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gerrit-dev.monogon.dev / monogon / a7dca8956f9e0182f51b74d1309f49f177416eef / . / build / toolchain / musl-host-gcc
tree: bdd476e48bf5466b282bcf78c1be33e266debd33 [path history] [tgz]
  1. sysroot/
  2. BUILD
  3. gcc-wrapper.sh
  4. musl.spec
  5. README.md
  6. sysroot.bzl
  7. sysroot.tar.xz
  8. sysroot_repository.bzl
build/toolchain/musl-host-gcc/README.md

musl-host-gcc

musl-host-gcc is a Bazel C++ toolchain that uses the machine's host gcc in combination with a pre-built musl, musl headers, and Linux headers.

It is currently used to build the few C binaries we need in Smalltown' runtime.

At some point, this toolchain should be replaced by a fully hermetic toolchain that doesn't depend on the host environment.

Usage

To use this toolchain explicitely while building a cc_binary, do:

bazel build --crosstool_top=//build/toolchain/musl-host-gcc:musl_host_cc_suite //foo/bar

During an actual build however, the right toolchain should be selected using aspects or other Bazel configurability features, instead of a hardcoded --crosstool_top.

Building Toolchain Sysroot Tarball

The toolchain's musl/linux components are currently built ahead of time and committed to this repository as //build/toolchain/musl-host-gcc/toolchain.tar.xz. This is the 'sysroot' tarball, that contains all headers and libraries required to build against Smalltown.

To build this tarball, run the following commands:

bazel build //build/toolchain/musl-host-gcc/sysroot
cp -f bazel-bin/build/toolchain/musl-host-gcc/sysroot/sysroot.tar.xz build/toolchain/musl-host-gcc/sysroot.tar.xz

Internals

The toolchain is implemented in the following way:

  1. //build/toolchain/musl-host-gcc/sysroot is used to build //build/toolchain/musl-host-gcc/sysroot.tar.xz which is a tarball that contains all include and binary library files for building against musl for Smalltown (x86_64 / k8) - thes are musl headers, musl libraries, and linux headers. This tarball is commited to source control.
  2. When building a target that uses the toolchain, the sysroot.tar.xz tarball is extracted into an external repository @musl_sysroot, via sysroot.bzl and sysroot_repository.bzl.
  3. A toolchain config is built using //build/toolchain:cc_toolchain_config.bzl, which points at gcc-wrapper.sh as its gcc entrypoint. gcc-wrapper.sh expects to be able to call the host gcc with musl.spec.
  4. A toolchain is built in //build/toolchain/musl-host-gcc:musl_host_cc_suite, which uses the previously mentioned config, and builds it to contain gcc-wrapper.sh, musl.spec, and the sysroot tarball.

Quirks

As mentioned above, the musl sysroot is kept in a tarball in this repository. This is obviously suboptimal, but on the other hand gives us an effectively pre-built part of a toolchain. In the future, once we have a hermetic toolchain, a similar tarball might actually contain a fully hermetic toolchain pre-built for k8.