enso/docs/runtime-roadmap.md

layout	title	category	tags	order
developer-doc	High-Level Runtime Roadmap	summary	contributing	6

High-Level Runtime Roadmap

This roadmap consists of longer, open-ended tasks that are required to make Enso better in the long term. The tasks here are not in any order that indicates priority, but the dependencies between tasks are described.

Technology Choices

Enso interpreter is written in a mixture of Scala and Java. Scala was originally used due to the capabilities of its type system in comparison to Java's. Modern Java (as provided by JDK 21 or Frgaal compiler) meets most of the needs too. The ultimate goal is to write everything in Java and also keep up with most recent long term supported JDK/GraalVM releases.

Static Analysis

Enso is a fairly dynamic language, but this doesn't mean that it doesn't admit static analysis. There are a number of areas that can be made better (read: more intuitive, more performant, and so on). These, again, are not in order of priority, but where there are dependencies these are indicated.

Purpose-Built IR

The current compiler IR is a bit of a mess. Due to time constraints, we ended up moving on with it though it was firmly unsuited to the direction we wanted to evolve the compiler. While many of the features listed below are possible in the current IR, they are difficult and inelegant compared to doing them on an IR suited to the task.

Currently, the IR is:

• Very verbose and difficult to add a new node to. Adding a new node requires adding ~100 lines of code that could likely be automated away. Lots of boilerplate.
• Of poor performance as witnessed by static compiler benchmarks
• Partially mutable, making it confusing as to which things are shared.

A new IR for Enso would have to:

• Be able to be serialized to disk (the current one can).
• Remove the verbosity and repetition when adding new nodes.
• Be built with performance and easy traversal in mind.

While it is a daunting task to wholesale move the entire compiler to a new IR, it can instead be done in an incremental fashion. First, it makes sense to design and build the new IR, and then write a translation from the current IR to the new IR. With that done, the boundary between the two in the compiler can be gradually shuffled, starting with codegen (IrToTruffle), until no usages of the old IR remain.

If it were up to us, we'd consider basing the new IR on a mutable graph as this easily admits many common compiler operations, and also is likely to reduce memory usage of the compiler overall. Care should be taken with introducing mutability, however. While the current IR is mutable in limited ways (primarily the metadata on nodes), a fully mutable IR will have to have comprehensive utilities for deep copying and dealing with cycles. That said, Marcin thinks that it may be worthwhile to stick to an immutable structure.

These two approaches offer a trade-off in terms of what they make easy. While it's very easy to reason about tree-like structures (within a module), it makes certain operations (e.g. Alias Analysis) more painful than they would otherwise be (we had to make a graph on top of the tree to get this working).

Unreliably, we can guestimate at:

• A tree with less verbosity and fixing some niggles would be approximately a month to implement the new IR and migrate the compiler and passes.
• A more novel graph-based IR would be more complex to implement (a couple of months perhaps), but also to migrate the passes due to the change in underlying principles. While it would make certain passes (e.g. dataflow analysis, alias analysis) easier to maintain and understand, the underlying principle still changes.

Improving Static Analysis Capabilities

Though we're not suggesting moving to a fully-type-checked language any time soon, the current system doesn't make use of most of the information contained in the type signatures. This should involve:

• Processing and resolving the existing type signatures for use in the compiler. We want to use them to provide accurate and helpful suggestions in the IDE. The type signatures are currently ignored by the compiler. They are only kept in their original almost-AST form. They are currently used primarily for documentation, though can also be used to indicate lazy arguments, and perform some role in automated parallelism analysis.
• Performing forward-only type propagation. This is a big win for comparatively low effort: if we have the standard library liberally type-hinted, forward-only propagation for workflows in the IDE means that you can have type information for a majority of the program without having to implement backwards inference rules for Enso (which are very complex). This win is for user programs, and requires type hinting of libraries to work well.
• Using that information to perform certain optimisations (see below).

While you do not need to update the IR to do this analysis and subsequent optimisation, it would certainly make many of them easier. If you are writing more passes on top of the old IR, it's just piling on technical debt. Please be aware of this.

Static Optimization Passes

With improved static analysis capabilities, we gain the ability to do lots more optimisations statically.

Scope Flattening

There are multiple points in the language where we create new scopes where this isn't strictly necessary. Eliminating these extra scopes eliminates the need for allocations and dynamic calls.

• Many types of pattern matches can, instead of treating each branch as a lambda, flatten these into an almost JS-style if-then-else. Rather than inserting a function call for each branch, we can hoist (with renaming) variables into the same scope. This means we don't need to perform a function call or allocate a new scope.
• With type inference, there are many cases where a lazy argument doesn't need to be made lazy (currently they are evaluated in a separate scope). This would improve performance significantly. In our opinion, this is the biggest performance pitfall of the language implementation.

For simple programs, GraalVM can usually optimise these additional scopes away. However, doing this flattening process removes the need to optimise these things and may actually admit more optimisations (claim unverified). This means that we think Graal will spend more time optimising the parts of the programs that matter.

Pattern Match Optimisation

Currently we don't perform any optimisation when desugaring nested pattern matches. This means that the IR (and resultant generated truffle code) is far larger than it needs to be.

• Deduplicating and flattening case expressions will bring a large win in terms of memory usage.
• This will likely also improve performance as less if branches need to occur to resolve the actual target function of the pattern match.
• It may be useful to look at dotty's implementation of pattern match desugaring and optimisation as Ara finds it very readable.

Liveness Analysis

Currently Enso keeps every variable alive for as long as it's in scope. This means that we have two major pitfalls:

1. We retain large data for far longer than is necessary (until the end of the enclosing scope, rather than until the last usage), ballooning the language's memory usage.
2. We accidentally capture these bloated scopes when creating closures, further retaining unnecessary data for the lifetime of the closure.

While we originally proposed to perform scope pruning when capturing variables in closures, a far more sensible approach is to perform liveness analysis:

• Use the information to free variables as soon as they are no longer used. Look into the Truffle APIs (Frame#clear) for informing GraalVM about this for increased performance in compiled code.
• This will allow them to be garbage collected when not needed.
• Furthermore, this will also mean that extraneous values are not captured in closures and further kept alive.
• This process needs to account for the fact that Debug.breakpoint, Debug.eval may be used in this code. Under such circumstances, all in-scope variables should be retained for the duration of the call.

Note that scope pruning could still be a win in rarer circumstances, but is not needed for the majority of improvement here.

Devirtualisation

There are multiple features in Enso that generate dynamic calls that do not always need to (e.g. when the concrete type of an atom is known at compile time, its accessors can be inlined, or when the types of a is known in a + b are known, we can devirtualise the + implementation that specializes based on the type of b. If we know the type of b we can do even better and compile the specific add implementation). In conjunction with the better static analysis it should become possible to devirtualise multiple types of calls statically, and allow you to inline the generated code instead.

• The cheapest way to do this is to retain the call, but make the call static with pre-sorted arguments. This behaves nicely in the IDE.
• The more expensive way to do this is with deep analysis in the compiler and direct inlining of method bodies wherever they match a heuristic. This would have to only occur outside introspected scopes, as it does not behave well with the IDE (without specific handling, at least).

We recommend a combination of the two, using the latter for non-introspected scopes, and the former for scopes being observed by the IDE. That said, if the first brings enough of a win, there may be little point to the second.

Language Semantics

While Enso is fairly semantically complete, there are still a number of things that have proven awkward to work with.

Shadow Definitions

Enso has a concept of extension methods. These are methods that are not defined "alongside" the type (in the same compilation unit). Currently, we have no way to define methods that are not extensions on builtin types without defining them in Java. This is awkward, and leads to a poor experience for both developers of Enso, and the users (where there is a special case rule for certain types, and also a hacky form of documentation for these same types).

For types defined in Java, their methods defined in Enso are extensions and are hence not available without importing Base. Currently if I have a Text and don't have Base imported, I can't call split on it as it's an extension.

This is particularly important for polyglot, as polyglot calls are not handed extension methods. Polyglot calls only have access to the methods defined on the type.

To rectify this situation, we recommend implementing a system we have termed "shadow definitions":

• Instead of creating builtins into their own module, provide an annotation-based system for linking definitions in Enso source code to built-in implementations and types in the compiler.
• For builtin types, the compiler should be informed that the type for a builtin is actually defined in a source file, despite being implemented elsewhere.
• You can see the existing design for this annotation-based system in Builtins.enso.
• Implementing this has a knock-on effect on what can be done later. For example, Vector and Time are currently defined in Base, and are therefore not (Truffle) interop friendly. With this system, we could implement these types in such a way that they can be handled properly in interop, making it much more seamless to use them with other truffle languages.
• Doing this will also improve the situation around the Builtins IR. Currently it is not really a library as it exists purely for documentation purposes. This means that it doesn't have a library location into which we can precompile the builtins before distribution (so right now it gets compiled on user machines in the first run).

With this done, it may still be necessary to create a Java DSL for implementing built-in methods and types, but that is unclear at this point.

Runtime Performance

While Enso is performant when it gets JITted by GraalVM, the performance when running in purely interpreted mode is poor. That said, there are still performance improvements that can be made that will benefit compiled code as well.

Interpreter Performance

This can be greatly improved.

• Start by measuring the performance with compilation disabled (no graal, only Java code running).
• Analyse the performance for bottlenecks in the interpreted code and fix the problems. See the brief guide to Graal document.
• Keeping methods in HashMap and similar implementation decisions can easily be improved.
• Many of the above-listed static optimisations will greatly help here.

IDE

As Enso's primary mode of use is in the IDE, there are a number of important improvements to the runtime and compiler that will greatly improve the user experience there.

Caching and User-Defined Types

Currently it is virtually impossible to define types for users in the IDE. This is due to a semantic issue with the IDE's value cache. When defining a type and creating an instance of it, the value of that instance is cached. When later defining a method on it, the cached value is retained with the old scope.

• Improve the heuristics for cache eviction in the IDE.
• Where no other strategy is possible, fall back to evicting the entire cache.

See #1662 for more details and options.

Dynamic Caches

Currently, the IDE cache is fairly dumb, maintaining soft references to as many in-scope values as possible. When memory runs out, the entire cache gets evicted, which is costly.

• Implement more sophisticated profiling information that can track allocations, LRU, and so on.
• Improve the cache eviction behaviour based on this.
• Ensure that, no matter what, the runtime should not go out of memory due to the cache.
• These eviction strategies should account for changes such as those described above

Lazy Visualization Support

Currently, IDE visualizations are evaluated eagerly on their candidate data. This is a nightmare when working with huge amounts of data (e.g. tables with millions of rows), and can easily lock up both the runtime and IDE. The current solution artificially limits the amount of data sent to the IDE.

In the future, we want to support the ability to cache inside visualization code such that the preprocessor doesn't have to be recomputed every time the IDE changes the parameters. This will enable the ability to view the full data in the IDE without having to send it all at once, or recompute potentially costly preprocessors.

• Implement caching support for the visualization expression processing.
• This cache should, much like the IDE's introspection cache, track and save the values of all top-level bindings in the visualization preprocessor.