Binaryen

Binaryen is a compiler and toolchain infrastructure library for WebAssembly,
written in C++. It aims to make compiling to WebAssembly easy, fast, and
effective:

• Easy: Binaryen has a simple C API in a single header, and can also be
  used from JavaScript. It accepts input in WebAssembly-like
  form but also accepts a general control flow graph for
  compilers that prefer that.

• Fast: Binaryen's internal IR uses compact data structures and is designed
  for completely parallel codegen and optimization, using all available CPU
  cores. Binaryen's IR also compiles down to WebAssembly extremely easily and
  quickly because it is essentially a subset of WebAssembly.

• Effective: Binaryen's optimizer has many passes that can improve code
  very significantly (e.g. local coloring to coalesce local variables; dead
  code elimination; precomputing expressions when possible at compile time;
  etc.). These optimizations aim to make Binaryen powerful enough to be used as
  a compiler backend by itself. One specific area of focus is on
  WebAssembly-specific optimizations (that general-purpose compilers might not
  do), which you can think of as wasm minification , similar to minification
  for JavaScript, CSS, etc., all of which are language-specific (an example of
  such an optimization is block return value generation in SimplifyLocals).

Compilers built using Binaryen include

• asm2wasm which compiles asm.js to WebAssembly
• AssemblyScript which compiles TypeScript to Binaryen IR
• wasm2js which compiles WebAssembly to JS
• Asterius which compiles Haskell to WebAssembly

Binaryen also provides a set of toolchain utilities that can

• Parse and emit WebAssembly. In particular this lets you load
  WebAssembly, optimize it using Binaryen, and re-emit it, thus implementing a
  wasm-to-wasm optimizer in a single command.
• Interpret WebAssembly as well as run the WebAssembly spec tests.
• Integrate with Emscripten in order to provide a
  complete compiler toolchain from C and C++ to WebAssembly.
• Polyfill WebAssembly by running it in the interpreter compiled to
  JavaScript, if the browser does not yet have native support (useful for
  testing).

Consult the contributing instructions if you're interested in
participating.

Binaryen IR

Binaryen's internal IR is designed to be

• Flexible and fast for optimization.
• As close as possible to WebAssembly so it is simple and fast to convert
  it to and from WebAssembly.

There are a few differences between Binaryen IR and the WebAssembly language:

• Tree structure
  • Binaryen IR is a tree, i.e., it has hierarchical structure,
    for convenience of optimization. This differs from the WebAssembly binary
    format which is a stack machine.
  • Consequently Binaryen's text format allows only s-expressions.
    WebAssembly's official text format is primarily a linear instruction list
    (with s-expression extensions). Binaryen can't read the linear style, but
    it can read a wasm text file if it contains only s-expressions.
  • Binaryen uses Stack IR to optimize "stacky" code (that can't be
    represented in structured form).
  • In rare cases stacky code must be represented in Binaryen IR as well, like
    popping a value in an exception catch. To support that Binaryen IR has
    push and pop instructions.
• Types and unreachable code
  • WebAssembly limits block/if/loop types to none and the concrete value types
    (i32, i64, f32, f64). Binaryen IR has an unreachable type, and it allows
    block/if/loop to take it, allowing local transforms that don't need to
    know the global context. As a result, Binaryen's default
    text output is not necessarily valid wasm text. (To get valid wasm text,
    you can do --generate-stack-ir --print-stack-ir, which prints Stack IR,
    this is guaranteed to be valid for wasm parsers.)
  • Binaryen ignores unreachable code when reading WebAssembly binaries. That
    means that if you read a wasm file with unreachable code, that code will be
    discarded as if it were optimized out (often this is what you want anyhow,
    and optimized programs have no unreachable code anyway, but if you write an
    unoptimized file and then read it, it may look different). The reason for
    this behavior is that unreachable code in WebAssembly has corner cases that
    are tricky to handle in Binaryen IR (it can be very unstructured, and
    Binaryen IR is more structured than WebAssembly as noted earlier). Note
    that Binaryen does support unreachable code in .wat text files, since as we
    saw Binaryen only supports s-expressions there, which are structured.
• Blocks
  • Binaryen IR has only one node that contains a variable-length list of
    operands: the block. WebAssembly on the other hand allows lists in loops,
    if arms, and the top level of a function. Binaryen's IR has a single
    operand for all non-block nodes; this operand may of course be a block.
    The motivation for this property is that many passes need special code
    for iterating on lists, so having a single IR node with a list simplifies
    them.
  • As in wasm, blocks and loops may have names. Branch targets in the IR are
    resolved by name (as opposed to nesting depth). This has 2 consequences:
    • Blocks without names may not be branch targets.
    • Names are required to be unique. (Reading .wat files with duplicate names
      is supported; the names are modified when the IR is constructed).
  • As an optimization, a block that is the child of a loop (or if arm, or
    function toplevel) and which has no branches targeting it will not be
    emitted when generating wasm. Instead its list of operands will be directly
    used in the containing node. Such a block is sometimes called an "implicit
    block".
• Multivalue
  • Binaryen will not represent multivalue instructions and values directly.
    Binaryen's main focus is on optimization of wasm, and therefore the question
    of whether we should have multivalue in the main IR is whether it justifes
    the extra complexity there. Experiments show that the shrinking of code
    size thanks to multivalue is useful but small, just 1-3% or so. Given that,
    we prefer to keep the main IR simple, and focus on multivalue optimizations
    in Stack IR, which is more suitable for such things.
  • Binaryen does still need to implement the "ABI" level of multivalue, that
    is, we need multivalue calls because those may cross module boundaries,
    and so they are observable externally. To support that, Binaryen may use
    push and pop as mentioned earlier; another option is to add LLVM-like
    extractvalue/composevalue instructions.

As a result, you might notice that round-trip conversions (wasm => Binaryen IR
=> wasm) change code a little in some corner cases.

• When optimizing Binaryen uses an additional IR, Stack IR (see
  src/wasm-stack.h). Stack IR allows a bunch of optimizations that are
  tailored for the stack machine form of WebAssembly's binary format (but Stack
  IR is less efficient for general optimizations than the main Binaryen IR). If
  you have a wasm file that has been particularly well-optimized, a simple
  round-trip conversion (just read and write, without optimization) may cause
  more noticeable differences, as Binaryen fits it into Binaryen IR's more
  structured format. If you also optimize during the round-trip conversion then
  Stack IR opts will be run and the final wasm will be better optimized.

Notes when working with Binaryen IR:

• As mentioned above, Binaryen IR has a tree structure. As a result, each
  expression should have exactly one parent - you should not "reuse" a node by
  having it appear more than once in the tree. The motivation for this
  limitation is that when we optimize we modify nodes, so if they appear more
  than once in the tree, a change in one place can appear in another
  incorrectly.
• For similar reasons, nodes should not appear in more than one functions.

Tools

This repository contains code that builds the following tools in bin/:

• wasm-opt: Loads WebAssembly and runs Binaryen IR passes on it.
• wasm-as: Assembles WebAssembly in text format (currently S-Expression
  format) into binary format (going through Binaryen IR).
• wasm-dis: Un-assembles WebAssembly in binary format into text format
  (going through Binaryen IR).
• wasm2js: A WebAssembly-to-JS compiler. This is used by Emscripten to
  generate JavaScript as an alternative to WebAssembly.
• wasm-reduce: A testcase reducer for WebAssembly files. Given a wasm file
  that is interesting for some reason (say, it crashes a specific VM),
  wasm-reduce can find a smaller wasm file that has the same property, which is
  often easier to debug. See the
  docs
  for more details.
• wasm-shell: A shell that can load and interpret WebAssembly code. It can
  also run the spec test suite.
• wasm-emscripten-finalize: Takes a wasm binary produced by llvm+lld and
  performs emscripten-specific passes over it.
• asm2wasm: An asm.js-to-WebAssembly compiler, using Emscripten's asm
  optimizer infrastructure. This is used by Emscripten in Binaryen mode when it
  uses Emscripten's fastcomp asm.js backend.
• wasm-ctor-eval: A tool that can execute C++ global constructors ahead of
  time. Used by Emscripten.
• binaryen.js: A standalone JavaScript library that exposes Binaryen methods for creating and optimizing WASM modules. For builds, see binaryen.js on npm (or download it directly from github, rawgit, or unpkg).

Usage instructions for each are below.

Building

cmake . && make

Note that you can also use ninja as your generator: cmake -G Ninja . && ninja

• A C++11 compiler is required.
• The JavaScript components can be built using build-js.sh, see notes inside. Normally this is not needed as builds are provided in this repo already.

If you also want to compile C/C++ to WebAssembly (and not just asm.js to WebAssembly), you'll need Emscripten. You'll need the incoming branch there (which you can get via the SDK), for more details see the wiki.

Visual C++

1. Using the Microsoft Visual Studio Installer, install the "Visual C++ tools for CMake" component.

2. Generate the projects:

   mkdir build
   cd build
   "%VISUAL_STUDIO_ROOT%\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin\cmake.exe" ..
   

   Substitute VISUAL_STUDIO_ROOT with the path to your Visual Studio
   installation. In case you are using the Visual Studio Build Tools, the path
   will be "C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools".

3. From the Developer Command Prompt, build the desired projects:

   msbuild binaryen.vcxproj
   

   CMake generates a project named "ALL_BUILD.vcxproj" for conveniently building all the projects.

Running

wasm-opt

Run

bin/wasm-opt [.wasm or .wat file] [options] [passes, see --help] [--help]

The wasm optimizer receives WebAssembly as input, and can run transformation
passes on it, as well as print it (before and/or after the transformations). For
example, try

bin/wasm-opt test/passes/lower-if-else.wat --print

That will pretty-print out one of the test cases in the test suite. To run a
transformation pass on it, try

bin/wasm-opt test/passes/lower-if-else.wat --print --lower-if-else

The lower-if-else pass lowers if-else into a block and a break. You can see
the change the transformation causes by comparing the output of the two print
commands.

It's easy to add your own transformation passes to the shell, just add .cpp
files into src/passes, and rebuild the shell. For example code, take a look at
the lower-if-else pass.

Some more notes:

• See bin/wasm-opt --help for the full list of options and passes.
• Passing --debug will emit some debugging info.

wasm2js

Run

bin/wasm2js [input.wasm file]

This will print out JavaScript to the console.

For example, try

$ bin/wasm2js test/hello_world.wat

That output contains

 function add(x, y) {
  x = x, 0;
  y = y, 0;
  return x + y, 0;
 }

as a translation of

 (func $add (; 0 ;) (type $0) (param $x i32) (param $y i32) (result i32)
  (i32.add
   (local.get $x)
   (local.get $y)
  )
 )

wasm2js's output is in ES6 module format - basically, it converts a wasm
module into an ES6 module (to run on older browsers and Node.js versions
you can use Babel etc. to convert it to ES5). Let's look at a full example
of calling that hello world wat; first, create the main JS file:

// main.mjs
import { add } from "./hello_world.mjs";
console.log('the sum of 1 and 2 is:', add(1, 2));

The run this (note that you need a new enough Node.js with ES6 module
support):

$ bin/wasm2js test/hello_world.wat -o hello_world.mjs
$ node --experimental-modules main.mjs
the sum of 1 and 2 is: 3

Things keep to in mind with wasm2js's output:

• You should run wasm2js with optimizations for release builds, using -O
  or another optimization level. That will optimize along the entire pipeline
  (wasm and JS). It won't do everything a JS minifer would, though, like
  minify whitespace, so you should still run a normal JS minifer afterwards.
• It is not possible to match WebAssembly semantics 100% precisely with fast
  JavaScript code. For example, every load and store may trap, and to make
  JavaScript do the same we'd need to add checks everywhere, which would be
  large and slow. Instead, wasm2js assumes loads and stores do not trap, that
  int/float conversions do not trap, and so forth. There may also be slight
  differences in corner cases of conversions, like non-trapping float to int.

asm2wasm

Run

bin/asm2wasm [input.asm.js file]

This will print out a WebAssembly module in s-expression format to the console.

For example, try

$ bin/asm2wasm test/hello_world.asm.js

That input file contains

function () {
  "use asm";
  function add(x, y) {
    x = x, 0;
    y = y, 0;
    return x + y, 0;
  }
  return { add: add };
}

You should see something like this:

By default you should see pretty colors as in that image. Set COLORS=0 in the
env to disable colors if you prefer that. On Linux and Mac, you can set
COLORS=1 in the env to force colors (useful when piping to more, for
example). For Windows, pretty colors are only available when stdout/stderr are
not redirected/piped.

Pass --debug on the command line to see debug info, about asm.js functions as
they are parsed, etc.

C/C++ Source ⇒ asm2wasm ⇒ WebAssembly

When using emcc with the BINARYEN option, it will use Binaryen to build to
WebAssembly. This lets you compile C and C++ to WebAssembly, with emscripten
using asm.js internally as a build step. Since emscripten's asm.js generation is
very stable, and asm2wasm is a fairly simple process, this method of compiling C
and C++ to WebAssembly is usable already. See the emscripten
wiki for more details
about how to use it.

Testing

./check.py

(or python check.py) will run wasm-shell, wasm-opt, asm2wasm, etc. on the testcases in test/, and verify their outputs.

The check.py script supports some options:

./check.py [--interpreter=/path/to/interpreter] [TEST1] [TEST2]..

• If an interpreter is provided, we run the output through it, checking for
  parse errors.
• If tests are provided, we run exactly those. If none are provided, we run
  them all. To see what tests are available, run ./check.py --list-suites.
• Some tests require emcc or nodejs in the path. They will not run if the
  tool cannot be found, and you'll see a warning.
• We have tests from upstream in tests/spec, in git submodules. Running
  ./check.py should update those.

Design Principles

• Interned strings for names: It's very convenient to have names on nodes,
  instead of just numeric indices etc. To avoid most of the performance
  difference between strings and numeric indices, all strings are interned,
  which means there is a single copy of each string in memory, string
  comparisons are just a pointer comparison, etc.
• Allocate in arenas: Based on experience with other
  optimizing/transformating toolchains, it's not worth the overhead to
  carefully track memory of individual nodes. Instead, we allocate all elements
  of a module in an arena, and the entire arena can be freed when the module is
  no longer needed.

FAQ

• Why the weird name for the project?

"Binaryen" is a combination of binary - since WebAssembly is a binary format
for the web - and Emscripten - with which it can integrate in order to
compile C and C++ all the way to WebAssembly, via asm.js. Binaryen began as
Emscripten's WebAssembly processing library (wasm-emscripten).

"Binaryen" is pronounced in the same manner as "Targaryen": bi-NAIR-ee-in. Or something like that? Anyhow, however Targaryen is correctly pronounced, they should rhyme. Aside from pronunciation, the Targaryen house words, "Fire and Blood", have also inspired Binaryen's: "Code and Bugs."

• Does it compile under Windows and/or Visual Studio?

Yes, it does. Here's a step-by-step tutorial on how to compile it
under Windows 10 x64 with with CMake and Visual Studio 2015. Help
would be appreciated on Windows and OS X as most of the core devs are on Linux.