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Uniqueness Types and In-Place Updates

Posted on June 13, 2022

I recently wrote about how Futhark had been relieved of its last design defect and was now perfect. It turns out that wings made of wax melt quite easily. While I am falling, I might as well write about one part of Futhark’s design that, while not crucially flawed, is somewhat unpleasant. This dubious feature is Futhark’s uniqueness type system, supporting the language feature we sometimes (unfortunately) call in-place updates. Although it serves a purpose, its interaction with other language features is problematic.

Basics of uniqueness types

The existence of uniqueness types in Futhark is ultimately rooted in a desire to accommodate a certain style of programming. Boiled down to a single construct, we want to support with-expressions:

A with [i] = v

Semantically, this expression produces an array with the same elements as A, except that the element at index i is replaced by v. Nothing fancy, purely functional, and you could easily write such a function on lists in any functional language. But in Futhark, we want to make a further guarantee: the run-time cost of a with expression must be proportional to the size of the value v, not the size of the full array A. In practice, this means that copying all of A is not allowed, and so the only straightforward implementation technique is to perform a write directly to the storage of A - what we call an in-place update. But since Futhark is a purely functional language, we don’t want the user to observe that a destructive update has been performed. This feature is about the cost model, not of the semantics of the language.

So how do we support such a feature, which we wish to implement with a destructive in-place update, without turning Futhark into an imperative language? Our approach is to introduce type rules that check that the “old” value of A is never used again, meaning that the effect of the in-place-update cannot possibly be observed. This is precisely what Futharks’s uniqueness type system makes possible: after A with [i] = v we mark A as consumed and mark any subsequent use as a compile-time error.

This has some overlap with linear and affine type systems, where values must be used exactly once (linear) or at most once (affine). Most operations on linear values return a new linear value, that you can use for further operations. Linear type systems are particularly useful for resource management - by requiring that a value must be used exactly once, you make it impossible to forget to free a resource (such as memory). Only the deallocation function will accept a linear argument without returning a new linear value.

Linear arrays would permit efficient with expressions, but the type system would be cumbersome. For example, while linear types have only a single notion of use, in Futhark we wish to distinguish a non-destructive observation from a destructive consumption. An array can be observed multiple times, but only consumed once. This is needed to allow expressions such as the following:

A with [i] = A[i] + 1

All observations of an array must happen before a consumption (if any). The notions of “before” and “after” are defined syntactically, by specifying an evaluation order. It could also be done by defining rules based on actual data dependencies, which is what the compiler actually does internally, but we judged that at the surface language level, it’s more important to have simple than flexible rules. This is not an issue that is particularly important in practice. It does mean that the following code fragment will not type-check:

let B = A with [i] = v
let x = A[i]

Here A is used after it has been consumed. It works if we flip the two lines:

let x = A[i]
let B = A with [i] = v

Alias analysis

What makes this entire business particularly hairy is that consumption of A also applies to aliases of A. Two values are aliased if they might share storage at run-time. The rules for aliasing are syntax-driven and mostly based on intuition (because we have neglected to document them properly):

And so on. There is a rule for each syntactic construct, and the type checker has to carefully track aliases even when names go out of scope, but it is ultimately more convoluted than truly complicated. And of course, it is always possible to copy a value to obtain an alias-free version, with no semantic impact. I want to emphasise this part: aliases never affect the semantics of the program, only whether it will type check. An alias-related type error can always be fixed by copying.

Functions

The real problem occurs for function applications, as syntax-driven alias analysis is intra-procedural. This is not just for implementation simplicity, but also to ensure compositionality. The types of functions should make it obvious whether they can be composed, and not be up to some complicated black-box alias analysis. This means that function types need to contain the following information:

  1. To which extent a function consumes its argument(s), e.g. a function that uses with-expressions.

  2. How the result of the function aliases its arguments, e.g. consider a function like transpose, which runs in constant time by merely returning a “view” of the original array, without copying it.

It is for this purpose, describing the behaviour of functions, that we need uniqueness types, although I think we have diverged so much from their origin in Clean that the name is no longer appropriate.

As a starting point, if a function f has type a -> b, then it does not consume its argument (of type a), and its result (of type b) may alias the argument.

If f has type *a -> b, then f consumes its argument, and the result may have aliases. This last part doesn’t matter for a function that takes a single argument, but for a function of type a -> *a -> b, the result may alias the first a. Such a function that consumes its argument behaves a lot like a with expression. After an application f A, we may no longer use A.

If a function f has type a -> *b, then it does not consume its argument, and the result has no aliases.

These rules are relatively simple, but we need an additional constraint: the result of a function may not alias a global variable. This is because the type rules only allow two cases: either the result of a function call to alias the function arguments, or it doesn’t alias anything. We have no way to express that it may alias some global variable. As with all other alias-related errors, this can be fixed by using copy to break the aliasing.

Uniqueness?

So why are we using the term “uniqueness types” at all? In the original design, a type of form *a was called unique. It was supposed to denote that when you had a variable of this type, you held the only reference to that type. When this is the case, you can clearly change the value without affecting anyone else. But we almost immediately diverged from this simple concept, as we wanted to permit a style of programming where an array can be “observed” (e.g. indexed) many times before it is “consumed”. So gradually we have moved to a design that is more focused on the notions of consumption and aliases, as this is what really matters.

Complicating the types

In principle, we could complicate the type system further. Imagine a function that accepts two arguments and returns a pair, each component of which will alias a corresponding argument. This is currently not expressible. Either a function result aliases nothing or it aliases all arguments. It is not difficult to conceive of a more elaborate type system where the potential aliases of return values are more explicit:

(x: a) -> (y: b) -> (a@{x}, b@{y})

In practice, this has not yet proven necessary or even useful. Most uses of uniqueness in real Futhark programs tends to be quite straightforward and localised.

A more significant problem is that uniqueness types do not interact well with higher-order functions. Consider an application function:

apply : (a -> b) -> a -> b

By its type, apply accepts a function that does not consume its argument. We can define a “consuming apply” of the following type:

capply : (*a -> b) -> *a -> b

The programmer then has to pick the right one for each situation. Although perhaps clumsy, it has not proven a major problem in practice. One reason, as usual, is that you can always obtain a “unique” (alias-free) array through copy. Since this entire business is solely about the cost model, not semantics, copying is always viable, as long as you don’t do it inside loops in ways that would affect the asymptotic cost of the program.

A slightly more subtle constraint is that a function is not allowed to consume any free variables. Example:

let B = copy A                   -- Fresh, no aliases
let f (i: i64) = B with [i] = 0  -- No!

At the point of definition, we have no idea how often f will be applied, and since an array must be consumed at most once, we forbid the definition of the function f. As an example of how it could go wrong, passing f to map would obviously result in f being applied multiple times. On the other hand, passing f to apply would result in only a single application.

This could perhaps be solved by using an effect system to indicate which variable would be consumed. Then f would have type

f : i64 ->{B} []i64

indicating that it consumes B. The map function would then have type

map : (a ->{} b) ->{} []a ->{} []b

indicating that the function passed in must have no effects. On the other hand, apply would be

apply : (a ->{E} b) ->{} a ->{E} b

where E is an effect variable, indicating that apply has the same effect as the function being applied. For brevity, we’d probably write the effect-free function arrow ->{} as just ->. This may look like an esoteric case, but in Futhark we want to support a programming style based on pipelines:

x |> f |> g |> h

The pipeline operator |> is exactly apply, although with its arguments flipped. Thus, in order to allow such pipelines where some of the functions consume their arguments, we would need some sort of consumption polymorphism. Alternatively, we need dedicated consuming operators.

Design goals

Apart from enabling a certain implementation technique while remaining purely functional, the design of Futhark’s uniqueness types was also shaped by other goals. The main one is simplicity, and particularly that programmers can ignore this entire business whenever it is not needed. We are particularly concerned with not making function types too complicated in the common case. The current situation is mostly tolerable this regard, with the only blemish being that most functions have a * in their return type to indicate that the result has no aliases. It turns out that most functions produce “fresh” arrays, and if this was not part of the types of standard functions, using with would be very annoying as most things would alias each other. Alternatively, we could change the design to require an annotation for those functions that produce aliased results, which would instead complicate the types of transpose, take, flatten, id, etc.

One could argue that for a function

val map 'a 'b : (a -> b) -> []a -> *[]b

we could infer that the result cannot possibly alias the arguments, as the types differ. There is no way to construct an array []b that aliases an array []a unless a = b, which map cannot assume. This would cut down on noise for certain polymorphic functions, but it would remain for many other functions.

Generally, the language complexity budget for Futhark is that it should be at about the level of Standard ML or Haskell 98. There are many complex parallel languages. Our research and design hypothesis is that a usable and effective high-level data parallel language does not have to be particularly complicated. We wish to preserve a semantics that is basically the lambda calculus with arrays, and in which programs are expressed using straightforward transformations of values. The with construct is a good example of this idea, and I have long regretted that we used the term “in-place updates” for this language feature, as it needlessly conflates the implementation with the semantics.

An open question is whether this feature is really needed. After all, Futhark is a parallel and functional language. Why do we need a feature that is mostly useful when writing imperative-ish code? The answer is that many parallel programs contain a sequential core inside all of those parallel loops, and if that core is not efficient, then neither is the program as a whole. The with expression is also not the only ultimate source of consumption - scatter is a kind of parallel with, and as far as we know it is needed to express certain highly irregular algorithms, such as graph operations, with the right asymptotic efficiency. I really wish we could get rid of in-place updates, uniqueness types, consumption, aliasing, or whatever you want to call it, but so far it appears necessary to support the kinds of programs that we want to write.