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, 98 insertions, 0 deletions

diff --git a/yjit/src/yjit.rs b/yjit/src/yjit.rs
new file mode 100644
index 0000000000..24a6b426bf
--- /dev/null
+++ b/yjit/src/yjit.rs
@@ -0,0 +1,98 @@
+use crate::codegen::*;
+use crate::core::*;
+use crate::cruby::*;
+use crate::invariants::*;
+use crate::options::*;
+
+use std::os::raw;
+use std::sync::atomic::{AtomicBool, Ordering};
+
+/// For tracking whether the user enabled YJIT through command line arguments or environment
+/// variables. AtomicBool to avoid `unsafe`. On x86 it compiles to simple movs.
+/// See <https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html>
+/// See [rb_yjit_enabled_p]
+static YJIT_ENABLED: AtomicBool = AtomicBool::new(false);
+
+/// Parse one command-line option.
+/// This is called from ruby.c
+#[no_mangle]
+pub extern "C" fn rb_yjit_parse_option(str_ptr: *const raw::c_char) -> bool {
+    return parse_option(str_ptr).is_some();
+}
+
+/// Is YJIT on? The interpreter uses this function to decide whether to increment
+/// ISEQ call counters. See mjit_exec().
+/// This is used frequently since it's used on every method call in the interpreter.
+#[no_mangle]
+pub extern "C" fn rb_yjit_enabled_p() -> raw::c_int {
+    // Note that we might want to call this function from signal handlers so
+    // might need to ensure signal-safety(7).
+    YJIT_ENABLED.load(Ordering::Acquire).into()
+}
+
+/// Like rb_yjit_enabled_p, but for Rust code.
+pub fn yjit_enabled_p() -> bool {
+    YJIT_ENABLED.load(Ordering::Acquire)
+}
+
+/// After how many calls YJIT starts compiling a method
+#[no_mangle]
+pub extern "C" fn rb_yjit_call_threshold() -> raw::c_uint {
+    get_option!(call_threshold) as raw::c_uint
+}
+
+/// This function is called from C code
+#[no_mangle]
+pub extern "C" fn rb_yjit_init_rust() {
+    // TODO: need to make sure that command-line options have been
+    // initialized by CRuby
+
+    // Catch panics to avoid UB for unwinding into C frames.
+    // See https://doc.rust-lang.org/nomicon/exception-safety.html
+    // TODO: set a panic handler so the we don't print a message
+    //       everytime we panic.
+    let result = std::panic::catch_unwind(|| {
+        Invariants::init();
+        CodegenGlobals::init();
+
+        // YJIT enabled and initialized successfully
+        YJIT_ENABLED.store(true, Ordering::Release);
+    });
+
+    if let Err(_) = result {
+        println!("YJIT: rb_yjit_init_rust() panicked. Aborting.");
+        std::process::abort();
+    }
+}
+
+/// Called from C code to begin compiling a function
+/// NOTE: this should be wrapped in RB_VM_LOCK_ENTER(), rb_vm_barrier() on the C side
+#[no_mangle]
+pub extern "C" fn rb_yjit_iseq_gen_entry_point(iseq: IseqPtr, ec: EcPtr) -> *const u8 {
+    let maybe_code_ptr = gen_entry_point(iseq, ec);
+
+    match maybe_code_ptr {
+        Some(ptr) => ptr.raw_ptr(),
+        None => std::ptr::null(),
+    }
+}
+
+/// Simulate a situation where we are out of executable memory
+#[no_mangle]
+pub extern "C" fn rb_yjit_simulate_oom_bang(_ec: EcPtr, _ruby_self: VALUE) -> VALUE {
+    // If YJIT is not enabled, do nothing
+    if !yjit_enabled_p() {
+        return Qnil;
+    }
+
+    // Enabled in debug mode only for security
+    #[cfg(debug_assertions)]
+    {
+        let cb = CodegenGlobals::get_inline_cb();
+        let ocb = CodegenGlobals::get_outlined_cb().unwrap();
+        cb.set_pos(cb.get_mem_size() - 1);
+        ocb.set_pos(ocb.get_mem_size() - 1);
+    }
+
+    return Qnil;
+}