Add guide to add new attributes support in libgccjit

Author: GuillaumeGomez

doc/add-attribute.md

@@ -0,0 +1,17 @@
+# Add support for a new function attribute
+
+To add support for a new function attribute in libgccjit, you need to do the following steps:
+
+ 1. Copy the corresponding function from `c-family/c-attribs.cc` into `jit/dummy-frontend.cc`. For example if you add the `target` attribute, the function name will be `handle_target_attribute`.
+ 2. Copy the corresponding entry from the `c_common_attribute_table` variable in the `c-family/c-attribs.cc` file into the `jit_attribute_table` variable in `jit/dummy-frontend.cc`.
+ 3. Add a new variant in the `gcc_jit_fn_attribute` enum in the `jit/libgccjit.h` file.
+ 4. Add a test to ensure the attribute is correctly applied in `gcc/testsuite/jit.dg/`. Take a look at `gcc/testsuite/jit.dg/test-nonnull.c` if you want an example.
+ 5. Run the example like this (in your `gcc-build` folder): `make check-jit RUNTESTFLAGS="-v -v -v jit.exp=jit.dg/test-nonnull.c"`
+
+Once done, you need to update the [gccjit.rs] crate to add the new enum variant in the corresponding enum (`FnAttribute`).
+
+Finally, you need to update this repository by calling the relevant API you added in [gccjit.rs].
+
+To test it, build `gcc`, run `cargo update -p gccjit` and then you can test the generated output for a given Rust crate.
+
+[gccjit.rs]: https://github.com/antoyo/gccjit.rs