GCC codegen backend

If you ran into an error related to tests executed with the GCC codegen backend on CI, you can use the following command to run UI tests locally using the GCC backend:

./x test tests/ui \
  --set 'rust.codegen-backends = ["llvm", "gcc"]' \
  --set 'rust.debug-assertions = false' \
  --test-codegen-backend gcc

If a different test suite has failed on CI, you will have to modify the tests/ui part.

Below, you can find more information about how to configure the GCC backend in bootstrap.

Choosing which codegen backends are built

The rust.codegen-backends = [...] bootstrap option affects which codegen backends will be built and included in the sysroot of the produced rustc. To use the GCC codegen backend, "gcc" has to be included in this array in bootstrap.toml:

rust.codegen-backends = ["llvm", "gcc"]

If you don't want to change your bootstrap.toml file, you can alternatively run your x commands with --set 'rust.codegen-backends=["llvm", "gcc"]'. For example:

./x build --set 'rust.codegen-backends=["llvm", "gcc"]'

The first backend in the codegen-backends array will determine which backend will be used as the default backend of the built rustc. This also determines which backend will be used to compile the stage 1 standard library (or anything built in stage 2+). To produce rustc that uses the GCC backend by default, you can thus put "gcc" as the first element of this array:

./x build --set 'rust.codegen-backends=["gcc"]' library

Choosing the codegen backend used in tests

To run compiler tests with the GCC codegen backend being used to build the test Rust programs, you can use the --test-codegen-backend flag:

./x test tests/ui --test-codegen-backend gcc

Note that in order for this to work, the tested compiler must have the GCC codegen backend available in its sysroot directory. You can achieve that using the instructions above.

Downloading GCC from CI

The gcc.download-ci-gcc bootstrap option controls if GCC (which is a dependency of the GCC codegen backend) will be downloaded from CI or built locally. The default value is true, which will download GCC from CI if there are no local changes to the GCC sources and the given host target is available on CI.

Note that GCC can currently only be downloaded from CI for the x86_64-unknown-linux-gnu target.