ed2035a3cc
This migrates the bazel integration to the new Bazel dependency system "bzlmod". bzlmod is becoming mandatory this year (see second sentence here: https://bazel.build/external/migration). This is a backwards incompatible migration, directly removing the old WORKSPACE based approach. Users will have to change how they depend on bzlmod, however I assume pretty much every user will be happy about it, because they are forced to use bzlmod anyway or add extra flags to continue to build with newer Bazel versions. Given that users normally depend on specific git commits in the old system, they won't be hit with this until they decide to upgrade emsdk. The basic principle here is simple: I took everything that WORKSPACE did and searched an alternative in bzlmod. Some interesting bits: - We have less worries about multiple versions and people depending on emscripten multiple times in different ways. This is resolved by the new system: Bazel first iterates through the MODULE.bazel files recursively, then finds the minimum version needed for each module and then executes the module extensions that define repos exactly once at that version. So no more ifs needed to detect multiple inclusions. - A bunch of nodejs stuff moves to MODULE.bazel, because that is how the nodejs module works now. As their module extension gets executed only once you need to declare everything that you could need before that in the MODULE.bazel file. A side effect of that is that we have to make a fake repository when emscripten doesn't have an arm64 binary for linux, because we can't actually figure that out in MODULE.bazel, so we have to declare that it always exists and then create one in all cases. There is a bunch of autoformatter changes in here as well, I could try to revert them if you prefer. Closes #1509
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. Now we only support inclusion as a bzlmod module.
In your MODULE.bazel file, put:
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.
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.
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.