Bazel Emscripten toolchain

Setup Instructions

In WORKSPACE file, put:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
    name = "emsdk",
    strip_prefix = "emsdk-c1589b55641787d55d53e883852035beea9aec3f/bazel",
    url = "https://github.com/emscripten-core/emsdk/archive/c1589b55641787d55d53e883852035beea9aec3f.tar.gz",
    sha256 = "7a58a9996b113d3e0675df30b5f17e28aa47de2e684a844f05394fe2f6f12e8e",
)

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

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

Building

Using --config=wasm

Put the following lines into your .bazelrc:

build:wasm --crosstool_top=//emscripten_toolchain:everything
build:wasm --cpu=wasm
build:wasm --host_crosstool_top=@bazel_tools//tools/cpp:toolchain

Simply pass --config=wasm when building a normal cc_binary. The result of this build will be a tar archive containing any files produced by emscripten.

Using wasm_cc_binary

First, 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.