71 lines
2.4 KiB
Markdown
71 lines
2.4 KiB
Markdown
# Bazel Emscripten toolchain
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## Setup Instructions
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In `WORKSPACE` file, put:
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```
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load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
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http_archive(
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name = "emsdk",
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sha256 = "d55e3c73fc4f8d1fecb7aabe548de86bdb55080fe6b12ce593d63b8bade54567",
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strip_prefix = "emsdk-3891e7b04bf8cbb3bc62758e9c575ae096a9a518/bazel",
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url = "https://github.com/emscripten-core/emsdk/archive/3891e7b04bf8cbb3bc62758e9c575ae096a9a518.tar.gz",
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)
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load("@emsdk//:deps.bzl", emsdk_deps = "deps")
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emsdk_deps()
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load("@emsdk//:emscripten_deps.bzl", emsdk_emscripten_deps = "emscripten_deps")
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emsdk_emscripten_deps(emscripten_version = "2.0.31")
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```
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The SHA1 hash in the above `strip_prefix` and `url` parameters correspond to the git revision of
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[emsdk 2.0.31](https://github.com/emscripten-core/emsdk/releases/tag/2.0.31). To get access to
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newer versions, you'll need to update those. To make use of older versions, change the
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parameter of `emsdk_emscripten_deps()`. Supported versions are listed in `revisions.bzl`
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## Building
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### Using wasm_cc_binary (preferred)
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First, write a new rule wrapping your `cc_binary`.
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```
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load("@rules_cc//cc:defs.bzl", "cc_binary")
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load("@emsdk//emscripten_toolchain:wasm_rules.bzl", "wasm_cc_binary")
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cc_binary(
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name = "hello-world",
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srcs = ["hello-world.cc"],
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)
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wasm_cc_binary(
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name = "hello-world-wasm",
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cc_target = ":hello-world",
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)
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```
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Now you can run `bazel build :hello-world-wasm`. The result of this build will
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be the individual files produced by emscripten. Note that some of these files
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may be empty. This is because bazel has no concept of optional outputs for
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rules.
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`wasm_cc_binary` uses transition to use emscripten toolchain on `cc_target`
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and all of its dependencies, and does not require amending `.bazelrc`. This
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is the preferred way, since it also unpacks the resulting tarball.
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See `test_external/` for an example using [embind](https://emscripten.org/docs/porting/connecting_cpp_and_javascript/embind.html).
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### Using --config=wasm
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Put the following lines into your `.bazelrc`:
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```
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build:wasm --crosstool_top=@emsdk//emscripten_toolchain:everything
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build:wasm --cpu=wasm
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build:wasm --host_crosstool_top=@bazel_tools//tools/cpp:toolchain
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```
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Simply pass `--config=wasm` when building a normal `cc_binary`. The result of
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this build will be a tar archive containing any files produced by emscripten.
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See the [Bazel documentation](https://docs.bazel.build/versions/main/tutorial/cc-toolchain-config.html)
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for more details
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