Notes

Compiling dynamic programming languages

September 2, 2018

It can be difficult to disassociate the idea that dynamically typed programming languages are tied to byte-code interpreters (e.g. YARV Ruby, CPython, V8, Zend Engine, etc.). But for many languages, a compiled implementation also exists. Cython, Chicken Scheme and SBCL are good examples.

In this post I will briefly describe how I built a compiler for my Scheme implementation using artifacts from the interpreter. In doing this, I learned a simple (not novel) technique for compiling dynamic languages. I'll introduce the Javascript to C++/V8 compiler I am developing using this technique.

BSDScheme

For the past year I've developed a Scheme implementation, BSDScheme. I started with an AST-interpreter (as opposed to a byte-code compiler and VM). A more detailed blog post on the first few steps writing BSDScheme can be found here.

As I built up support for the various objects and operations in the language, I had a sizeable base of D code for the BSDScheme runtime. This included an object representation for primitive types (and support for converting to and from types in D) as well as basic Scheme operations (+, -, car, cdr, etc.).

When the time came to implement a compiler backend, I only needed to do codegen since the parser already existed. Furthermore, the fundamental bits had already been written: object representation and much of the standard library. So I wrote the simplest compiler I could think of by targeting D and the objects/functions I had already written to support the interpreter.

Take, for example, the equals function in the standard library:

Value equals(Value arguments, void** rest) {
  auto tuple = valueToList(arguments);
  auto left = tuple[0];
  auto right = car(tuple[1]);

  bool b;

  switch (tagOfValue(left)) {
  case ValueTag.Integer:
    b = valueIsInteger(right) && valueToInteger(left) == valueToInteger(right);
    break;
  case ValueTag.Char:
    b = valueIsChar(right) && valueToChar(left) == valueToChar(right);
    break;
  case ValueTag.String:
    b = valueIsString(right) && valueToString(left) == valueToString(right);
    break;
  case ValueTag.Symbol:
    b = valueIsSymbol(right) && valueToSymbol(left) == valueToSymbol(right);
    break;
  case ValueTag.Function:
    b = valueIsFunction(right) && valueToFunction(left)[1] == valueToFunction(right)[1];
    break;
  case ValueTag.Bool:
    b = valueIsBool(right) && valueToBool(left) == valueToBool(right);
    break;
  default:
    b = false;
  }

  return makeBoolValue(b);
}

So long as my compiler generated code that used the Value object to represent Scheme data, I already had an equals function and large swaths of a Scheme standard library that I could share between the compiler and interpreter.

Ultimately I only needed to implement a few control structures to support compiling a large subset of what I supported in the interpreter. The key aspects here include: function definitions (in D), function calls (D function calls), if/else (if/else in D) and so on.

To give a concrete example of a whole program compiled, this Scheme program:

(define (exp base pow)
  (if (= pow 0)
      1
      (* base (exp base (- pow 1)))))

(define (main)
  (display (exp 2 16))
(newline))

when run through the BSDScheme compiler would become:

import std.stdio;
import lex;
import common;
import parse;
import utility;
import value;
import buffer;

Value exp(Value arguments, void** ctx) {
    Value[] tmp = listToVector(arguments);
    Value base = tmp[0];
    Value pow = tmp[1];

    Value equals_result = equals(vectorToList([pow, makeIntegerValue(0)]), null);
    Value if_result;
    if (truthy(equals_result)) {
    makeIntegerValue(1);
    if_result = makeIntegerValue(1);
    } else {

    Value minus_result = minus(vectorToList([pow, makeIntegerValue(1)]), null);

    Value exp_result = exp(vectorToList([base, minus_result]), null);

    Value times_result = times(vectorToList([base, exp_result]), null);
    if_result = times_result;
    };
    return if_result;
}

Value BSDScheme_main(Value arguments, void** ctx) {

    Value exp_result = exp(vectorToList([makeIntegerValue(2), makeIntegerValue(16)]), null);

    Value display_result = display(vectorToList([exp_result]), null);

    Value newline_result = newline(vectorToList([]), null);
    return newline_result;
}

void main() { BSDScheme_main(nilValue, cast(void**)0); }

Where every imported function had already been written for the interpreter. I had only to translate a few lines to D and import/call these existing libraries. Now I had a small binary of compiled Scheme.

It was at this point I realized I was using the same technique used by Cython to compile Python code.

...the Cython project has approached this problem by means of a source code compiler that translates Python code to equivalent C code. This code is executed within the CPython runtime environment, but at the speed of compiled C and with the ability to call directly into C libraries. http://docs.cython.org/en/latest/src/quickstart/overview.html

jsc

I played with many PL-research-y languages over the years and wanted to do build something a little more practical. So I took what I learned writing the BSDScheme compiler and decided to write a Javascript compiler. Specifically, it would target the easiest backend I could imagine: C++ using the V8 C++ library and generating a Node addon.

There already existed well-trodden guides/means of writing Node addons in C++ so I spent some time trying to hand-compile simple Javascript programs to C++ and V8. A string in Javascript would become a v8::String type in C++. A number in Javascript would become v8::Number in C++ and so forth.

I decided to write this compiler in Rust given its roots in (and my familiarity with) ML and Python. I found a Javascript parser by Dave Herman and after a few lazy weeks finally got a "Hello world!" program compiling. Getting my first program to compile has by far been the hardest part of building jsc.

Let's look at a concrete example of a recursive fibonacci program (example/recursion.js in the repo):

function fib(i) {
  if (i <= 1) {
    return i;
  }
  return fib(i - 1) + fib(i - 2);
}

function main() {
  console.log(fib(20));
}

Let's add a call to main() at the end and time this with Node to get a baseline:

$ time node example/recursion.js
6765
node example/recursion.js  0.06s user 0.02s system 97% cpu 0.083 total

Now let's install jsc to compare. We'll need Rust, Cargo, Node and Node-GYP.

$ git clone https:/github.com/eatonphil/jsc
$ cd jsc
$ make && make install
$ jsc example/recursion.js

jsc produces a Javascript entrypoint that imports our addon (build/recursion.js):

require("./build/Release/recursion").jsc_main();

And it produces a C++ file that represents the entire program (build/recursion.cc):

#include <string>

#include <node.h>

using v8::Boolean;
using v8::Context;
using v8::Exception;
using v8::Function;
using v8::FunctionTemplate;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Null;
using v8::Number;
using v8::Object;
using v8::String;
using v8::False;
using v8::True;
using v8::Value;

void fib(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Value> i = args[0];
tail_recurse_1:
  Local<Context> ctx_2 = isolate->GetCurrentContext();
  Local<Object> global_3 = ctx_2->Global();
  Local<Function> Boolean_4 = Local<Function>::Cast(global_3->Get(String::NewFromUtf8(isolate, "Boolean")));
  String::Utf8Value utf8value_tmp_5(i);
  std::string string_tmp_6(*utf8value_tmp_5);
  String::Utf8Value utf8value_tmp_7(Number::New(isolate, 1));
  std::string string_tmp_8(*utf8value_tmp_7);
  Local<Value> argv_9[] = { (i->IsNumber() || Number::New(isolate, 1)->IsNumber()) ? Boolean::New(isolate, i->ToNumber(isolate)->Value() <= Number::New(isolate, 1)->ToNumber(isolate)->Value()) : ((i->IsString() || Number::New(isolate, 1)->IsString()) ? Boolean::New(isolate, string_tmp_6 <= string_tmp_8) : (False(isolate))) };
  Local<Value> result_10 = Boolean_4->Call(Null(isolate), 1, argv_9);
  if (result_10->ToBoolean()->Value()) {
    args.GetReturnValue().Set(i);
    return;
    return;
  }
  Local<Value> arg_11 = (i->IsNumber() || Number::New(isolate, 1)->IsNumber()) ? (Number::New(isolate, i->ToNumber(isolate)->Value() - Number::New(isolate, 1)->ToNumber(isolate)->Value())) : Local<Number>::Cast(Null(isolate));
  Local<FunctionTemplate> ftpl_13 = FunctionTemplate::New(isolate, fib);
  Local<Function> fn_12 = ftpl_13->GetFunction();
  fn_12->SetName(String::NewFromUtf8(isolate, "fib"));
  Local<Value> argv_14[] = { arg_11 };
  Local<Value> result_15 = fn_12->Call(Null(isolate), 1, argv_14);
  Local<Value> arg_16 = (i->IsNumber() || Number::New(isolate, 2)->IsNumber()) ? (Number::New(isolate, i->ToNumber(isolate)->Value() - Number::New(isolate, 2)->ToNumber(isolate)->Value())) : Local<Number>::Cast(Null(isolate));
  Local<FunctionTemplate> ftpl_18 = FunctionTemplate::New(isolate, fib);
  Local<Function> fn_17 = ftpl_18->GetFunction();
  fn_17->SetName(String::NewFromUtf8(isolate, "fib"));
  Local<Value> argv_19[] = { arg_16 };
  Local<Value> result_20 = fn_17->Call(Null(isolate), 1, argv_19);
  args.GetReturnValue().Set((result_15->IsString() || result_20->IsString()) ? Local<Value>::Cast(String::Concat(result_15->ToString(), result_20->ToString())) : Local<Value>::Cast((result_15->IsNumber() || result_20->IsNumber()) ? (Number::New(isolate, result_15->ToNumber(isolate)->Value() + result_20->ToNumber(isolate)->Value())) : Local<Number>::Cast(Null(isolate))));
  return;
}

void jsc_main(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
tail_recurse_21:
  Local<Value> arg_22 = Number::New(isolate, 20);
  Local<FunctionTemplate> ftpl_24 = FunctionTemplate::New(isolate, fib);
  Local<Function> fn_23 = ftpl_24->GetFunction();
  fn_23->SetName(String::NewFromUtf8(isolate, "fib"));
  Local<Value> argv_25[] = { arg_22 };
  Local<Value> result_26 = fn_23->Call(Null(isolate), 1, argv_25);
  Local<Value> arg_27 = result_26;
  Local<Function> fn_28 = Local<Function>::Cast(Local<Object>::Cast(isolate->GetCurrentContext()->Global()->Get(String::NewFromUtf8(isolate, "console")))->Get(String::NewFromUtf8(isolate, "log")));
  Local<Value> argv_29[] = { arg_27 };
  Local<Value> result_30 = fn_28->Call(Null(isolate), 1, argv_29);
  result_30;
}

void Init(Local<Object> exports) {
  NODE_SET_METHOD(exports, "jsc_main", jsc_main);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

Let's time this version:

$ time node build/recursion.js
6765
node build/recursion.js  0.16s user 0.03s system 107% cpu 0.175 total

jsc, over twice as slow, is already falling behind Node. :)

As I incremented the number passed to my fibonacci function the compiled program time to completion get exponentially worse. Node stayed the same. I decided to try tail-call optimization to decrease the performance distance between Node and jsc.

I implemented tail-call optimization for the interpreter in BSDScheme by putting all functions in a loop that would break if tail-call elimination was not to happen. It took me a week to implement this and I never put it in place for the compiler. This time around I was able to add basic tail call elimination to jsc in two hours. It is done by labels and gotos instead of a tail call when applicable.

Here is a tail-call optimized version of the same program (example/tco.js):

function fib(n, a, b) {
    if (n == 0) {
        return a;
    }

    if (n == 1) {
        return b;
    }

    return fib(n - 1, b, a + b);
}

function main() {
  console.log(fib(50, 0, 1));
}

We add a call to main() again for Node and time it:

$ time node example/tco.js
12586269025
node example/tco.js  0.06s user 0.02s system 96% cpu 0.080 total

And compile it with jsc and time it:

$ jsc example/tco.js
$ time node build/tco.js
12586269025
node build/tco.js  0.07s user 0.02s system 95% cpu 0.087 total

Well that's not bad at all. :)

Next steps with jsc

jsc has very limited support for... everything. Today I added almost all primitive numeric operations + equality/inequality operations + unit tests. jsc does not yet support nested functions, callbacks, or closures. It supports while loops but not yet for loops. And I'm not sure if it supports else if. It does not support arrays or objects let alone constructors and prototypes. Adding support for these is low-hanging fruit.

After the low-hanging fruit, more interesting projects for jsc include: