82d41a76d3
This is a working solution for generating a separate Emscripten cache.
Note that this requires an additional entry in the workspace as follows:
```starlark
load("@emsdk//:emscripten_cache.bzl", emsdk_emscripten_cache = "emscripten_cache")
emsdk_emscripten_cache()
```
When used like this, the default Emscripten cache will be used. However,
if the entry is as follows:
```starlark
load("@emsdk//:emscripten_cache.bzl", emsdk_emscripten_cache = "emscripten_cache")
emsdk_emscripten_cache(flags = ["--lto"])
```
Then embuilder will be called to build all system libraries and ports
(i.e., the `ALL` option to embuilder) with the LTO option enabled. This
can take awhile, so I have also made possible to specify which libraries
you want to build explicitly:
```starlark
load("@emsdk//:emscripten_cache.bzl", emsdk_emscripten_cache = "emscripten_cache")
emsdk_emscripten_cache(
flags = ["--lto"],
libraries = [
"crtbegin",
"libprintf_long_double-debug",
"libstubs-debug",
"libnoexit",
"libc-debug",
"libdlmalloc",
"libcompiler_rt",
"libc++-noexcept",
"libc++abi-debug-noexcept",
"libsockets"
]
)
```
Resolves #807, resolves #971, resolves #1099, resolves #1362, resolves
#1401
96 lines
3.1 KiB
Markdown
96 lines
3.1 KiB
Markdown
# Bazel Emscripten toolchain
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## Setup Instructions
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In `WORKSPACE` file, put:
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```starlark
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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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load("@emsdk//:toolchains.bzl", "register_emscripten_toolchains")
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register_emscripten_toolchains()
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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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Bazel 7+ additionally requires `platforms` dependencies in the `MODULE.bazel` file.
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```starlark
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bazel_dep(name = "platforms", version = "0.0.9")
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```
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## Building
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Put the following line into your `.bazelrc`:
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```
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build --incompatible_enable_cc_toolchain_resolution
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```
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Then write a new rule wrapping your `cc_binary`.
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```starlark
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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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The Emscripten cache shipped by default does not include LTO, 64-bit or PIC
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builds of the system libraries and ports. If you wish to use these features you
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will need to declare the cache when you register the toolchain as follows. Note
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that the configuration consists of the same flags that can be passed to
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embuilder. If `targets` is not provided, all system libraries and ports will be
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built, i.e., the `ALL` option to embuilder.
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```starlark
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load("@emsdk//:toolchains.bzl", "register_emscripten_toolchains")
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register_emscripten_toolchains(cache = {
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"configuration": ["--lto"],
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"targets": [
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"crtbegin",
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"libprintf_long_double-debug",
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"libstubs-debug",
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"libnoexit",
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"libc-debug",
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"libdlmalloc",
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"libcompiler_rt",
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"libc++-noexcept",
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"libc++abi-debug-noexcept",
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"libsockets"
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]
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})
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```
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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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