Rust YJIT

In December 2021, we opened an [issue] to solicit feedback regarding the
porting of the YJIT codebase from C99 to Rust. There were some
reservations, but this project was given the go ahead by Ruby core
developers and Matz. Since then, we have successfully completed the port
of YJIT to Rust.

The new Rust version of YJIT has reached parity with the C version, in
that it passes all the CRuby tests, is able to run all of the YJIT
benchmarks, and performs similarly to the C version (because it works
the same way and largely generates the same machine code). We've even
incorporated some design improvements, such as a more fine-grained
constant invalidation mechanism which we expect will make a big
difference in Ruby on Rails applications.

Because we want to be careful, YJIT is guarded behind a configure
option:

```shell
./configure --enable-yjit # Build YJIT in release mode
./configure --enable-yjit=dev # Build YJIT in dev/debug mode
```

By default, YJIT does not get compiled and cargo/rustc is not required.
If YJIT is built in dev mode, then `cargo` is used to fetch development
dependencies, but when building in release, `cargo` is not required,
only `rustc`. At the moment YJIT requires Rust 1.60.0 or newer.

The YJIT command-line options remain mostly unchanged, and more details
about the build process are documented in `doc/yjit/yjit.md`.

The CI tests have been updated and do not take any more resources than
before.

The development history of the Rust port is available at the following
commit for interested parties:
https://github.com/Shopify/ruby/commit/1fd9573d8b4b65219f1c2407f30a0a60e537f8be

Our hope is that Rust YJIT will be compiled and included as a part of
system packages and compiled binaries of the Ruby 3.2 release. We do not
anticipate any major problems as Rust is well supported on every
platform which YJIT supports, but to make sure that this process works
smoothly, we would like to reach out to those who take care of building
systems packages before the 3.2 release is shipped and resolve any
issues that may come up.

[issue]: https://bugs.ruby-lang.org/issues/18481

Co-authored-by: Maxime Chevalier-Boisvert <maximechevalierb@gmail.com>
Co-authored-by: Noah Gibbs <the.codefolio.guy@gmail.com>
Co-authored-by: Kevin Newton <kddnewton@gmail.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 87aaeefa7e..2eb9fd8630 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -4926,8 +4926,13 @@ vm_opt_newarray_min(rb_execution_context_t *ec, rb_num_t num, const VALUE *ptr)
 
 #define IMEMO_CONST_CACHE_SHAREABLE IMEMO_FL_USER0
 
-// For each getconstant, associate the ID that corresponds to the first operand
-// to that instruction with the inline cache.
+// This is the iterator used by vm_ic_compile for rb_iseq_each. It is used as a
+// callback for each instruction within the ISEQ, and is meant to return a
+// boolean indicating whether or not to keep iterating.
+//
+// This is used to walk through the ISEQ and find all getconstant instructions
+// between the starting opt_getinlinecache and the ending opt_setinlinecache and
+// associating the inline cache with the constant name components on the VM.
 static bool
 vm_ic_compile_i(VALUE *code, VALUE insn, size_t index, void *ic)
 {