DVL

A prototype compiler for running the code you want to write as fast as the code you have to write (as long as it's numerical).

By which I mean that DVL is a language and compiler for writing the numerical kernel of your system in, instead of Fortran or C. The DVL input language is much nicer and higher level, a dialect of Lisp; but more important, high level DVL constructs compile to efficient code.

Why yet another compiler? Isn't it already possible to get within 3-5x of Fortran with a little annotation of Common Lisp or Haskell? Yes, but only by writing Fortran in parentheses (or in monads). I know of no compiler where modularity is costless (except research systems along the lines of DVL).

It's not costless in DVL either, but in DVL you pay for it with resource-intensive compilations rather than slow runtimes. In the numeric context, all the modularity constructs you know and love: data structures, closures, higher-order functions, abstraction boundaries of every stripe -- even automatic differentiation -- are just scaffolding for carrying numbers hither and yon to be added and multiplied. DVL solves the scaffolding at compile time and emits efficient code to do the arithmetic at runtime.

Disclaimer: DVL is a prototype. The language is minimal (but you don't need much to predict the positions of the planets); the error messages are inscrutable; and the commandline interface is inflexible. DVL is not curretnly polished enough to be really used by someone who does not have time to understand its internals. You have been warned.

• So where do I get it?
• So how do I use it?
• What's with the name?
• Where did this come from?
• I can haz legal?

Installation

DVL is written primarily in MIT Scheme and distributed in source form, so you will need to install that first. If you want to use any other backend besides compiling through MIT Scheme, you will need to install that system. In particular, the Haskell backend, in addition to generating Haskell, is written in Haskell, so you will need GHC twice for that.

When ready, just git clone this repository and then git submodule init and git submodule update to pull in code dependencies. Then

dysvunctional-language/dvl/dvl <dvl-program-file>

and you're off to the races. Or fire up your MIT Scheme and

(load "dysvunctional-language/dvl/load")

to use DVL as a library.

Usage

DVL is a prototype. The purpose of making it public is experimentation, collaboration, and elucidation. You are not expected to be able to use it without considerable source diving -- but the source is intended to be legible.

That said, read dvl/README to get started, or the dvl shell script to see the entry points.

The code tree is organized as follows:

Directories

• fol/ Implementation of FOL (intermediate target for VL and DVL)
• vl/ Implementation of VL (uses fol/)
• dvl/ Implementation of DVL (some refers to vl/; uses fol/)
• support/ General libraries
• support/pattern-case/ Schemely pattern-matching macro (submodule)
• support/rules/ Rule-based term-rewriting system (submodule)
• testing/ Unit test framework (submodule)
• */test/ Unit tests for *

Important Files

• */load.scm Scheme entry point for *
• ./README This document
• */README Entry point for * documentation
• fol/fol Commandline entry point for FOL
• vl/vl Commandline entry point for VL
• dvl/dvl Commandline entry point for DVL

Technical Summary

DVL consists of three main pieces:

1. DVL proper,
2. VL, a more expository and less capable compiler sharing a lot of code with DVL, and
3. FOL, which serves as a compilation target for DVL and VL, and in turn compiles to (low-level, efficient) Scheme, Lisp, or Haskell.

In reverse order,

3. FOL is the intermediate language for VL and DVL. Instead of being entirely internal, FOL is a standalone compiler for a statically typed first-order language with Scheme syntax (which makes it easy to generate programmatically). The FOL compiler is dominated by a FOL to FOL optimizer, which performs inlining, scalar replacement of aggregates, dead code elimination, common subexpression elimination, and some algebraic simplification. Optimized FOL can then be compiled to linkable libraries or executables via any of several backends.

4. VL is a compiler implementing union-free polyvariant flow analysis on a (almost) functional source language. It generates FOL, and therefore does not itself contain any post-analysis optimization.

5. DVL is a variation on VL extended to be able to reason about gensyms (unique values); DVL the language adds the primitives GENSYM and GENSYM= to VL. Gensyms are sufficient to implement forward mode AD correctly by operator overloading; the flow analysis suffices to migrate all the boxing, tagging, dispatching, and unboxing that this normally entails to compile time. DVL also adds a rudimentary foreign interface.

History

DVL is based on Stalingrad, which was developed by Professors Jeffrey Mark Siskind at Purdue University and Barak A. Pearlmutter at the Hamilton Institute, with work beginning at least as early as 2004.

Stalingrad is an explicitly automatic-differentiation-aware compiler for scientific computing, building on Professor Siskind's earlier work on the Stalin compiler for Scheme. The source language Stalingrad compiles is called VLAD, for Vunctional Language with Automatic Differentiation -- a language that, true to its name, is purely functional (except for a minimal i/o facility), and provides built-in operators for invoking AD.

I had the privilege of having a post-doctoral appointment with Professor Pearlmutter between the fall of 2010 and the summer of 2012, working on compilation for numerical computing and automatic differentiation. During that time, I rewrote the non-AD portion of VLAD in a system called, appropriately enough, VL (included in the vl/ directory) as a more expository implementation of the compilation technique at the heart of Stalingrad.

After VL, I found myself taking this same compilation method in a slightly different direction than VLAD and Stalingrad. I noticed that it essentially still worked even in the presence of the slight language impurity of being able to generate unique objects -- which is sufficient for an implementation of AD in user space, without having to increase the (already considerable) complexity of the compiler with it. Thus was born the DysVunctional Language.

I have been the primary author of DVL to date, but my coworkers at the Hamilton Institute cannot go unmentioned. I am indebted to Professor Pearlmutter himself, to David Rush, and to Oleksandr Manzyuk. In particular, the GHC and Javascript backends are due to him. The project also would not have been possible without access to Professor Siskind, both for inspiration from the Stalingrad sources, and for numerous technical conversations. The early and, so far, majority of the development of DVL was supported by Science Foundation Ireland Principal Investigator grant 09/IN.1/I2637.

--Alexey Radul, Boston, 2013