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+# Bazel Emscripten toolchain
+
+## Setup Instructions
+
+Support for depending on emsdk with a WORKSPACE file was removed and last available in [emsdk version 4.0.6](https://github.com/emscripten-core/emsdk/tree/24fc909c0da13ef641d5ae75e89b5a97f25e37aa). Now we only support inclusion as a bzlmod module.
+
+In your `MODULE.bazel` file, put:
+```starlark
+emsdk_version = "4.0.6"
+bazel_dep(name = "emsdk", version = emsdk_version)
+git_override(
+    module_name = "emsdk",
+    remote = "https://github.com/emscripten-core/emsdk.git",
+    strip_prefix = "bazel",
+    tag = emsdk_version,
+)
+```
+
+You can use a different version of this SDK by changing it in your `MODULE.bazel` file. The Emscripten version is by default the same as the SDK version, but you can use a different one as well by adding to your `MODULE.bazel`:
+
+```
+emscripten_deps = use_extension(
+    "@emsdk//:emscripten_deps.bzl",
+    "emscripten_deps",
+)
+emscripten_deps.config(version = "4.0.1")
+```
+
+## Building
+
+Put the following line into your `.bazelrc`:
+
+```
+build --incompatible_enable_cc_toolchain_resolution
+```
+
+Then write a new rule wrapping your `cc_binary`.
+
+```starlark
+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 in your `MODULE.bazel` as follows. Note
+that the configuration consists of the same flags that can be passed to
+embuilder. If `targets` is not set, all system libraries and ports will be
+built, i.e., the `ALL` option to embuilder.
+
+```starlark
+emscripten_cache = use_extension(
+    "@emsdk//:emscripten_cache.bzl",
+    "emscripten_cache",
+)
+emscripten_cache.configuration(flags = ["--lto"])
+emscripten_cache.targets(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](https://emscripten.org/docs/porting/connecting_cpp_and_javascript/embind.html).