Bazel Emscripten toolchain

Setup Instructions

In WORKSPACE file, put:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
    name = "emsdk",
    sha256 = "d55e3c73fc4f8d1fecb7aabe548de86bdb55080fe6b12ce593d63b8bade54567",
    strip_prefix = "emsdk-3891e7b04bf8cbb3bc62758e9c575ae096a9a518/bazel",
    url = "https://github.com/emscripten-core/emsdk/archive/3891e7b04bf8cbb3bc62758e9c575ae096a9a518.tar.gz",
)

load("@emsdk//:deps.bzl", emsdk_deps = "deps")
emsdk_deps()

load("@emsdk//:emscripten_deps.bzl", emsdk_emscripten_deps = "emscripten_deps")
emsdk_emscripten_deps(emscripten_version = "2.0.31")

load("@emsdk//:toolchains.bzl", "register_emscripten_toolchains")
register_emscripten_toolchains()

The SHA1 hash in the above strip_prefix and url parameters correspond to the git revision of emsdk 2.0.31. To get access to newer versions, you'll need to update those. To make use of older versions, change the parameter of emsdk_emscripten_deps(). Supported versions are listed in revisions.bzl

Bazel 7+ additionally requires platforms dependencies in the MODULE.bazel file.

bazel_dep(name = "platforms", version = "0.0.9")

Building

Put the following line into your .bazelrc:

build --incompatible_enable_cc_toolchain_resolution

Then write a new rule wrapping your cc_binary.

load("@rules_cc//cc:defs.bzl", "cc_binary")
load("@emsdk//emscripten_toolchain:wasm_rules.bzl", "wasm_cc_binary")

cc_binary(
    name = "hello-world",
    srcs = ["hello-world.cc"],
)

wasm_cc_binary(
    name = "hello-world-wasm",
    cc_target = ":hello-world",
)

Now you can run bazel build :hello-world-wasm. The result of this build will be the individual files produced by emscripten. Note that some of these files may be empty. This is because bazel has no concept of optional outputs for rules.

wasm_cc_binary uses transition to use emscripten toolchain on cc_target and all of its dependencies, and does not require amending .bazelrc. This is the preferred way, since it also unpacks the resulting tarball.

The Emscripten cache shipped by default does not include LTO, 64-bit or PIC builds of the system libraries and ports. If you wish to use these features you will need to declare the cache when you register the toolchain as follows. Note that the configuration consists of the same flags that can be passed to embuilder. If targets is not provided, all system libraries and ports will be built, i.e., the ALL option to embuilder.

load("@emsdk//:toolchains.bzl", "register_emscripten_toolchains")
register_emscripten_toolchains(cache = {
    "configuration": ["--lto"],
    "targets": [
        "crtbegin",
        "libprintf_long_double-debug",
        "libstubs-debug",
        "libnoexit",
        "libc-debug",
        "libdlmalloc",
        "libcompiler_rt",
        "libc++-noexcept",
        "libc++abi-debug-noexcept",
        "libsockets"
    ]
})

See test_external/ for an example using embind.