dart-sdk/pkg/dart2wasm

Compiling Dart to WebAssembly

WebAssembly (commonly abbreviated to Wasm) is a "binary instruction format for a stack-based virtual machine". Although Wasm was originally designed for running native code on the web, Wasm has since evolved into a general technology for running compiled code across multiple platforms.

The Dart team is currently investigating support for compiling Dart to Wasm, in conjunction with support in Flutter.

Note: This feature is under active development, and is currently considered experimental. The tracking issue is #32894.

Running dart2wasm

You don't need to build the Dart SDK to run dart2wasm, as long as you have a Dart SDK installed and have the Dart SDK repository checked out. NB: the SDK must be checked out using depot tools and not just cloned from this repo.

To compile a Dart file to Wasm, in a checkout of the Dart SDK repository, run:

dart --enable-asserts pkg/dart2wasm/bin/dart2wasm.dart options infile.dart outfile.wasm

where options include:

Option	Default	Description
--dart-sdk=path	relative to script	The location of the sdk directory inside the Dart SDK, containing the core library sources.
--platform=path	none	The location of the platform dill file containing the compiled core libraries.
--depfile=path	none	Write a Ninja depfile listing the input sources for the compilation.
--[no-]export-all	no	Export all functions; otherwise, just export main.
--[no-]import-shared-memory	no	Import a shared memory buffer. If this is on, --shared-memory-max-pages must also be specified.
--[no-]inlining	yes	Enable function inlining.
--inlining-limit size	0	Always inline functions no larger than this number of AST nodes, if inlining is enabled.
--[no-]name-section	yes	Emit Name Section with function names.
--[no-]omit-type-checks	no	Omit runtime type checks, such as covariance checks and downcasts.
--[no-]polymorphic-specialization	no	Do virtual calls by switching on the class ID instead of using call_indirect.
--[no-]print-kernel	no	Print IR for each function before compiling it.
--[no-]print-wasm	no	Print Wasm instructions of each compiled function.
--[no-]enable-asserts	no	Enable assertions at runtime.
--[no-]constant-branch-pruning	yes	Avoid emitting code for dead branches of conditionals based on constants.
--shared-memory-max-pages pagecount		Max size of the imported memory buffer. If --shared-import-memory is specified, this must also be specified.
--watch offset		Print stack trace leading to the byte at offset offset in the .wasm output file. Can be specified multiple times.

Dart2Wasm will output a wasm file, containing Dart compiled to Wasm, as well as an mjs file containing the runtime. The result can be run with:

d8 --experimental-wasm-gc --experimental-wasm-type-reflection pkg/dart2wasm/bin/run_wasm.js -- outfile.wasm /abs/path/to/*outfile*.mjs

Where d8 is the V8 developer shell.

Imports and exports

To import a function, declare it as a global, external function and mark it with a wasm:import pragma indicating the imported name (which must be two identifiers separated by a dot):

@pragma("wasm:import", "foo.bar")
external void fooBar(Object object);

which will call foo.bar on the host side:

var foo = {
    bar: function(object) { /* implementation here */ }
};

To export a function, mark it with a wasm:export pragma:

@pragma("wasm:export")
void foo(double x) { /* implementation here */  }

@pragma("wasm:export", "baz")
void bar(double x) { /* implementation here */  }

With the Wasm module instance in inst, these can be called as:

inst.exports.foo(1);
inst.exports.baz(2);

Types to use for interop

In the signatures of imported and exported functions, use the following types:

• For numbers, use double.
• For JS objects, use a JS interop type, e.g. JSAny, which translates to the Wasm externref type. These can be passed around and stored as opaque values on the Dart side.
• For Dart objects, use the corresponding Dart type. This will be emitted as anyref and automatically converted to and from the Dart type at the boundary.