Bril: An Intermediate Language for Teaching Compilers

I've ranted extensively [1] about how badly interpreters (among adjacent topics) are often taught, because so often decisions are just abruptly made without explanation of the reasoning (or, often, even that a decision is being made!).

One thing I just added was that there are (at least) 6 "arities" (which really should not be called that) of bytecode/IR:

* stack output, stack input, stack input - though often a few instructions do take arguments (push, jump)

* accumulator output/input, 1 explicit input (usually a local variable or constant)

* belt output, belt input, 1 explicit input. The first argument is always the output of the previous instruction; the explicit argument (if not a constant) specifies the output of some previous instruction. Backwards jumps will require storing many variables to a prior part of the belt, but this is simple and useful for mini expression evaluators.

* explicit output/input, explicit input - common for machine code

* explicit output, explicit input, explicit input - common for machine code

* implicit output, explicit input, explicit input - an easy way to save space for SSA (though you can also use it with mutability), though there's weirdness for phis.

[1]: https://gist.github.com/o11c/6b08643335388bbab0228db763f9921...

The original ANF is actually looser than this in that it permits anonymous functions as arguments. In practice, there is no canonical variant of ANF that people really refer to, but most people usually mean a variant of ANF that doesn't permit this (which, to my knowledge, was first published in David Tarditi's PhD thesis). See this table from Appel's "Modern Compiler Implementation in ML" for the comparisons made in the functional IR space: https://i.imgur.com/17nfGMI.png.

Usually what people in the functional compiler space mean when they mention ANF is some variant of ANF (with Tarditi's restriction) that retains nested functions in a tree-like structure. The tree structure is important because it practically necessitates the extension of "join point"s within the tree structure (to act as local, second-class, continuations: to avoid duplicating evaluation contexts for multi-way branching constructs, without using functions for this purpose). It just so happens that you hoist ANF into a bunch of function bodies (which were once nested) and blocks (which were once join points), you can easily construct a control flow graph. However, you could also say that lambda calculus would be "in SSA" throughout all of this (as it is originally, then normalised into ANF, and then hoisted into a control flow graph) - it just isn't usually what people mean when they talk about an SSA IR (they tend to imply the existence of specific provisions for the splitting of live ranges at join points, be it a phi-like pseudo-instruction or block arguments).

All this is to say that ANF is very tied to literature about the compilation of functional languages and its related analysis and comparison with CPS (such as whether it's closed under beta-reduction, for example), such that I think we need to be a bit more precise about the expected shape and properties of IRs to differentiate them, rather than just expecting compiler engineers to know what you're talking about - and, indeed, agree with your description - when you describe something as "an ANF language".

The linked MLIR documentation, in turn, credits Swift for that idea, but the earliest occurrence of phis as continuation arguments I know is in MLton. It’d be interesting to know where this idea comes from initially, because standard phis really are incredibly awkward.

I also like the decision to not have the IR always in SSA form. I think it helps you understand SSA form better to have to construct it, and you're going to have to eliminate SSA form anyway for any of the classic techniques for generating machine code.

Now the main choices seem to be CPS (which is seeing a bit of a resurgence!) and SSA.

So why teach ANF?

Education often has needs diametrically opposed to production. For example, the early Pascal compilers seem to have been optimised for throughput, not object codegen: after all, production code is compiled rarely and run often, but student code is compiled often and run almost never.

Frankly, I don’t see much point in a “special IR for teaching” because llvm text format is just really straightforward, and at the same time teaches the “real deal” (sure, normally you’d likely use bindings, but still).

You can still have your students reimplement optimizations that llvm would normally do by themselves (like inductive variable elimination, const propagation, dataflow analysis, using phi instead of alloc etc.).

At least I was really happy I could play with llvm for a university project.