derived mechanically from the state description, just as we do with
>> > constructors
>> > and accessors; there is little difference between record patterns and
>> > deconstruction patterns other than the ability to declare them explicitly.
>> > (There is an accidental difference in the translation, in that we currently
>> > implement record patterns by appealing to individual accessors rather than a
>> > single deconstructor, but this may eventually converge as well.)

>> > The use-site syntax of deconstruction bears a deliberate similarity to
>> > that of
>> > construction; `new Point(x, y)` is deconstructed by `case Point(var x,
>> > var y)`.

>> > #### Overload selection

>> > In the presence of overloaded deconstructors, we need to figure out which
>> > deconstructor a deconstruction pattern `C(P*)` is referring to. The
>> > details are
>> > similar to overload selection for methods, except that we operate on the
>> > bindings rather than the parameters. We first search for _applicable
>> > matchers_,
>> > using increasingly loose criteria (first excluding boxing, unboxing, and
>> > varargs; then including boxing and unboxing but not varargs; and
>> > finally, all
>> > candidates) and then selecting the most applicable.

>> > It is tempting to try and bypass the three-phase selection process and use a
>> > simpler notion of applicability (perhaps noting that we got this process for
>> > compatibility with existing overload selection decisions when autoboxing and
>> > varargs were added, and that there are few deconstructor invocations to be
>> > compatible with yet.) But because existing overloaded constructors use this
>> > mechanism, and there is significant value in pairing constructors and
>> > deconstructors, attempting to invent a simpler-but-different overload
>> > selection
>> > mechanism for deconstructors would inevitably undermine the duality between
>> > matching constructor-deconstructor pairs. So compatibility (this time, with
>> > existing overloaded constructors) once again forces our hand.

>> > The specification for overload selection is complicated significantly by
>> > poly
>> > expressions (e.g., lambdas); fortunately, there are no "poly patterns",
>> > and so,
>> > while the structure of JLS 15.12.2 is retained for overload selection of
>> > deconstruction patterns, much of the detail is left behind.

>> > ## Translation

>> > We translate patterns into synthetic methods with a `Matcher` attribute;
>> > this
>> > method implements the matcher behavior. The translation scheme derives
>> > from a
>> > number of requirements, only some of which are in play for deconstructors.

>> > The matcher method for a deconstructor is a final instance method that
>> > takes no
>> > parameters and returns `Object`, perhaps with a special name (just as
>> > constructors are called `<init>`.)

>> > #### Carriers

>> > Because the matcher methods implements the matcher behavior, but a
>> > matcher may
>> > "return" multiple bindings (or failure), we must encode the bindings in some
>> > way. For this, we use a _carrier object_. The choice of carrier is
>> > largely a
>> > footprint/specificity tradeoff. One could imagine a carrier class per
>> > matcher,
>> > or a carrier class per matcher descriptor, or using `Object[]` as a
>> > carrier for
>> > everything, or caching some number of common shapes (e.g, three ints and two
>> > refs). This sort of tuning should be separate from the protocol encoded
>> > in the
>> > bytecode of the pattern method and its clients.

>> > We use a small _carrier runtime_ to decouple pattern translation from
>> > carrier
>> > selection. (This same carrier runtime is used by string templates as well.)
>> > This allows tradeoffs in runtime characteristics (e.g., carrier per
>> > matcher vs
>> > sharing carriers across matchers, dropping carrier identity with value types
>> > later, etc) without affecting the translation. The carrier API consists
>> > of condy
>> > bootstraps like:

>> > ```
>> > static MethodHandle carrierFactory(MethodType matcherDescriptor) { ... }
>> > static MethodHandle carrierAccessor(MethodType matcherDescriptor, int
>> > bindingNo) { ... }
>> > ```

>> > The `matcherDescriptor` is a `MethodType` describing the binding types. The
>> > `carrierFactory` method returns a method handle which takes the bindings and
>> > produces a carrier object; the `carrierAccessor` method returns method
>> > handles
>> > that take the carrier object and return the corresponding binding. To
>> > indicate
>> > success, the matcher method invokes the carrier factory method handle and
>> > returns the result; to indicate failure (deconstructors cannot fail, but
>> > other
>> > matchers can) the matcher method returns null.

>> > We would translate the XY deconstructor from `Point` as follows
>> > (pseudo-code):

>> > ```
>> > #100: MethodType[(II)V]
>> > #101: Condy[bsm=Carriers::carrierFactory, args=[#100]]

>> > final synthetic Object Point$MANGLE() {
>> > aload_0
>> > getfield Point::x
>> > aload_0
>> > getfield Point::y
>> > LDC #101
>> > invokevirtual MethodHandle::invoke(II)V
>> > areturn
>> > }
>> > ```

>> > Constant `#100` contains a `MethodType` holding the binding descriptor;
>> > constant
>> > `#101` holds a method handle whose parameters are the parameter types of the
>> > binding descriptor and returns `Object`.

>> > At the use site, matching a deconstruction pattern is performed by
>> > invoking the
>> > matcher method on the appropriate target object, and then extracting the
>> > components with the carrier accessor method handles if the match is
>> > successful.
>> > (Deconstructors are total, so are always successful, but for other patterns,
>> > null is returned from the matcher method on failure to match.)

>> > #### Method names

>> > The name of the matcher method is mangled to support overloading. The JVM
>> > permits overloading on parameter types, but not return types (and overloaded
>> > matchers are effectively overloaded on return types.) We take the
>> > approach of
>> > encoding the erasure of the matcher descriptor in the name of the
>> > pattern. This
>> > has several desirable properties: it is stable (the name is derived
>> > solely from
>> > stable aspects of the declaration), for matchers with override-equivalent
>> > signatures (deconstructors can't be overridden, but other patterns can be),
>> > these map to true overrides in the translation, and valid overloads of
>> > matchers
>> > will always have distinct names.

>> > We use the ["Symbolic Freedom"]() encoding of the erasure of the matcher
>> > descriptor as the mangled disambiguator, which is exactly as stable as
>> > any other
>> > method descriptor derived from source declarations.

>> I do not think you need mangling, you need different name but matcher$0,
>> matcher$1, etc or something similar with a counter should be enough.

>> The Matcher attribute contains enough information to do the linking both at
>> compile time and at runtime.

>> The idea is that in the bytecode, you can derive the MethodType from the
>> invokeVirtual (or invokedynamic) and then ask the VM the method that has a
>> Matcher attribute that matches that MethodType.
>> So the exact name of the matcher method should not be present in the bytecode of
>> a switch that uses a deconstructor, only the corresponding MethodType should be
>> present.

>> Mangling method names appear in stack trace, if we can have "light mangling",
>> i.e. a counter likes with lambdas, instead of a C++ like mangling, i think it's
>> a win.

> A couple of thoughts:
> * counter based names aren't very friendly to the broader ecosystem, especially
> to native image / precompilation projects. Best to avoid them when possible.
> i.e. There was a recent PR [0] that removed the counter from the hidden classes
> created by the LambdaMetafactory to make the build more reproducible for native
> image generators.

Counter based class name *at runtime* (where the thread scheduling change from run to run) is not the same issue as counter based method name at *compile time*. 
I think the problem is not the same here. 

> * embedding the mangled descriptor in the name is a big aid to debugging for
> users and service engineers. It's much easier to tell which matcher was called
> from a stacktrace when you only have the stacktrace and the source code than
> when you need to figure out which matcher got numbered as matcher$1.

Yes, it helps VM/JDK engineers, not necessary end users. We have kept the name of the desugared lambdas as simple as possible even if having the type of the parameters would have helped people that knows exactly how the compiler desugars things for the runtime. 

> * stable names allow for existing override mechanisms (ie: vtables) to work. Not
> sure this is the intended strategy but if it is, then we need a name that
> ensures that matchers implemented in subclasses correctly override the ones in
> superclasses. While this can be done without the names, the VM is already
> really good at matching names and descriptors.

That a stronger argument. Not having stable name will cause trouble if the base class and the subclass are compiled separately. Like you, i am not sure of the exact strategy around overriding of matcher methods. During previous discussions, we talked about overridable deconstructors but it seems Brian has now rule them out. 

> I haven't thought through the proposal to avoid having the names in the bytecode
> enough to comment on that yet.

> --Dan

> [0] [ https://github.com/openjdk/jdk/pull/12579 |
> https://github.com/openjdk/jdk/pull/12579 ]

Rémi 
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