49ccc1ddeb
local=True marks a repository rule to unconditionally re-execute anytime bazel re-evaluates the workspace (almost every invocation) It works well for processes that execute in <10ms, not large ones like the embuilder cache generation. I have tested this locally and it seems to work fine, but I'll be honest that I don't understand the nuances of repository rules that well, nor do I know much of anything about emscripten or the circumstances in which the cache ought to be rebuilt. I checked the `external/@emscripten_cache.marker` file that Bazel uses to store the cache key for if it needs to re-evaluate, and changing any of the options passed into register_emscripten_toolchains resulted in the cache key changing, and it correctly resulted in the cache being regenerated. The big question I don't have the context to answer: are there files on disk other than these two that need to be watched for changes, so the repository rule should change if they change: ``` FILE:@@emscripten_bin_linux//BUILD.bazel FILE:@@emsdk//emscripten_toolchain/default_config ```
Bazel Emscripten toolchain
Setup Instructions
In WORKSPACE file, put:
load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
git_repository(
name = "emsdk",
remote = "https://github.com/emscripten-core/emsdk.git",
tag = "3.1.64",
strip_prefix = "bazel",
)
load("@emsdk//:deps.bzl", emsdk_deps = "deps")
emsdk_deps()
load("@emsdk//:emscripten_deps.bzl", emsdk_emscripten_deps = "emscripten_deps")
emsdk_emscripten_deps(emscripten_version = "3.1.64")
load("@emsdk//:toolchains.bzl", "register_emscripten_toolchains")
register_emscripten_toolchains()
The tag and emscripten_version parameters correspond to the git revision of
emsdk 3.1.64. To get access to
newer versions, you'll need to update those. To make use of older versions, change the
parameter of git_repository and 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.