GCC codegen backend

We run a subset of the compiler test suite with the GCC codegen backend on our CI, to help find changes that could break the integration of this backend with the compiler.

If you encounter any bugs or problems with the GCC codegen backend in general, don't hesitate to open issues on the rustc_codegen_gcc repository.

Note that the backend currently only supports the x86_64-unknown-linux-gnu target.

Running into GCC backend CI errors

If you ran into an error related to tests executed with the GCC codegen backend on CI in the x86_64-gnu-gcc job, you can use the following command to run UI tests locally using the GCC backend, which reproduces what happens on CI:

./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.

To reproduce the whole CI job locally, you can run cargo run --manifest-path src/ci/citool/Cargo.toml run-local x86_64-gnu-gcc. See Testing with Docker for more information.

What to do in case of a GCC job failure?

If the GCC job test fails and it seems like the failure could be caused by the GCC backend, you can ping the cg-gcc working group using @rust-lang/wg-gcc-backend

If fixing a compiler test that fails with the GCC backend is non-trivial, you can ignore that test when executed with cg_gcc using the //@ ignore-backends: gcc compiletest directive.

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.

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.

Running tests of the backend itself

In addition to running the compiler's test suites using the GCC codegen backend, you can also run the test suite of the backend itself.

Now you do that using the following command:

./x test rustc_codegen_gcc

The backend needs to be enabled for this to work.