roc/crates/compiler/builtins/bitcode

Bitcode for Builtins

Adding a bitcode builtin

To add a builtin:

1. Add the function to the relevant module. For Num builtin use it in src/num.zig, for Str builtins use src/str.zig, and so on. For anything you add, you must add tests for it! Not only does to make the builtins more maintainable, it's the the easiest way to test these functions on Zig. To run the test, run: zig build test
2. Make sure the function is public with the pub keyword and uses the C calling convention. This is really easy, just add pub and callconv(.C) to the function declaration like so: pub fn atan(num: f64) callconv(.C) f64 { ... }
3. In src/main.zig, export the function. This is also organized by module. For example, for a Num function find the Num section and add: comptime { exportNumFn(num.atan, "atan"); }. The first argument is the function, the second is the name of it in LLVM.
4. In compiler/builtins/src/bitcode.rs, add a constant for the new function. This is how we use it in Rust. Once again, this is organized by module, so just find the relevant area and add your new function.
5. You can now use your function in Rust using call_bitcode_fn in llvm/src/build.rs!

How it works

Roc's builtins are implemented in the compiler using LLVM only. When their implementations are simple enough (e.g. addition), they can be implemented directly in Inkwell.

When their implementations are complex enough, it's nicer to implement them in a higher-level language like Zig, then compile the result to LLVM bitcode, and import that bitcode into the compiler.

Compiling the bitcode happens automatically in a Rust build script at compiler/builtins/build.rs. Then builtins/src/bitcode/rs staticlly imports the compiled bitcode for use in the compiler.

You can find the compiled bitcode in target/debug/build/roc_builtins-[some random characters]/out/builtins.bc. There will be two directories like roc_builtins-[some random characters], look for the one that has an out directory as a child.

The bitcode is a bunch of bytes that aren't particularly human-readable. If you want to take a look at the human-readable LLVM IR, look at compiler/builtins/bitcode/builtins.ll

Calling bitcode functions

use the call_bitcode_fn function defined in llvm/src/build.rs to call bitcode functions.