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https://github.com/WebAssembly/binaryen

Optimizer and compiler/toolchain library for WebAssembly
https://github.com/WebAssembly/binaryen

c-plus-plus compilers emscripten hacktoberfest webassembly

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Optimizer and compiler/toolchain library for WebAssembly

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# 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][JS_API]. It accepts input in [WebAssembly-like

   form][compile_to_wasm] 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 (see an overview later

   down) that can improve code size and speed. These optimizations aim to make

   Binaryen powerful enough to be used as a [compiler backend][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.



Toolchains using Binaryen as a **component** (typically running `wasm-opt`) include:



  * [`Emscripten`](http://emscripten.org) (C/C++)

  * [`wasm-pack`](https://github.com/rustwasm/wasm-pack) (Rust)

  * [`J2CL`](https://j2cl.io/) (Java; [`J2Wasm`](https://github.com/google/j2cl/tree/master/samples/wasm))

  * [`Kotlin`](https://kotl.in/wasmgc) (Kotlin/Wasm)

  * [`Dart`](https://flutter.dev/wasm) (Flutter)

  * [`wasm_of_ocaml`](https://github.com/ocaml-wasm/wasm_of_ocaml) (OCaml)



For more on how some of those work, see the toolchain architecture parts of

the [V8 WasmGC porting blogpost](https://v8.dev/blog/wasm-gc-porting).



Compilers using Binaryen as a **library** include:



 * [`AssemblyScript`](https://github.com/AssemblyScript/assemblyscript) which compiles a variant of TypeScript to WebAssembly

 * [`wasm2js`](https://github.com/WebAssembly/binaryen/blob/main/src/wasm2js.h) which compiles WebAssembly to JS

 * [`Asterius`](https://github.com/tweag/asterius) which compiles Haskell to WebAssembly

 * [`Grain`](https://github.com/grain-lang/grain) which compiles Grain 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](http://emscripten.org)** 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](Contributing.md) 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][binaryen_ir], 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).

   * When stacky code must be represented in Binaryen IR, such as with

     multivalue instructions and blocks, it is represented with tuple types that

     do not exist in the WebAssembly language. In addition to multivalue

     instructions, locals and globals can also have tuple types in Binaryen IR

     but not in WebAssembly. Experiments show that better support for

     multivalue could enable useful but small code size savings of 1-3%, so it

     has not been worth changing the core IR structure to support it better.

   * Block input values (currently only supported in `catch` blocks in the

     exception handling feature) are represented as `pop` subexpressions.

 * 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][unreachable]. 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".

 * Reference Types

  * The wasm text and binary formats require that a function whose address is

    taken by `ref.func` must be either in the table, or declared via an

    `(elem declare func $..)`. Binaryen will emit that data when necessary, but

    it does not represent it in IR. That is, IR can be worked on without needing

    to think about declaring function references.

  * Binaryen IR allows non-nullable locals in the form that the wasm spec does,

    (which was historically nicknamed "1a"), in which a `local.get` must be

    structurally dominated by a `local.set` in order to validate (that ensures

    we do not read the default value of null). Despite being aligned with the

    wasm spec, there are some minor details that you may notice:

    * A nameless `Block` in Binaryen IR does not interfere with validation.

      Nameless blocks are never emitted into the binary format (we just emit

      their contents), so we ignore them for purposes of non-nullable locals. As

      a result, if you read wasm text emitted by Binaryen then you may see what

      seems to be code that should not validate per the spec (and may not

      validate in wasm text parsers), but that difference will not exist in the

      binary format (binaries emitted by Binaryen will always work everywhere,

      aside for bugs of course).

    * The Binaryen pass runner will automatically fix up validation after each

      pass (finding things that do not validate and fixing them up, usually by

      demoting a local to be nullable). As a result you do not need to worry

      much about this when writing Binaryen passes. For more details see the

      `requiresNonNullableLocalFixups()` hook in `pass.h` and the

      `LocalStructuralDominance` class.

  * Binaryen IR uses the most refined types possible for references,

    specifically:

    * The IR type of a `ref.func` is always a specific function type, and not

      plain `funcref`. It is also non-nullable.

    * Non-nullable types are also used for the type that `try_table` sends

      on branches (if we branch, a null is never sent), that is, it sends

      (ref exn) and not (ref null exn).

    In both cases if GC is not enabled then we emit the less-refined type in the

    binary. When reading a binary, the more refined types will be applied as we

    build the IR.

  * `br_if` output types are more refined in Binaryen IR: they have the type of

    the value, when a value flows in. In the wasm spec the type is that of the

    branch target, which may be less refined. Using the more refined type here

    ensures that we optimize in the best way possible, using all the type

    information, but it does mean that some roundtripping operations may look a

    little different. In particular, when we emit a `br_if` whose type is more

    refined in Binaryen IR then we emit a cast right after it, so that the

    output has the right type in the wasm spec. That may cause a few bytes of

    extra size in rare cases (we avoid this overhead in the common case where

    the `br_if` value is unused).

 * Strings

   * Binaryen allows string views (`stringview_wtf16` etc.) to be cast using

     `ref.cast`. This simplifies the IR, as it allows `ref.cast` to always be

     used in all places (and it is lowered to `ref.as_non_null` where possible

     in the optimizer). The stringref spec does not seem to allow this though,

     and to fix that the binary writer will replace `ref.cast` that casts a

     string view to a non-nullable type to `ref.as_non_null`. A `ref.cast` of a

     string view that is a no-op is skipped entirely.



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.



### Intrinsics



Binaryen intrinsic functions look like calls to imports, e.g.,



```wat

(import "binaryen-intrinsics" "foo" (func $foo))

```



Implementing them that way allows them to be read and written by other tools,

and it avoids confusing errors on a binary format error that could happen in

those tools if we had a custom binary format extension.



An intrinsic method may be optimized away by the optimizer. If it is not, it

must be **lowered** before shipping the wasm, as otherwise it will look like a

call to an import that does not exist (and VMs will show an error on not having

a proper value for that import). That final lowering is *not* done

automatically. A user of intrinsics must run the pass for that explicitly,

because the tools do not know when the user intends to finish optimizing, as the

user may have a pipeline of multiple optimization steps, or may be doing local

experimentation, or fuzzing/reducing, etc. Only the user knows when the final

optimization happens before the wasm is "final" and ready to be shipped. Note

that, in general, some additional optimizations may be possible after the final

lowering, and so a useful pattern is to optimize once normally with intrinsics,

then lower them away, then optimize after that, e.g.:



```bash

wasm-opt input.wasm -o output.wasm -O --intrinsic-lowering -O

```



Each intrinsic defines its semantics, which includes what the optimizer is

allowed to do with it and what the final lowering will turn it to. See

[intrinsics.h](https://github.com/WebAssembly/binaryen/blob/main/src/ir/intrinsics.h)

for the detailed definitions. A quick summary appears here:



* `call.without.effects`: Similar to a `call_ref` in that it receives

  parameters, and a reference to a function to call, and calls that function

  with those parameters, except that the optimizer can assume the call has no

  side effects, and may be able to optimize it out (if it does not have a

  result that is used, generally).



## Tools



This repository contains code that builds the following tools in `bin/` (see the [building instructions](#building)):



 * **`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](https://github.com/WebAssembly/binaryen/wiki/Fuzzing#reducing)

   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.

 * **`wasm-ctor-eval`**: A tool that can execute functions (or parts of functions)

   at compile time.

 * **`wasm-merge`**: Merges multiple wasm files into a single file, connecting

   corresponding imports to exports as it does so. Like a bundler for JS, but

   for wasm.

 * **`wasm-metadce`**: A tool to remove parts of Wasm files in a flexible way

   that depends on how the module is used.

 * **`binaryen.js`**: A standalone JavaScript library that exposes Binaryen methods for [creating and optimizing Wasm modules](https://github.com/WebAssembly/binaryen/blob/main/test/binaryen.js/hello-world.js). For builds, see [binaryen.js on npm](https://www.npmjs.com/package/binaryen) (or download it directly from [GitHub](https://raw.githubusercontent.com/AssemblyScript/binaryen.js/master/index.js) or [unpkg](https://unpkg.com/binaryen@latest/index.js)). Minimal requirements: Node.js v15.8 or Chrome v75 or Firefox v78.



All of the Binaryen tools are deterministic, that is, given the same inputs you should always get the same outputs. (If you see a case that behaves otherwise, please file an issue.)



Usage instructions for each are below.



## Binaryen Optimizations



Binaryen contains

[a lot of optimization passes](https://github.com/WebAssembly/binaryen/tree/main/src/passes)

to make WebAssembly smaller and faster. You can run the Binaryen optimizer by

using ``wasm-opt``, but also they can be run while using other tools, like

``wasm2js`` and ``wasm-metadce``.



* The default optimization pipeline is set up by functions like

  [`addDefaultFunctionOptimizationPasses`](https://github.com/WebAssembly/binaryen/blob/369b8bdd3d9d49e4d9e0edf62e14881c14d9e352/src/passes/pass.cpp#L396).

* There are various

  [pass options](https://github.com/WebAssembly/binaryen/blob/369b8bdd3d9d49e4d9e0edf62e14881c14d9e352/src/pass.h#L85)

  that you can set, to adjust the optimization and shrink levels, whether to

  ignore unlikely traps, inlining heuristics, fast-math, and so forth. See

  ``wasm-opt --help`` for how to set them and other details.



See each optimization pass for details of what it does, but here is a quick

overview of some of the relevant ones:



* **CoalesceLocals** - Key "register allocation" pass. Does a live range

  analysis and then reuses locals in order to minimize their number, as well as

  to remove copies between them.

* **CodeFolding** - Avoids duplicate code by merging it (e.g. if two `if` arms

  have some shared instructions at their end).

* **CodePushing** - "Pushes" code forward past branch operations, potentially

  allowing the code to not be run if the branch is taken.

* **DeadArgumentElimination** - LTO pass to remove arguments to a function if it

  is always called with the same constants.

* **DeadCodeElimination**

* **Directize** - Turn an indirect call into a normal call, when the table index

  is constant.

* **DuplicateFunctionElimination** - LTO pass.

* **Inlining** - LTO pass.

* **LocalCSE** - Simple local common subexpression elimination.

* **LoopInvariantCodeMotion**

* **MemoryPacking** - Key "optimize data segments" pass that combines segments,

  removes unneeded parts, etc.

* **MergeBlocks** - Merge a `block` to an outer one where possible, reducing

  their number.

* **MergeLocals** - When two locals have the same value in part of their

  overlap, pick in a way to help CoalesceLocals do better later (split off from

  CoalesceLocals to keep the latter simple).

* **MinifyImportsAndExports** - Minifies them to "a", "b", etc.

* **OptimizeAddedConstants** - Optimize a load/store with an added constant into

  a constant offset.

* **OptimizeInstructions** - Key peephole optimization pass with a constantly

  increasing list of patterns.

* **PickLoadSigns** - Adjust whether a load is signed or unsigned in order to

  avoid sign/unsign operations later.

* **Precompute** - Calculates constant expressions at compile time, using the

  built-in interpreter (which is guaranteed to be able to handle any constant

  expression).

* **ReReloop** - Transforms wasm structured control flow to a CFG and then goes

  back to structured form using the Relooper algorithm, which may find more

  optimal shapes.

* **RedundantSetElimination** - Removes a `local.set` of a value that is already

  present in a local. (Overlaps with CoalesceLocals; this achieves the specific

  operation just mentioned without all the other work CoalesceLocals does, and

  therefore is useful in other places in the optimization pipeline.)

* **RemoveUnsedBrs** - Key "minor control flow optimizations" pass, including

  jump threading and various transforms that can get rid of a `br` or `br_table`

  (like turning a `block` with a `br` in the middle into an `if` when possible).

* **RemoveUnusedModuleElements** - "Global DCE", an LTO pass that removes

  imports, functions, globals, etc., when they are not used.

* **ReorderFunctions** - Put more-called functions first, potentially allowing

  the LEB emitted to call them to be smaller (in a very large program).

* **ReorderLocals** - Put more-used locals first, potentially allowing the LEB

  emitted to use them to be smaller (in a very large function). After the

  sorting, it also removes locals not used at all.

* **SimplifyGlobals** - Optimizes globals in various ways, for example,

  coalescing them, removing mutability from a global never modified, applying a

  constant value from an immutable global, etc.

* **SimplifyLocals** - Key "`local.get/set/tee`" optimization pass, doing things

  like replacing a set and a get with moving the set’s value to the get (and

  creating a tee) where possible. Also creates `block/if/loop` return values

  instead of using a local to pass the value.

* **Vacuum** - Key "remove silly unneeded code" pass, doing things like removing

  an `if` arm that has no contents, a drop of a constant value with no side

  effects, a `block` with a single child, etc.



"LTO" in the above means an optimization is Link Time Optimization-like in that

it works across multiple functions, but in a sense Binaryen is always "LTO" as

it usually is run on the final linked wasm.



Advanced optimization techniques in the Binaryen optimizer include

[SSAification](https://github.com/WebAssembly/binaryen/blob/main/src/passes/SSAify.cpp),

[Flat IR](https://github.com/WebAssembly/binaryen/blob/main/src/ir/flat.h), and

[Stack/Poppy IR](https://github.com/WebAssembly/binaryen/blob/main/src/ir/stack-utils.h).



See the

[Optimizer Cookbook wiki page](https://github.com/WebAssembly/binaryen/wiki/Optimizer-Cookbook)

for more on how to use the optimizer effectively.



Binaryen also contains various passes that do other things than optimizations,

like

[legalization for JavaScript](https://github.com/WebAssembly/binaryen/blob/main/src/passes/LegalizeJSInterface.cpp),

[Asyncify](https://github.com/WebAssembly/binaryen/blob/main/src/passes/Asyncify.cpp),

etc.



## Building



Binaryen uses git submodules (at time of writing just for gtest), so before you build you will have to initialize the submodules:



```bash

git submodule init

git submodule update

```



After that you can build with CMake:



```bash

cmake . && make

```



A C++17 compiler is required. On macOS, you need to install `cmake`, for example, via `brew install cmake`. Note that you can also use `ninja` as your generator: `cmake -G Ninja . && ninja`.



To avoid the gtest dependency, you can pass `-DBUILD_TESTS=OFF` to cmake.



Binaryen.js can be built using Emscripten, which can be installed via [the SDK](http://kripken.github.io/emscripten-site/docs/getting_started/downloads.html).



- Building for Node.js:

  ```bash

  emcmake cmake . && emmake make binaryen_js

  ```

- Building for the browser:

  ```bash

  emcmake cmake -DBUILD_FOR_BROWSER=ON . && emmake make

  ```



### Visual C++



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



1. Generate the projects:



   ```bash

   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".



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



   ```bash

   msbuild binaryen.vcxproj

   ```



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



## Releases



Builds are distributed by the various toolchains that use Binaryen, like

Emscripten, `wasm-pack`, etc. There are also official releases on GitHub:



https://github.com/WebAssembly/binaryen/releases



Currently builds of the following platforms are included:



 * `Linux-x86_64`

 * `Linux-arm64`

 * `MacOS-x86_64`

 * `MacOS-arm64`

 * `Windows-x86_64`

 * `Node.js` (experimental): A port of `wasm-opt` to JavaScript+WebAssembly.

   Run `node wasm-opt.js` as a drop-in replacement for a native build of

   `wasm-opt`, on any platform that Node.js runs on. Requires Node.js 18+ (for

   Wasm EH and Wasm Threads). (Note that this build may also run in Deno, Bun,

   or other JavaScript+WebAssembly environments, but is tested only on Node.js.)



## Running



### wasm-opt



Run



```bash

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



```bash

bin/wasm-opt test/lit/passes/name-types.wast -all -S -o -

```



That will output one of the test cases in the test suite. To run a

transformation pass on it, try



```bash

bin/wasm-opt test/lit/passes/name-types.wast --name-types -all -S -o -

```



The `name-types` pass ensures each type has a name and renames exceptionally long type names. You can see

the change the transformation causes by comparing the output of the two 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 [`name-types` pass](https://github.com/WebAssembly/binaryen/blob/main/src/passes/NameTypes.cpp).



Some more notes:



 * See `bin/wasm-opt --help` for the full list of options and passes.

 * Passing `--debug` will emit some debugging info. Individual debug channels

   (defined in the source code via `#define DEBUG_TYPE xxx`) can be enabled by

   passing them as list of comma-separated strings. For example: `bin/wasm-opt

   --debug=binary`. These debug channels can also be enabled via the

   `BINARYEN_DEBUG` environment variable.



### wasm2js



Run



```bash

bin/wasm2js [input.wasm file]

```



This will print out JavaScript to the console.



For example, try



```bash

bin/wasm2js test/hello_world.wat

```



That output contains



```js

 function add(x, y) {

  x = x | 0;

  y = y | 0;

  return x + y | 0 | 0;

 }

```



as a translation of



```wat

 (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:



```js

// 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):



```bash

$ 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.



### wasm-ctor-eval



`wasm-ctor-eval` executes functions, or parts of them, at compile time.

After doing so it serializes the runtime state into the wasm, which is like

taking a "snapshot". When the wasm is later loaded and run in a VM, it will

continue execution from that point, without re-doing the work that was already

executed.



For example, consider this small program:



```wat

(module

 ;; A global variable that begins at 0.

 (global $global (mut i32) (i32.const 0))



 (import "import" "import" (func $import))



 (func "main"

  ;; Set the global to 1.

  (global.set $global

   (i32.const 1))



  ;; Call the imported function. This *cannot* be executed at

  ;; compile time.

  (call $import)



  ;; We will never get to this point, since we stop at the

  ;; import.

  (global.set $global

   (i32.const 2))

 )

)

```



We can evaluate part of it at compile time like this:



```bash

wasm-ctor-eval input.wat --ctors=main -S -o -

```



This tells it that there is a single function that we want to execute ("ctor"

is short for "global constructor", a name that comes from code that is executed

before a program's entry point) and then to print it as text to `stdout`. The

result is this:



```bash

trying to eval main

  ...partial evalling successful, but stopping since could not eval: call import: import.import

  ...stopping

(module

 (type $none_=>_none (func))

 (import "import" "import" (func $import))

 (global $global (mut i32) (i32.const 1))

 (export "main" (func $0_0))

 (func $0_0

  (call $import)

  (global.set $global

   (i32.const 2)

  )

 )

)

```



The logging shows us managing to eval part of `main()`, but not all of it, as

expected: We can eval the first `global.get`, but then we stop at the call to

the imported function (because we don't know what that function will be when the

wasm is actually run in a VM later). Note how in the output wasm the global's

value has been updated from 0 to 1, and that the first `global.get` has been

removed: the wasm is now in a state that, when we run it in a VM, will seamlessly

continue to run from the point at which `wasm-ctor-eval` stopped.



In this tiny example we just saved a small amount of work. How much work can be

saved depends on your program. (It can help to do pure computation up front, and

leave calls to imports to as late as possible.)



Note that `wasm-ctor-eval`'s name is related to global constructor functions,

as mentioned earlier, but there is no limitation on what you can execute here.

Any export from the wasm can be executed, if its contents are suitable. For

example, in Emscripten `wasm-ctor-eval` is even run on `main()` when possible.



### wasm-merge



`wasm-merge` combines wasm files together. For example, imagine you have a

project that uses wasm files from multiple toolchains. Then it can be helpful to

merge them all into a single wasm file before shipping, since in a single wasm

file the calls between the modules become just normal calls inside a module,

which allows them to be inlined, dead code eliminated, and so forth, potentially

improving speed and size.



`wasm-merge` operates on normal wasm files. It differs from `wasm-ld` in that

respect, as `wasm-ld` operates on wasm *object* files. `wasm-merge` can help

in multi-toolchain situations where at least one of the toolchains does not use

wasm object files.



For example, imagine we have these two wasm files:



```wat

;; a.wasm

(module

  (import "second" "bar" (func $second.bar))



  (export "main" (func $func))



  (func $func

    (call $second.bar)

  )

)

```



```wat

;; b.wasm

(module

  (import "outside" "log" (func $log (param i32)))



  (export "bar" (func $func))



  (func $func

    (call $log

      (i32.const 42)

    )

  )

)

```



The filenames on your local drive are `a.wasm` and `b.wasm`, but for merging /

bundling purposes let's say that the first is known as `"first"` and the second

as `"second"`. That is, we want the first module's import of `"second.bar"` to

call the function `$func` in the second module. Here is a wasm-merge command for

that:



```bash

wasm-merge a.wasm first b.wasm second -o output.wasm

```



We give it the first wasm file, then its name, and then the second wasm file

and then its name. The merged output is this:



```wat

(module

  (import "outside" "log" (func $log (param i32)))



  (export "main" (func $func))

  (export "bar" (func $func_2))



  (func $func

    (call $func_2)

  )



  (func $func_2

    (call $log

      (i32.const 42)

    )

  )

)

```



`wasm-merge` combined the two files into one, merging their functions, imports,

etc., all while fixing up name conflicts and connecting corresponding imports to

exports. In particular, note how `$func` calls `$func_2`, which is exactly what

we wanted: `$func_2` is the function from the second module (renamed to avoid a

name collision).



Note that the wasm output in this example could benefit from additional

optimization. First, the call to `$func_2` can now be easily inlined, so we can

run `wasm-opt -O3` to do that for us. Also, we may not need all the imports and

exports, for which we can run

[wasm-metadce](https://github.com/WebAssembly/binaryen/wiki/Pruning-unneeded-code-in-wasm-files-with-wasm-metadce#example-pruning-exports).

A good workflow could be to run `wasm-merge`, then `wasm-metadce`, then finish

with `wasm-opt`.



`wasm-merge` is kind of like a bundler for wasm files, in the sense of a "JS

bundler" but for wasm. That is, with the wasm files above, imagine that we had

this JS code to instantiate and connect them at runtime:



```js

// Compile the first module.

var first = await fetch("a.wasm");

first = new WebAssembly.Module(first);



// Compile the first module.

var second = await fetch("b.wasm");

second = new WebAssembly.Module(second);



// Instantiate the second, with a JS import.

second = new WebAssembly.Instance(second, {

  outside: {

    log: (value) => {

      console.log('value:', value);

    }

  }

});



// Instantiate the first, importing from the second.

first = new WebAssembly.Instance(first, {

  second: second.exports

});



// Call the main function.

first.exports.main();

```



What `wasm-merge` does is basically what that JS does: it hooks up imports to

exports, resolving names using the module names you provided. That is, by

running `wasm-merge` we are moving the work of connecting the modules from

runtime to compile time. As a result, after running `wasm-merge` we need a lot

less JS to get the same result:



```js

// Compile the single module.

var merged = await fetch("merged.wasm");

merged = new WebAssembly.Module(merged);



// Instantiate it with a JS import.

merged = new WebAssembly.Instance(merged, {

  outside: {

    log: (value) => {

      console.log('value:', value);

    }

  }

});



// Call the main function.

merged.exports.main();

```



We still need to fetch and compile the merged wasm, and to provide it the JS

import, but the work to connect two wasm modules is not needed any more.



#### Handling exports



By default `wasm-merge` errors if there are overlapping export names. That is,

`wasm-merge` will automatically handle overlapping function names and so forth,

because those are not externally visible (the code still behaves the same), but

if we renamed exports then the outside would need to be modified to expect the

new export names, and so we error instead on such name conflicts.



If you do want exports to be renamed, run `wasm-merge` with

`--rename-export-conflicts`. Later exports will have a suffix appended to them

to ensure they do not overlap with previous exports. The suffixes are

deterministic, so once you see what they are you can call them from the outside.



Another option is to use `--skip-export-conflicts` which will simply skip later

exports that have conflicting names. For example, this can be useful in the

case where the first module is the only one that interacts with the outside and

the later modules just interact with the first module.



#### Features



`wasm-merge` uses the multi-memory and multi-table features. That is, if

multiple input modules each have a memory then the output wasm will have several

memories, and will depend on the multi-memory feature, which means that older

wasm VMs might not be able to run the wasm. (As a workaround for such older VMs

you can run `wasm-opt --multi-memory-lowering` to lower multiple memories into a

single one.)



## Testing



```bash

./check.py

```



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



The `check.py` script supports some options:



```bash

./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.



Note that we are trying to gradually port the legacy wasm-opt tests to use `lit`

and `filecheck` as we modify them. For `passes` tests that output wast, this

can be done automatically with `scripts/port_passes_tests_to_lit.py` and for

non-`passes` tests that output wast, see

https://github.com/WebAssembly/binaryen/pull/4779 for an example of how to do a

simple manual port.



For lit tests the test expectations (the CHECK lines) can often be automatically

updated as changes are made to binaryen. See `scripts/update_lit_checks.py`.



Non-lit tests can also be automatically updated in most cases. See

`scripts/auto_update_tests.py`.



### Setting up dependencies



```bash

./third_party/setup.py [mozjs|v8|wabt|all]

```



(or `python third_party/setup.py`) installs required dependencies like the SpiderMonkey JS shell, the V8 JS shell

and WABT in `third_party/`. Other scripts automatically pick these up when installed.



Run `pip3 install -r requirements-dev.txt` to get the requirements for the `lit`

tests. Note that you need to have the location `pip` installs to in your `$PATH`

(on linux, `~/.local/bin`).



### Fuzzing



```bash

./scripts/fuzz_opt.py [--binaryen-bin=build/bin]

```



(or `python scripts/fuzz_opt.py`) will run various fuzzing modes on random inputs with random passes until it finds

a possible bug. See [the wiki page](https://github.com/WebAssembly/binaryen/wiki/Fuzzing) for all the details.



## 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.



## Debug Info Support



### Source Maps



Binaryen can read and write source maps (see the `-ism` and `-osm` flags to

`wasm-opt`). It can also read and read source map annotations in the text

format, that is,



```wat

;;@ src.cpp:100:33

(i32.const 42)

```



That 42 constant is annotated as appearing in a file called `src.cpp` at line

`100` and column `33`. Source maps and text format annotations are

interchangeable, that is, they both lead to the same IR representation, so you

can start with an annotated wat and have Binaryen write that to a binary + a

source map file, or read a binary + source map file and print text which will

contain those annotations.



The IR representation of source map info is simple: in each function we have a

map of expressions to their locations. Optimization passes should update the

map as relevant. Often this "just works" because the optimizer tries to reuse

nodes when possible, so they keep the same debug info.



#### Shorthand notation



The text format annotations support a shorthand in which repeated annotations

are not necessary. For example, children are tagged with the debug info of the

parent, if they have no annotation of their own:



```wat

;;@ src.cpp:100:33

(i32.add

  (i32.const 41)      ;; This receives an annotation of src.cpp:100:33

  ;;@ src.cpp:111:44

  (i32.const 1)

)

```



The first const will have debug info identical to the parent, because it has

none specified, and generally such nesting indicates a "bundle" of instructions

that all implement the same source code.



Note that text printing will not emit such repeated annotations, which can be

confusing. To print out all the annotations, set `BINARYEN_PRINT_FULL=1` in the

environment. That will print this for the above `add`:



```wat

[i32] ;;@ src.cpp:100:33

(i32.add

 [i32] ;;@ src.cpp:100:33

 (i32.const 41)

 [i32] ;;@ src.cpp:111:44

 (i32.const 1)

)

```



(full print mode also adds a `[type]` for each expression, right before the

debug location).



The debug information is also propagated from an expression to its

next sibling:

```wat

;;@ src.cpp:100:33

(local.set $x

 (i32.const 0)

)

(local.set $y ;; This receives an annotation of src.cpp:100:33

 (i32.const 0)

)

```



You can prevent the propagation of debug info by explicitly mentioning

that an expression has not debug info using the annotation `;;@` with

nothing else:

```wat

;;@ src.cpp:100:33

(local.set $x

 ;;@

 (i32.const 0) ;; This does not receive any annotation

)

;;@

(local.set $y ;; This does not receive any annotation

 (i32.const 7)

)

```

This stops the propagatation to children and siblings as well. So,

expression `(i32.const 7)` does not have any debug info either.



There is no shorthand in the binary format. That is, roundtripping (writing and

reading) through a binary + source map should not change which expressions have

debug info on them or the contents of that info.



#### Implementation Details



The [source maps format](https://sourcemaps.info/spec.html) defines a mapping

using *segments*, that is, if a segment starts at binary offset 10 then it

applies to all instructions at that offset and until another segment begins (or

the end of the input is reached). Binaryen's IR represents a mapping from

expressions to locations, as mentioned, so we need to map to and from the

segment-based format when writing and reading source maps.



That is mostly straightforward, but one thing we need to do is to handle the

lack of debug info in between things that have it. If we have `A B C` where `B`

lacks debug info, then just emitting a segment for `A` and `C` would lead `A`'s

segment to also cover `B`, since in source maps segments do not have a size -

rather they end when a new segment begins. To avoid `B` getting smeared in this

manner, we emit a source maps entry to `B` of size 1, which just marks the

binary offset it has, and without the later 3 fields of the source file, line

number, and column. (This appears to be the intent of the source maps spec, and

works in browsers and tools.)



### DWARF



Binaryen also has optional support for DWARF. This primarily just tracks the

locations of expressions and rewrites the DWARF's locations accordingly; it does

not handle things like re-indexing of locals, and so passes that might break

DWARF are disabled by default. As a result, this mode is not suitable for a

fully optimized release build, but it can be useful for local debugging.



## FAQ



* Why the weird name for the project?



Binaryen's name was inspired by *Emscripten*'s: Emscripten's name

[suggests](https://en.wikipedia.org/wiki/Lisa_the_Iconoclast#Embiggen_and_cromulent)

it converts something into a **script** - specifically *JavaScript* - and

Binaryen's suggests it converts something into a **binary** - specifically

*WebAssembly*. Binaryen began as Emscripten's WebAssembly generation and

optimization tool, so the name fit as it moved Emscripten from something that

emitted the text-based format JavaScript (as it did from its early days) to the

binary format WebAssembly (which it has done since WebAssembly launched).



"Binaryen" is pronounced [in the same manner](https://www.makinggameofthrones.com/production-diary/2011/2/11/official-pronunciation-guide-for-game-of-thrones.html#:~:text=Targaryen%20%2D%20AIR%2Deez-,Tar%2DGAIR%2Dee%2Din,-Alliser%20Thorne%20%2D%20AL)

as

"[Targaryen](https://en.wikipedia.org/wiki/List_of_A_Song_of_Ice_and_Fire_characters#House_Targaryen)".



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



Yes, it does. Here's a step-by-step [tutorial][win32] on how to compile it

under **Windows 10 x64** with with **CMake** and **Visual Studio 2015**.

However, Visual Studio 2017 may now be required. Help would be appreciated on

Windows and OS X as most of the core devs are on Linux.



[compiling to WebAssembly]: https://github.com/WebAssembly/binaryen/wiki/Compiling-to-WebAssembly-with-Binaryen

[win32]: https://github.com/brakmic/bazaar/blob/master/webassembly/COMPILING_WIN32.md

[C API]: https://github.com/WebAssembly/binaryen/wiki/Compiling-to-WebAssembly-with-Binaryen#c-api

[control flow graph]: https://github.com/WebAssembly/binaryen/wiki/Compiling-to-WebAssembly-with-Binaryen#cfg-api

[JS_API]: https://github.com/WebAssembly/binaryen/wiki/binaryen.js-API

[compile_to_wasm]: https://github.com/WebAssembly/binaryen/wiki/Compiling-to-WebAssembly-with-Binaryen#what-do-i-need-to-have-in-order-to-use-binaryen-to-compile-to-webassembly

[backend]: https://kripken.github.io/talks/binaryen.html#/9

[minification]: https://kripken.github.io/talks/binaryen.html#/2

[unreachable]: https://github.com/WebAssembly/binaryen/issues/903

[binaryen_ir]: https://github.com/WebAssembly/binaryen/issues/663