tvl-depot/tvix/eval/src/observer.rs

//! Implements a trait for things that wish to observe internal state
//! changes of tvix-eval.
//!
//! This can be used to gain insights from compilation, to trace the
//! runtime, and so on.

use codemap::CodeMap;
use std::io::Write;
use std::rc::Rc;
use tabwriter::TabWriter;

use crate::chunk::Chunk;
use crate::opcode::{CodeIdx, OpCode};
use crate::value::Lambda;
use crate::Value;

/// Implemented by types that wish to observe internal happenings of
/// Tvix.
///
/// All methods are optional, that is, observers can implement only
/// what they are interested in observing.
pub trait Observer {
    /// Called when the compiler finishes compilation of the top-level
    /// of an expression (usually the root Nix expression of a file).
    fn observe_compiled_toplevel(&mut self, _: &Rc<Lambda>) {}

    /// Called when the compiler finishes compilation of a
    /// user-defined function.
    ///
    /// Note that in Nix there are only single argument functions, so
    /// in an expression like `a: b: c: ...` this method will be
    /// called three times.
    fn observe_compiled_lambda(&mut self, _: &Rc<Lambda>) {}

    /// Called when the compiler finishes compilation of a thunk.
    fn observe_compiled_thunk(&mut self, _: &Rc<Lambda>) {}

    /// Called when the runtime enters a new call frame.
    fn observe_enter_frame(&mut self, _arg_count: usize, _: &Rc<Lambda>, _call_depth: usize) {}

    /// Called when the runtime exits a call frame.
    fn observe_exit_frame(&mut self, _frame_at: usize) {}

    /// Called when the runtime enters a builtin.
    fn observe_enter_builtin(&mut self, _name: &'static str) {}

    /// Called when the runtime exits a builtin.
    fn observe_exit_builtin(&mut self, _name: &'static str) {}

    /// Called when the runtime *begins* executing an instruction. The
    /// provided stack is the state at the beginning of the operation.
    fn observe_execute_op(&mut self, _ip: usize, _: &OpCode, _: &[Value]) {}
}

#[derive(Default)]
pub struct NoOpObserver {}

impl Observer for NoOpObserver {}

/// An observer that prints disassembled chunk information to its
/// internal writer whenwever the compiler emits a toplevel function,
/// closure or thunk.
pub struct DisassemblingObserver<W: Write> {
    codemap: Rc<CodeMap>,
    writer: TabWriter<W>,
}

impl<W: Write> DisassemblingObserver<W> {
    pub fn new(codemap: Rc<CodeMap>, writer: W) -> Self {
        Self {
            codemap,
            writer: TabWriter::new(writer),
        }
    }

    fn lambda_header(&mut self, kind: &str, lambda: &Rc<Lambda>) {
        let _ = writeln!(
            &mut self.writer,
            "=== compiled {} @ {:p} ({} ops) ===",
            kind,
            lambda,
            lambda.chunk.code.len()
        );
    }

    fn disassemble_chunk(&mut self, chunk: &Chunk) {
        // calculate width of the widest address in the chunk
        let width = format!("{:#x}", chunk.code.len() - 1).len();

        for (idx, _) in chunk.code.iter().enumerate() {
            let _ = chunk.disassemble_op(&mut self.writer, &self.codemap, width, CodeIdx(idx));
        }
    }
}

impl<W: Write> Observer for DisassemblingObserver<W> {
    fn observe_compiled_toplevel(&mut self, lambda: &Rc<Lambda>) {
        self.lambda_header("toplevel", lambda);
        self.disassemble_chunk(&lambda.chunk);
        let _ = self.writer.flush();
    }

    fn observe_compiled_lambda(&mut self, lambda: &Rc<Lambda>) {
        self.lambda_header("lambda", lambda);
        self.disassemble_chunk(&lambda.chunk);
        let _ = self.writer.flush();
    }

    fn observe_compiled_thunk(&mut self, lambda: &Rc<Lambda>) {
        self.lambda_header("thunk", lambda);
        self.disassemble_chunk(&lambda.chunk);
        let _ = self.writer.flush();
    }
}