Merge pull request #2587 from GuillaumeGomez/gcc-backend-missing-docs

Add missing documentation for running tests with GCC backend

src/doc/rustc-dev-guide/src/tests/codegen-backend-tests/cg_gcc.md

@@ -1,3 +1,56 @@
-# GCC codegen backend tests
+# GCC codegen backend
 
-TODO: please add some more information to this page.
+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 tests locally using the GCC backend:
+
+```bash
+./x test tests/ui --set 'rust.codegen-backends = ["llvm", "gcc"]' --test-codegen-backend gcc
+```
+
+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`:
+
+```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:
+
+```bash
+./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:
+
+```bash
+./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:
+
+```bash
+./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](#choosing-which-codegen-backends-are-built).
+
+## 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.