ICFP 2024 Artifact

Name: The Long Way to Deforestation: A Type Inference and Elaboration Technique for Removing Intermediate Data Structures

Abstract

This artifact consists of an sbt project with a Scala implementation of Lumberhack as introduced in the corresponding paper. Lumberhack aims to improve the efficiency of programs by eliminating intermediate data structures. We provide a test suite containing all the examples in the paper and all the nofib benchmarks we have ported and presented in the paper. Related scripts in bash, R and Python to generate the figures and tables in the paper are also contained in this artifact.

Artifact Instructions

Setting up the Artifact

• Using the Docker Image

  We have built Docker images containing all necessary dependencies compiled for both amd64 and arm64 platforms and exported them to archived files. One can load the amd64 image and launch a container with the following command:

  docker load -i lumberhack-docker-amd64.tar
  docker run -it --rm lumberhack/lumberhack-docker:v0-amd64
  

  If one wants to use the arm64 image, the commands are:

  docker load -i lumberhack-docker-arm64.tar
  docker run -it --rm lumberhack/lumberhack-docker:v0-arm64
  

  The user will be attached to the shell of the container after the image gets pulled and the container is launched. Please cd to lumberhack/ and launch the SBT shell by typing sbt.

• Setting up from Scratch

  1. To compile the project and run the tests: you need JDK (≥11) supported by Scala, sbt, java-tree-sitter (Java binding of tree-sitter) and tree-sitter-haskell (Haskell syntax for tree-sitter). java-tree-sitter and tree-sitter-haskell are needed to parse input Haskell programs into our core language AST so that Lumberhack can perform optimization on them.

     We recommend you install JDK and sbt via coursier.

     Then, install java-tree-sitter (including its submodules) and tree-sitter-haskell by cloning them from GitHub. After that, some modifications are needed before we start to compile the tree-sitter library due to some compatibility issues. The changes are:

     • Enabling c++ compilation for java-tree-sitter (explained at point 3 from this PR). This can be done by executing

       sed -i "s/if cpp/if cpp or True/g" ./build.py

       at the root directory of java-tree-sitter
     • Checking out to commit b6ec26f181dd059eedd506fa5fbeae1b8e5556c8 for tree-sitter-haskell

     After the above changes are made, run at the root directory of java-tree-sitter:

     ./build.py -o libjava-tree-sitter-haskell -v ../tree-sitter-haskell

     to compile the dynamic library. Then copy the output file to java.library.path (which can be shown by executing java -XshowSettings:properties). On linux machine, this can be done by the following command because /lib should be a java.library.path.

     cp ./libjava-tree-sitter-haskell.so /lib/libjava-tree-sitter-haskell.so

     After installing the prerequisites, change your working directory to the root of this repository and launch the SBT shell by typing sbt in the terminal.

  2. To compile the output OCaml programs and run the benchmarks: you need the optimizing OCaml compiler flambda (the version we use is 5.0.0), the library core_bench and zarith.

     We recommend you install the above dependencies via opam:

     opam switch create artifact --packages=ocaml-variants.5.0.0+options,ocaml-option-flambda -y
     eval $(opam env)
     opam install zarith core_bench -y

  3. To generate the plots we used in the paper: you need R (≥3.5) and R packages ggplot2, RColorBrewer, gridExtra.

     After installing R, run the following command in your shell to install the required packages:

     R -e "install.packages(c('RColorBrewer', 'ggplot2', 'gridExtra'), repos='http://cran.rstudio.com/')"

Evaluation Instructions

This section describes how to run Lumberhack and its generated OCaml program, as well as how to generate the plots and tables we used in the paper. For the correspondence between this artifact and the paper, please refer to the section below.

• To perform and test Lumberhack's optimization on the nofib benchmarks we presented in the paper:

  run the following command in your shell:

  sbt 'testOnly mlscript.lumberhack.DiffTestLumberhack'

  This will start the sbt shell and execute the sbt command testOnly mlscript.lumberhack.DiffTestLumberhack. Alternatively, you can start sbt shell first and then execute manually the command testOnly mlscript.lumberhack.DiffTestLumberhack.

  The output OCaml programs will be located in new-nofib-ocaml-gen, grouped by sub-directories following their names. These sub-directories contain both the unoptimized programs and the optimized ones, and each of the sub-directories also includes a main.ml that contains codes utilizing OCaml's benchmark library core_bench to benchmark both the original program and the optimized ones and show the execution time and GC data.

  After OCaml programs are generated, run the following command to compile them using flambda and execute them.

  ./bench.sh

  The raw output from core_bench about the execution stats and the compiled binary size information will be printed to stdout, plot/time.txt and plot/size.txt.

  Examples in the paper are located in lumberhack/shared/src/test/resources/PaperExamples.mls. For some of the examples, their corresponding OCaml programs are generated and located in the folder ocaml-gen. These OCaml programs are not compiled and executed by running ./bench.sh. Please refer to the instructions below for running them individually.

  NOTE: some programs require longer running duration to enable core_bench to report reliable 95% confidence intervals, and their test durations are adjusted accordingly in bench.sh based on the results of executing the tests from the authors' machines (at the current setting, benchmarking all the 38 programs should finish within around 3 hours). Depending on the machine executing the tests, the numbers may need to be further adjusted. To make the adjustment, modify bench.sh and add an elif case after line 35:

  ...
  elif [ "$dir" = "<benchmark_name>" ]
  then
    c=$cmd" -q"" <duration_in_seconds>"
  ...

• To test output OCaml benchmark programs individually: cd to the corresponding sub-directory in new-nofib-ocaml-gen and run the following command to show the suggested command for compiling and running the generated OCaml programs.

  echo $(head -2 ./main.ml | tail -1)

  The suggested command is composed of two commands, the first compiles the OCaml programs using ocamlopt -O3 to produce an executable <benchmark_name>.out, the second launches the generated executable: ./<benchmark_name>.out +time. The +time flag tells core_bench to measure the 95% confidence interval. The default benchmarking duration is 10 seconds, and it can be changed by passing -q <time_in_seconds> to the executable. More options are also provided by core_bench, and they can be listed by running ./<benchmark_name>.out --help.

• To write one's own programs and test Lumberhack's optimization on them:

  The testing infrastructure (DiffTest) for Lumberhack's sbt project is set up so that if there are any unstaged changes (as determined by git) in any test file (those in lumberhack/shared/src/test/resources), only the corresponding files will be processed. So one can make selected modifications to some test files and run the test command (testOnly mlscript.lumberhack.DiffTestLumberhack) again in sbt shell, and only your modified tests will be run. There is a lumberhack/shared/src/test/resources/PaperExamples.mls file with examples for you to start editing and getting new results from Lumberhack.

  Test output (program after expansion, producer/consumer matches and program after optimization) is inserted in place in the test file after each corresponding block, as comments beginning with //│. This makes it very easy and convenient to see the test results for each code block. For this reason, we recommend using an editor that automatically reloads open files on changes. VSCode and Sublime Text work well for this.

  To let Lumberhack also generate OCaml programs in addition to inserting the output to the test files, a DiffTest flag :lhGenOCaml is needed to be inserted at the first line of the input program. Also, a function with name test<benchmark_name> accepting exactly one parameter should be defined and called in the top level of the input program. The call to that function should follow the form test<benchmark_name>(primId(arg)), where primId is needed to ensure that the data produced by arg, which is meant to be the unknown input at compile time, will not be fused by Lumberhack. There are examples in lumberhack/shared/src/test/resources/PaperExamples.mls for your reference.

  Currently, we support input programs using a subset of either MLscript syntax (explained below) or Haskell syntax (so that we can port the nofib benchmarks). We recommend using MLscript syntax to manually write programs as inputs to Lumberhack, because the generated output tends to be more readable due to less name mangling.

  • The supported MLscript syntax is described below. We also implemented the example programs we described in the paper using this syntax in lumberhack/shared/src/test/resources/PaperExamples.mls. One can refer to that for a general idea of the MLscript syntax.

  • The Haskell syntax is supported to the extent that suffices to enable us to port the related nofib benchmarks. In the list below (including but not limited to), we outline some unsupported Haskell features that are more related to programs testing a deforestation optimizer. During the porting, these are manually desugared to more elementary language constructs.

    • the "as" pattern: ls@(x:xs)
    • lambda definition with patterns as its parameters: \(x:xs) -> ...
    • let groups: let a = x; b = x in ...
    • let bindings with patterns as its binder: let (x:xs) = [1] in ...
    • where clauses that refer to variables defined at the outer scope: f x = g x where g _ = x

    One can also refer to the programs we have ported (located in lumberhack/shared/src/test/resources/nofib_benchmarks) for more detail. When a Haskell program is provided as input for Lumberhack in a DiffTest .mls file, a DiffTest flag :lhInHaskell is needed to be inserted at the first line of the input Haskell program.

  Note that Lumberhack ignores definitions that are never used. So, for example, Lumberhack will not do anything to the following input program:

  let rec map(f, xs) = if xs is
    Nil then Nil
    Cons(x, xs) then Cons(f(x), map(f, xs))
  let main(ls) = map(x => x * 2, map(x => x + 1, ls))
  

  If we refer to main in the top-level expression like below, Lumberhack will work.

  let rec map(f, xs) = if xs is
    Nil then Nil
    Cons(x, xs) then Cons(f(x), map(f, xs))
  let main(ls) = map(x => x * 2, map(x => x + 1, ls))
  main
  

  Also, due to how DiffTest works, the input programs cannot contain any empty lines.

• To generate plots in the paper:

  After the execution of bench.sh, plot/time.txt and plot/size.txt should be ready. Then run

  ./plot.sh

  The scripts in plot/ will be automatically called, and the plots will be generated as plot/size.pdf and plot/time.pdf. plot/size.csv and plot/time.csv will also be generated along the way as intermediate files, which collect information in csv format from the raw outputs from core_bench.

  Additionally, to generate the tables (in csv format) we present in Appendix D (table 1 and table 2), run

  ./table.sh

Correspondence with the Paper

• All the examples shown throughout the paper are presented in MLscript syntax in lumberhack/shared/src/test/resources/PaperExamples.mls.

• After ./bench.sh successfully finishes, ./plot.sh will output Fig. 8 and Fig. 9 in section 6.3 Analysis of Results.

• An csv table aggregating the data in Table 1 and Table 2 in Appendix D can be generated by ./table.sh.

Supported MLscript Syntax

• Function definition

  let f(param1, param2) = ...
  

  This defines a non-recursive function f taking two parameters param1. and param2.

  Function and variable names should start with non-capitalized letters. To define a recursive function, use let rec f(param1, param2) = ....

• Lambda definition

  x => body
  

  This defines a lambda function taking x as its parameter with body as its body.

• Function call

  f(arg1, arg2)
  

  This calls the function f with two arguments arg1 and arg2

• let binding

  let x = rhs in body
  

  This binds the value of rhs to the variable x, which will be visible in body. To define recursive let rec bindings, use let rec x = rhs in body.

• Data constructor calls

  Cons(arg1, arg2)
  Nil
  

  The first line creates a Cons data structure with arg1 and arg2 as its arguments. The second line creates a Nil data structure, which takes no argument. Data constructor names should start with capitalized letters.

• Pattern matching

  if x is
    Cons(h, t) then branch1
    Nil then branch2
  

  The above MLscript program shares the same semantics with the following OCaml program:

  match x with
    | `Cons(h, t) -> branch1
    | `Nil -> branch2

  NOTE: we do not support nested patterns for input programs written in the MLscript syntax

• Integer and operations on integers are totally conventional

• Special symbols

  • primitive stands for any code that is unknown or irrelevant for Lumberhack
  • primId: this "primitive identity" annotation takes one argument and prevents Lumberhack from fusing values produced by its argument with the rest of the program by treating its argument as something unknown. It is usually used by us to mark the input of programs.

Additional Artifact Description

In this artifact, the implementation of Lumberhack is presented together with MLscript, a nascent functional programming language intended for real-world usage. This allows Lumberhack to leverage the DiffTests utility implemented for MLscript. Additionally, this also enables Lumberhack to use the lexer and parser for MLscript, such that MLscript programs can be taken as inputs for Lumberhack. Although the support for generating MLscript programs is not available yet, it is anticipated that Lumberhack will be integrated into the MLscript compiler in the future.

The implementation of Lumberhack is self-contained, meaning that there is no dependency on MLscript after input MLscript programs are translated into Lumberhack's core AST. The shared directory contains the sources for MLscript, and the sources for Lumberhack are located in lumberhack, whose project structure is illustrated as follows.

• lumberhack/shared/src/main/scala/CodeGen.scala contains the implementation of the translation from Haskell's parse tree from tree-sitter to Lumberhack's AST. The implementation of the generation of OCaml programs from the optimized programs in Lumberhack's core AST is also implemented in this file.

• lumberhack/shared/src/main/scala/Deforest.scala contains the implementation of Lumberhack's fusion strategy inference algorithm.

• lumberhack/shared/src/main/scala/Expr.scala contains the implementation of Lumberhack's core Language AST.

• lumberhack/shared/src/main/scala/Main.scala contains a dummy implementation of the entry point function for Lumberhack, because currently Lumberhack takes its inputs solely from the DiffTest files located in lumberhack/shared/src/test/resources.

  The overall workflow of Lumberhack can be found at lumberhack/shared/src/test/scala/DiffTestLumberhack.scala.

• lumberhack/shared/src/main/scala/Rewrite.scala contains the implementation of Lumberhack's transformation, including expansion, fusion and floating-out.

• lumberhack/shared/src/main/scala/Uid.scala contains the implementation of a Uid class which helps the assignment of unique id to expressions, type variables and identifiers.

• lumberhack/shared/src/main/scala/utils/Document.scala contains the implementation of a Document class and several helper functions that facilitate the pretty printing of programs.