Add "rustc_codegen_gcc" page

Signed-off-by: Antoni Boucher <bouanto@zoho.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

SUMMARY.md

@@ -23,6 +23,7 @@
 ## Tools
 
   - [Coccinelle for Rust](Coccinelle-for-Rust.md)
+  - [`rustc_codegen_gcc`](rustc_codegen_gcc.md)
 
 ## Users
 

src/rustc_codegen_gcc.md

@@ -0,0 +1,44 @@
+# `rustc_codegen_gcc`
+
+[`rustc_codegen_gcc`](https://github.com/rust-lang/rustc_codegen_gcc) is a GCC ahead-of-time codegen for rustc, meaning that it can be loaded by the existing rustc frontend, but benefits from GCC by having more architectures supported and having access to GCC’s optimizations.
+
+Rust for Linux can be compiled with `rustc_codegen_gcc` which allows to have a GCC-compiled Linux kernel with Rust kernel modules compiled with GCC as well.
+
+[Blog with updates about the progress of the GCC codegen.](https://blog.antoyo.xyz/)
+
+## Building `rustc_codegen_gcc` and the sysroot
+
+The following have been tested with GCC commit [b334f15ed21c95868059a16484a1948be08e26a3](https://github.com/antoyo/gcc/commit/b334f15ed21c95868059a16484a1948be08e26a3) and `rustc_codegen_gcc` commit [c6bc7ecd65046ee502118664f42637ca318cdfb5](https://github.com/rust-lang/rustc_codegen_gcc/commit/c6bc7ecd65046ee502118664f42637ca318cdfb5).
+
+Follow the build instructions [here](https://github.com/rust-lang/rustc_codegen_gcc#building).
+
+## Building Rust for Linux
+
+Follow the [Rust for Linux Quick Start instructions](https://docs.kernel.org/rust/quick-start.html) with a few changes as explained below.
+
+Since the GCC codegen might not work on every nightly version (that should soon be fixed now that we run some tests in the Rust CI), we're going to use [the same nightly version as the GCC codegen](https://github.com/rust-lang/rustc_codegen_gcc/blob/master/rust-toolchain) instead of using the version recommended by Rust for Linux:
+
+```sh
+rustup override set nightly-2023-10-21 # Adjust to the version used by the GCC codegen.
+```
+
+Now, you need to set some variables to build Rust for Linux with the GCC codegen (do not forget to adjust your path to `rustc_codegen_gcc`):
+
+```sh
+make -j20 KRUSTFLAGS="-Zcodegen-backend=/path/to/rustc_codegen_gcc/target/debug/librustc_codegen_gcc.so --sysroot /path/to/rustc_codegen_gcc/build_sysroot/sysroot" HOSTRUSTFLAGS="-Zcodegen-backend=/path/to/rustc_codegen_gcc/target/debug/librustc_codegen_gcc.so --sysroot /path/to/rustc_codegen_gcc/build_sysroot/sysroot -Clto=no"
+```
+
+Since we use a differently nightly version, you might need to adjust the code of the `alloc` crate built by Rust for Linux.
+You'll see some errors when running the above command in this case.
+
+## Troubleshooting
+
+If that didn't build the Rust object files, run `make menuconfig` again and check if the "Rust support" is available.
+It could be that you have `RUST_IS_AVAILABLE [=n]`.
+In that case, run `make rustavailable` with the `KRUSTFLAGS` you used above.
+That should give you the correct error, which could be one of those:
+
+ * `libgccjit.so.0: cannot open shared object file: No such file or directory`
+   - In this case, make sure you set `LD_LIBRARY_PATH` and `LIBRARY_PATH`.
+ * `Source code for the 'core' standard library could not be found`
+   - In this case, make sure you used a recent enough version of `rustc_codegen_gcc` (c6bc7ecd65046ee502118664f42637ca318cdfb5 or more recent should be good) that copies the source of the sysroot at the correct location.