refactor(tvix/eval): ensure VM operations fit in a single byte r/8519

This replaces the OpCode enum with a new Op enum which is guaranteed to fit in a
single byte. Instead of carrying enum variants with data, every variant that has
runtime data encodes it into the `Vec<u8>` that a `Chunk` now carries.

This has several advantages:

* Less stack space is required at runtime, and fewer allocations are required
  while compiling.
* The OpCode doesn't need to carry "weird" special-cased data variants anymore.
* It is faster (albeit, not by much). On my laptop, results consistently look
  approximately like this:

  Benchmark 1: ./before -E '(import <nixpkgs> {}).firefox.outPath' --log-level ERROR --no-warnings
  Time (mean ± σ):      8.224 s ±  0.272 s    [User: 7.149 s, System: 0.688 s]
  Range (min … max):    7.759 s …  8.583 s    10 runs

  Benchmark 2: ./after -E '(import <nixpkgs> {}).firefox.outPath' --log-level ERROR --no-warnings
  Time (mean ± σ):      8.000 s ±  0.198 s    [User: 7.036 s, System: 0.633 s]
  Range (min … max):    7.718 s …  8.334 s    10 runs

  See notes below for why the performance impact might be less than expected.
* It is faster while at the same time dropping some optimisations we previously
  performed.

This has several disadvantages:

* The code is closer to how one would write it in C or Go.
* Bit shifting!
* There is (for now) slightly more code than before.

On performance I have the following thoughts at the moment:

In order to prepare for adding GC, there's a couple of places in Tvix where I'd
like to fence off certain kinds of complexity (such as mutating bytecode, which,
for various reaons, also has to be part of data that is subject to GC). With
this change, we can drop optimisations like retroactively modifying existing
bytecode and *still* achieve better performance than before.

I believe that this is currently worth it to pave the way for changes that are
more significant for performance.

In general this also opens other avenues of optimisation: For example, we can
profile which argument sizes actually exist and remove the copy overhead of
varint decoding (which does show up in profiles) by using more adequately sized
types for, e.g., constant indices.

Known regressions:

* Op::Constant is no longer printing its values in disassembly (this can be
  fixed, I just didn't get around to it, will do separately).

Change-Id: Id9b3a4254623a45de03069dbdb70b8349e976743
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12191
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

9 files changed, 748 insertions, 572 deletions

diff --git a/tvix/eval/src/chunk.rs b/tvix/eval/src/chunk.rs
index e9c65db72590..2a5446a782ed 100644
--- a/tvix/eval/src/chunk.rs
+++ b/tvix/eval/src/chunk.rs
@@ -1,9 +1,10 @@
+use crate::opcode::{CodeIdx, ConstantIdx, Op, OpArg};
+use crate::value::Value;
+use crate::{CoercionKind, SourceCode};
 use std::io::Write;
-use std::ops::{Index, IndexMut};
 
-use crate::opcode::{CodeIdx, ConstantIdx, OpCode};
-use crate::value::Value;
-use crate::SourceCode;
+/// Maximum size of a u64 encoded in the vu128 varint encoding.
+const U64_VARINT_SIZE: usize = 9;
 
 /// Represents a source location from which one or more operations
 /// were compiled.
@@ -30,39 +31,69 @@ struct SourceSpan {
 /// emitted by the compiler.
 #[derive(Debug, Default)]
 pub struct Chunk {
-    pub code: Vec<OpCode>,
+    pub code: Vec<u8>,
     pub constants: Vec<Value>,
     spans: Vec<SourceSpan>,
+
+    /// Index of the last operation (i.e. not data) written to the code vector.
+    /// Some operations (e.g. jump patching) need to know this.
+    last_op: usize,
 }
 
-impl Index<ConstantIdx> for Chunk {
-    type Output = Value;
+impl Chunk {
+    pub fn push_op(&mut self, op: Op, span: codemap::Span) -> usize {
+        self.last_op = self.code.len();
+        self.code.push(op as u8);
+        self.push_span(span, self.last_op);
+        self.last_op
+    }
 
-    fn index(&self, index: ConstantIdx) -> &Self::Output {
-        &self.constants[index.0]
+    pub fn push_uvarint(&mut self, data: u64) {
+        let mut encoded = [0u8; U64_VARINT_SIZE];
+        let bytes_written = vu128::encode_u64(&mut encoded, data);
+        self.code.extend_from_slice(&encoded[..bytes_written]);
     }
-}
 
-impl Index<CodeIdx> for Chunk {
-    type Output = OpCode;
+    pub fn read_uvarint(&self, idx: usize) -> (u64, usize) {
+        debug_assert!(
+            idx < self.code.len(),
+            "invalid bytecode (missing varint operand)",
+        );
 
-    fn index(&self, index: CodeIdx) -> &Self::Output {
-        &self.code[index.0]
+        if self.code.len() - idx >= U64_VARINT_SIZE {
+            vu128::decode_u64(
+                &self.code[idx..idx + U64_VARINT_SIZE]
+                    .try_into()
+                    .expect("size statically checked"),
+            )
+        } else {
+            let mut tmp = [0u8; U64_VARINT_SIZE];
+            tmp[..self.code.len() - idx].copy_from_slice(&self.code[idx..]);
+            vu128::decode_u64(&tmp)
+        }
     }
-}
 
-impl IndexMut<CodeIdx> for Chunk {
-    fn index_mut(&mut self, index: CodeIdx) -> &mut Self::Output {
-        &mut self.code[index.0]
+    pub fn push_u16(&mut self, data: u16) {
+        self.code.extend_from_slice(&data.to_le_bytes())
     }
-}
 
-impl Chunk {
-    pub fn push_op(&mut self, data: OpCode, span: codemap::Span) -> CodeIdx {
-        let idx = self.code.len();
-        self.code.push(data);
-        self.push_span(span, idx);
-        CodeIdx(idx)
+    /// Patches the argument to the jump operand of the jump at the given index
+    /// to point to the *next* instruction that will be emitted.
+    pub fn patch_jump(&mut self, idx: usize) {
+        let offset = (self.code.len() - idx - /* arg idx = */ 1 - /* jump arg size = */ 2) as u16;
+        self.code[idx + 1..idx + 3].copy_from_slice(&offset.to_le_bytes())
+    }
+
+    pub fn read_u16(&self, idx: usize) -> u16 {
+        if idx + 2 > self.code.len() {
+            panic!("Tvix bug: invalid bytecode (expected u16 operand not found)")
+        }
+
+        let byte_array: &[u8; 2] = &self.code[idx..idx + 2]
+            .try_into()
+            .expect("fixed-size slice can not fail to convert to array");
+
+        u16::from_le_bytes(*byte_array)
     }
 
     /// Get the first span of a chunk, no questions asked.
@@ -70,23 +101,13 @@ impl Chunk {
         self.spans[0].span
     }
 
-    /// Return a reference to the last op in the chunk, if any
-    pub fn last_op(&self) -> Option<&OpCode> {
-        self.code.last()
-    }
-
-    /// Pop the last operation from the chunk and clean up its tracked
-    /// span. Used when the compiler backtracks.
-    pub fn pop_op(&mut self) {
-        // Simply drop the last op.
-        self.code.pop();
-
-        if let Some(span) = self.spans.last() {
-            // If the last span started at this op, drop it.
-            if span.start == self.code.len() {
-                self.spans.pop();
-            }
+    /// Return the last op in the chunk together with its index, if any.
+    pub fn last_op(&self) -> Option<(Op, usize)> {
+        if self.code.is_empty() {
+            return None;
         }
+
+        Some((self.code[self.last_op].into(), self.last_op))
     }
 
     pub fn push_constant(&mut self, data: Value) -> ConstantIdx {
@@ -100,8 +121,6 @@ impl Chunk {
         self.constants.get(constant.0)
     }
 
-    // Span tracking implementation
-
     fn push_span(&mut self, span: codemap::Span, start: usize) {
         match self.spans.last_mut() {
             // We do not need to insert the same span again, as this
@@ -136,76 +155,88 @@ impl Chunk {
     }
 
     /// Write the disassembler representation of the operation at
-    /// `idx` to the specified writer.
+    /// `idx` to the specified writer, and return how many bytes in the code to
+    /// skip for the next op.
     pub fn disassemble_op<W: Write>(
         &self,
         writer: &mut W,
         source: &SourceCode,
         width: usize,
         idx: CodeIdx,
-    ) -> Result<(), std::io::Error> {
+    ) -> Result<usize, std::io::Error> {
         write!(writer, "{:#width$x}\t ", idx.0, width = width)?;
 
         // Print continuation character if the previous operation was at
         // the same line, otherwise print the line.
         let line = source.get_line(self.get_span(idx));
-        if idx.0 > 0 && source.get_line(self.get_span(CodeIdx(idx.0 - 1))) == line {
+        if idx.0 > 0 && source.get_line(self.get_span(idx - 1)) == line {
             write!(writer, "   |\t")?;
         } else {
             write!(writer, "{:4}\t", line)?;
         }
 
-        let a = |idx| match &self[idx] {
+        let _fmt_constant = |idx: ConstantIdx| match &self.constants[idx.0] {
             Value::Thunk(t) => t.debug_repr(),
             Value::Closure(c) => format!("closure({:p})", c.lambda),
             Value::Blueprint(b) => format!("blueprint({:p})", b),
             val => format!("{}", val),
         };
 
-        match self[idx] {
-            OpCode::OpConstant(idx) => {
-                writeln!(writer, "OpConstant({}@{})", a(idx), idx.0)
-            }
-            OpCode::OpClosure(idx) => {
-                writeln!(writer, "OpClosure({}@{})", a(idx), idx.0)
-            }
-            OpCode::OpThunkClosure(idx) => {
-                writeln!(writer, "OpThunkClosure({}@{})", a(idx), idx.0)
+        let op: Op = self.code[idx.0].into();
+
+        match op.arg_type() {
+            OpArg::None => {
+                writeln!(writer, "Op{:?}", op)?;
+                Ok(1)
             }
-            OpCode::OpThunkSuspended(idx) => {
-                writeln!(writer, "OpThunkSuspended({}@{})", a(idx), idx.0)
+
+            OpArg::Fixed => {
+                let arg = self.read_u16(idx.0 + 1);
+                writeln!(writer, "Op{:?}({})", op, arg)?;
+                Ok(3)
             }
-            op => writeln!(writer, "{:?}", op),
-        }?;
 
-        Ok(())
-    }
+            OpArg::Uvarint => {
+                let (arg, size) = self.read_uvarint(idx.0 + 1);
+                writeln!(writer, "Op{:?}({})", op, arg)?;
+                Ok(1 + size)
+            }
 
-    /// Extend this chunk with the content of another, moving out of the other
-    /// in the process.
-    ///
-    /// This is used by the compiler when it detects that it unnecessarily
-    /// thunked a nested expression.
-    pub fn extend(&mut self, other: Self) {
-        // Some operations need to be modified in certain ways before being
-        // valid as part of the new chunk.
-        let const_count = self.constants.len();
-        for (idx, op) in other.code.iter().enumerate() {
-            let span = other.get_span(CodeIdx(idx));
-            match op {
-                // As the constants shift, the index needs to be moved relatively.
-                OpCode::OpConstant(ConstantIdx(idx)) => {
-                    self.push_op(OpCode::OpConstant(ConstantIdx(idx + const_count)), span)
+            _ => match op {
+                Op::CoerceToString => {
+                    let kind: CoercionKind = self.code[idx.0 + 1].into();
+                    writeln!(writer, "Op{:?}({:?})", op, kind)?;
+                    Ok(2)
                 }
 
-                // Other operations either operate on relative offsets, or no
-                // offsets, and are safe to keep as-is.
-                _ => self.push_op(*op, span),
-            };
-        }
+                Op::Closure | Op::ThunkClosure | Op::ThunkSuspended => {
+                    let mut cidx = idx.0 + 1;
 
-        self.constants.extend(other.constants);
-        self.spans.extend(other.spans);
+                    let (bp_idx, size) = self.read_uvarint(cidx);
+                    cidx += size;
+
+                    let (packed_count, size) = self.read_uvarint(cidx);
+                    cidx += size;
+
+                    let captures_with = packed_count & 0b1 == 1;
+                    let count = packed_count >> 1;
+
+                    write!(writer, "Op{:?}(BP @ {}, ", op, bp_idx)?;
+                    if captures_with {
+                        write!(writer, "captures with, ")?;
+                    }
+                    writeln!(writer, "{} upvalues)", count)?;
+
+                    for _ in 0..count {
+                        let (_, size) = self.read_uvarint(cidx);
+                        cidx += size;
+                    }
+
+                    Ok(cidx - idx.0)
+                }
+                _ => panic!("Tvix bug: don't know how to format argument for Op{:?}", op),
+            },
+        }
     }
 }
 
@@ -221,79 +252,49 @@ mod tests {
     #[test]
     fn push_op() {
         let mut chunk = Chunk::default();
-        chunk.push_op(OpCode::OpAdd, dummy_span());
-        assert_eq!(chunk.code.last().unwrap(), &OpCode::OpAdd);
+        let idx = chunk.push_op(Op::Add, dummy_span());
+        assert_eq!(*chunk.code.last().unwrap(), Op::Add as u8);
+        assert_eq!(chunk.code[idx], Op::Add as u8);
     }
 
     #[test]
-    fn extend_empty() {
+    fn push_op_with_arg() {
         let mut chunk = Chunk::default();
-        chunk.push_op(OpCode::OpAdd, dummy_span());
+        let mut idx = chunk.push_op(Op::Constant, dummy_span());
+        chunk.push_uvarint(42);
 
-        let other = Chunk::default();
-        chunk.extend(other);
+        assert_eq!(chunk.code[idx], Op::Constant as u8);
 
-        assert_eq!(
-            chunk.code,
-            vec![OpCode::OpAdd],
-            "code should not have changed"
-        );
+        idx += 1;
+        let (arg, size) = chunk.read_uvarint(idx);
+        assert_eq!(idx + size, chunk.code.len());
+        assert_eq!(arg, 42);
     }
 
     #[test]
-    fn extend_simple() {
-        let span = dummy_span();
+    fn push_jump() {
         let mut chunk = Chunk::default();
-        chunk.push_op(OpCode::OpAdd, span);
 
-        let mut other = Chunk::default();
-        other.push_op(OpCode::OpSub, span);
-        other.push_op(OpCode::OpMul, span);
+        chunk.push_op(Op::Constant, dummy_span());
+        chunk.push_uvarint(0);
 
-        let expected_code = vec![OpCode::OpAdd, OpCode::OpSub, OpCode::OpMul];
+        let idx = chunk.push_op(Op::Jump, dummy_span());
+        chunk.push_u16(0);
 
-        chunk.extend(other);
+        chunk.push_op(Op::Constant, dummy_span());
+        chunk.push_uvarint(1);
 
-        assert_eq!(chunk.code, expected_code, "code should have been extended");
-    }
+        chunk.patch_jump(idx);
+        chunk.push_op(Op::Return, dummy_span());
 
-    #[test]
-    fn extend_with_constant() {
-        let span = dummy_span();
-        let mut chunk = Chunk::default();
-        chunk.push_op(OpCode::OpAdd, span);
-        let cidx = chunk.push_constant(Value::Integer(0));
-        assert_eq!(
-            cidx.0, 0,
-            "first constant in main chunk should have index 0"
-        );
-        chunk.push_op(OpCode::OpConstant(cidx), span);
-
-        let mut other = Chunk::default();
-        other.push_op(OpCode::OpSub, span);
-        let other_cidx = other.push_constant(Value::Integer(1));
-        assert_eq!(
-            other_cidx.0, 0,
-            "first constant in other chunk should have index 0"
-        );
-        other.push_op(OpCode::OpConstant(other_cidx), span);
-
-        chunk.extend(other);
-
-        let expected_code = vec![
-            OpCode::OpAdd,
-            OpCode::OpConstant(ConstantIdx(0)),
-            OpCode::OpSub,
-            OpCode::OpConstant(ConstantIdx(1)), // <- note: this was rewritten
+        #[rustfmt::skip]
+        let expected: Vec<u8> = vec![
+            Op::Constant as u8, 0,
+            Op::Jump as u8, 2, 0,
+            Op::Constant as u8, 1,
+            Op::Return as u8,
         ];
 
-        assert_eq!(
-            chunk.code, expected_code,
-            "code should have been extended and rewritten"
-        );
-
-        assert_eq!(chunk.constants.len(), 2);
-        assert!(matches!(chunk.constants[0], Value::Integer(0)));
-        assert!(matches!(chunk.constants[1], Value::Integer(1)));
+        assert_eq!(chunk.code, expected);
     }
 }
diff --git a/tvix/eval/src/compiler/bindings.rs b/tvix/eval/src/compiler/bindings.rs
index f5c6376eb1b3..6a3ae485936c 100644
--- a/tvix/eval/src/compiler/bindings.rs
+++ b/tvix/eval/src/compiler/bindings.rs
@@ -605,7 +605,7 @@ impl Compiler<'_, '_> {
                         c.emit_force(&namespace);
 
                         c.emit_constant(name.as_str().into(), &span);
-                        c.push_op(OpCode::OpAttrsSelect, &span);
+                        c.push_op(Op::AttrsSelect, &span);
                     })
                 }
 
@@ -632,7 +632,8 @@ impl Compiler<'_, '_> {
             if self.scope()[idx].needs_finaliser {
                 let stack_idx = self.scope().stack_index(idx);
                 let span = self.scope()[idx].span;
-                self.push_op(OpCode::OpFinalise(stack_idx), &OrEntireFile(span));
+                self.push_op(Op::Finalise, &OrEntireFile(span));
+                self.push_uvarint(stack_idx.0 as u64)
             }
         }
     }
@@ -667,11 +668,8 @@ impl Compiler<'_, '_> {
         self.bind_values(bindings);
 
         if kind.is_attrs() {
-            self.push_op(OpCode::OpAttrs(Count(count)), node);
-        }
-
-        if count == 0 {
-            self.unthunk();
+            self.push_op(Op::Attrs, node);
+            self.push_uvarint(count as u64);
         }
     }
 
@@ -697,7 +695,7 @@ impl Compiler<'_, '_> {
         self.scope_mut().end_scope();
 
         self.emit_constant("body".into(), node);
-        self.push_op(OpCode::OpAttrsSelect, node);
+        self.push_op(Op::AttrsSelect, node);
     }
 
     /// Is the given identifier defined *by the user* in any current scope?
@@ -718,8 +716,9 @@ impl Compiler<'_, '_> {
         match self.scope_mut().resolve_local(ident) {
             LocalPosition::Unknown => {
                 // Are we possibly dealing with an upvalue?
-                if let Some(idx) = self.resolve_upvalue(self.contexts.len() - 1, ident, node) {
-                    self.push_op(OpCode::OpGetUpvalue(idx), node);
+                if let Some(idx) = self.resolve_upvalue(self.contexts.len() - 1, ident) {
+                    self.push_op(Op::GetUpvalue, node);
+                    self.push_uvarint(idx.0 as u64);
                     return;
                 }
 
@@ -742,7 +741,7 @@ impl Compiler<'_, '_> {
                     self.thunk(slot, node, |c, _| {
                         c.context_mut().captures_with_stack = true;
                         c.emit_constant(ident.into(), node);
-                        c.push_op(OpCode::OpResolveWith, node);
+                        c.push_op(Op::ResolveWith, node);
                     });
                     return;
                 }
@@ -753,18 +752,17 @@ impl Compiler<'_, '_> {
 
             LocalPosition::Known(idx) => {
                 let stack_idx = self.scope().stack_index(idx);
-                self.push_op(OpCode::OpGetLocal(stack_idx), node);
+                self.push_op(Op::GetLocal, node);
+                self.push_uvarint(stack_idx.0 as u64);
             }
 
             // This identifier is referring to a value from the same scope which
             // is not yet defined. This identifier access must be thunked.
             LocalPosition::Recursive(idx) => self.thunk(slot, node, move |compiler, _| {
-                let upvalue_idx = compiler.add_upvalue(
-                    compiler.contexts.len() - 1,
-                    node,
-                    UpvalueKind::Local(idx),
-                );
-                compiler.push_op(OpCode::OpGetUpvalue(upvalue_idx), node);
+                let upvalue_idx =
+                    compiler.add_upvalue(compiler.contexts.len() - 1, UpvalueKind::Local(idx));
+                compiler.push_op(Op::GetUpvalue, node);
+                compiler.push_uvarint(upvalue_idx.0 as u64);
             }),
         };
     }
@@ -777,12 +775,7 @@ impl Compiler<'_, '_> {
 
 /// Private compiler helpers related to bindings.
 impl Compiler<'_, '_> {
-    fn resolve_upvalue<N: ToSpan>(
-        &mut self,
-        ctx_idx: usize,
-        name: &str,
-        node: &N,
-    ) -> Option<UpvalueIdx> {
+    fn resolve_upvalue(&mut self, ctx_idx: usize, name: &str) -> Option<UpvalueIdx> {
         if ctx_idx == 0 {
             // There can not be any upvalue at the outermost context.
             return None;
@@ -795,7 +788,7 @@ impl Compiler<'_, '_> {
             // stack (i.e. in the right position) *during* their runtime
             // construction
             LocalPosition::Known(idx) | LocalPosition::Recursive(idx) => {
-                return Some(self.add_upvalue(ctx_idx, node, UpvalueKind::Local(idx)))
+                return Some(self.add_upvalue(ctx_idx, UpvalueKind::Local(idx)))
             }
 
             LocalPosition::Unknown => { /* continue below */ }
@@ -803,19 +796,14 @@ impl Compiler<'_, '_> {
 
         // If the upvalue comes from even further up, we need to recurse to make
         // sure that the upvalues are created at each level.
-        if let Some(idx) = self.resolve_upvalue(ctx_idx - 1, name, node) {
-            return Some(self.add_upvalue(ctx_idx, node, UpvalueKind::Upvalue(idx)));
+        if let Some(idx) = self.resolve_upvalue(ctx_idx - 1, name) {
+            return Some(self.add_upvalue(ctx_idx, UpvalueKind::Upvalue(idx)));
         }
 
         None
     }
 
-    fn add_upvalue<N: ToSpan>(
-        &mut self,
-        ctx_idx: usize,
-        node: &N,
-        kind: UpvalueKind,
-    ) -> UpvalueIdx {
+    fn add_upvalue(&mut self, ctx_idx: usize, kind: UpvalueKind) -> UpvalueIdx {
         // If there is already an upvalue closing over the specified index,
         // retrieve that instead.
         for (idx, existing) in self.contexts[ctx_idx].scope.upvalues.iter().enumerate() {
@@ -824,11 +812,7 @@ impl Compiler<'_, '_> {
             }
         }
 
-        let span = self.span_for(node);
-        self.contexts[ctx_idx]
-            .scope
-            .upvalues
-            .push(Upvalue { kind, span });
+        self.contexts[ctx_idx].scope.upvalues.push(Upvalue { kind });
 
         let idx = UpvalueIdx(self.contexts[ctx_idx].lambda.upvalue_count);
         self.contexts[ctx_idx].lambda.upvalue_count += 1;
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 4878db1f1a84..33b70b87ce84 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -29,7 +29,7 @@ use std::rc::{Rc, Weak};
 use crate::chunk::Chunk;
 use crate::errors::{CatchableErrorKind, Error, ErrorKind, EvalResult};
 use crate::observer::CompilerObserver;
-use crate::opcode::{CodeIdx, ConstantIdx, Count, JumpOffset, OpCode, UpvalueIdx};
+use crate::opcode::{CodeIdx, Op, Position, UpvalueIdx};
 use crate::spans::ToSpan;
 use crate::value::{Closure, Formals, Lambda, NixAttrs, Thunk, Value};
 use crate::warnings::{EvalWarning, WarningKind};
@@ -52,7 +52,6 @@ struct LambdaCtx {
     lambda: Lambda,
     scope: Scope,
     captures_with_stack: bool,
-    unthunk: bool,
 }
 
 impl LambdaCtx {
@@ -61,7 +60,6 @@ impl LambdaCtx {
             lambda: Lambda::default(),
             scope: Default::default(),
             captures_with_stack: false,
-            unthunk: false,
         }
     }
 
@@ -70,7 +68,6 @@ impl LambdaCtx {
             lambda: Lambda::default(),
             scope: self.scope.inherit(),
             captures_with_stack: false,
-            unthunk: false,
         }
     }
 }
@@ -270,13 +267,37 @@ impl Compiler<'_, '_> {
 
     /// Push a single instruction to the current bytecode chunk and
     /// track the source span from which it was compiled.
-    fn push_op<T: ToSpan>(&mut self, data: OpCode, node: &T) -> CodeIdx {
+    fn push_op<T: ToSpan>(&mut self, data: Op, node: &T) -> CodeIdx {
         if self.dead_scope > 0 {
             return CodeIdx(0);
         }
 
         let span = self.span_for(node);
-        self.chunk().push_op(data, span)
+        CodeIdx(self.chunk().push_op(data, span))
+    }
+
+    fn push_u8(&mut self, data: u8) {
+        if self.dead_scope > 0 {
+            return;
+        }
+
+        self.chunk().code.push(data);
+    }
+
+    fn push_uvarint(&mut self, data: u64) {
+        if self.dead_scope > 0 {
+            return;
+        }
+
+        self.chunk().push_uvarint(data);
+    }
+
+    fn push_u16(&mut self, data: u16) {
+        if self.dead_scope > 0 {
+            return;
+        }
+
+        self.chunk().push_u16(data);
     }
 
     /// Emit a single constant to the current bytecode chunk and track
@@ -287,7 +308,8 @@ impl Compiler<'_, '_> {
         }
 
         let idx = self.chunk().push_constant(value);
-        self.push_op(OpCode::OpConstant(idx), node);
+        self.push_op(Op::Constant, node);
+        self.push_uvarint(idx.0 as u64);
     }
 }
 
@@ -400,7 +422,7 @@ impl Compiler<'_, '_> {
                 Value::UnresolvedPath(Box::new(home_relative_path.into())),
                 node,
             );
-            self.push_op(OpCode::OpResolveHomePath, node);
+            self.push_op(Op::ResolveHomePath, node);
             return;
         } else if raw_path.starts_with('<') {
             // TODO: decide what to do with findFile
@@ -416,7 +438,7 @@ impl Compiler<'_, '_> {
             // Make a thunk to resolve the path (without using `findFile`, at least for now?)
             return self.thunk(slot, node, move |c, _| {
                 c.emit_constant(Value::UnresolvedPath(Box::new(path.into())), node);
-                c.push_op(OpCode::OpFindFile, node);
+                c.push_op(Op::FindFile, node);
             });
         } else {
             let mut buf = self.root_dir.clone();
@@ -452,13 +474,15 @@ impl Compiler<'_, '_> {
                 ast::InterpolPart::Interpolation(ipol) => {
                     self.compile(slot, ipol.expr().unwrap());
                     // implicitly forces as well
-                    self.push_op(
-                        OpCode::OpCoerceToString(CoercionKind {
-                            strong: false,
-                            import_paths: true,
-                        }),
-                        ipol,
-                    );
+                    self.push_op(Op::CoerceToString, ipol);
+
+                    let encoded: u8 = CoercionKind {
+                        strong: false,
+                        import_paths: true,
+                    }
+                    .into();
+
+                    self.push_u8(encoded);
                 }
 
                 ast::InterpolPart::Literal(lit) => {
@@ -468,7 +492,8 @@ impl Compiler<'_, '_> {
         }
 
         if parts.len() != 1 {
-            self.push_op(OpCode::OpInterpolate(Count(parts.len())), parent_node);
+            self.push_op(Op::Interpolate, parent_node);
+            self.push_uvarint(parts.len() as u64);
         }
     }
 
@@ -494,8 +519,8 @@ impl Compiler<'_, '_> {
         self.emit_force(op);
 
         let opcode = match op.operator().unwrap() {
-            ast::UnaryOpKind::Invert => OpCode::OpInvert,
-            ast::UnaryOpKind::Negate => OpCode::OpNegate,
+            ast::UnaryOpKind::Invert => Op::Invert,
+            ast::UnaryOpKind::Negate => Op::Negate,
         };
 
         self.push_op(opcode, op);
@@ -526,21 +551,21 @@ impl Compiler<'_, '_> {
         self.emit_force(&op.rhs().unwrap());
 
         match op.operator().unwrap() {
-            BinOpKind::Add => self.push_op(OpCode::OpAdd, op),
-            BinOpKind::Sub => self.push_op(OpCode::OpSub, op),
-            BinOpKind::Mul => self.push_op(OpCode::OpMul, op),
-            BinOpKind::Div => self.push_op(OpCode::OpDiv, op),
-            BinOpKind::Update => self.push_op(OpCode::OpAttrsUpdate, op),
-            BinOpKind::Equal => self.push_op(OpCode::OpEqual, op),
-            BinOpKind::Less => self.push_op(OpCode::OpLess, op),
-            BinOpKind::LessOrEq => self.push_op(OpCode::OpLessOrEq, op),
-            BinOpKind::More => self.push_op(OpCode::OpMore, op),
-            BinOpKind::MoreOrEq => self.push_op(OpCode::OpMoreOrEq, op),
-            BinOpKind::Concat => self.push_op(OpCode::OpConcat, op),
+            BinOpKind::Add => self.push_op(Op::Add, op),
+            BinOpKind::Sub => self.push_op(Op::Sub, op),
+            BinOpKind::Mul => self.push_op(Op::Mul, op),
+            BinOpKind::Div => self.push_op(Op::Div, op),
+            BinOpKind::Update => self.push_op(Op::AttrsUpdate, op),
+            BinOpKind::Equal => self.push_op(Op::Equal, op),
+            BinOpKind::Less => self.push_op(Op::Less, op),
+            BinOpKind::LessOrEq => self.push_op(Op::LessOrEq, op),
+            BinOpKind::More => self.push_op(Op::More, op),
+            BinOpKind::MoreOrEq => self.push_op(Op::MoreOrEq, op),
+            BinOpKind::Concat => self.push_op(Op::Concat, op),
 
             BinOpKind::NotEqual => {
-                self.push_op(OpCode::OpEqual, op);
-                self.push_op(OpCode::OpInvert, op)
+                self.push_op(Op::Equal, op);
+                self.push_op(Op::Invert, op)
             }
 
             // Handled by separate branch above.
@@ -561,20 +586,22 @@ impl Compiler<'_, '_> {
         self.compile(slot, node.lhs().unwrap());
         self.emit_force(&node.lhs().unwrap());
 
-        let throw_idx = self.push_op(OpCode::OpJumpIfCatchable(JumpOffset(0)), node);
+        let throw_idx = self.push_op(Op::JumpIfCatchable, node);
+        self.push_u16(0);
         // If this value is false, jump over the right-hand side - the
         // whole expression is false.
-        let end_idx = self.push_op(OpCode::OpJumpIfFalse(JumpOffset(0)), node);
+        let end_idx = self.push_op(Op::JumpIfFalse, node);
+        self.push_u16(0);
 
         // Otherwise, remove the previous value and leave the
         // right-hand side on the stack. Its result is now the value
         // of the whole expression.
-        self.push_op(OpCode::OpPop, node);
+        self.push_op(Op::Pop, node);
         self.compile(slot, node.rhs().unwrap());
         self.emit_force(&node.rhs().unwrap());
 
         self.patch_jump(end_idx);
-        self.push_op(OpCode::OpAssertBool, node);
+        self.push_op(Op::AssertBool, node);
         self.patch_jump(throw_idx);
     }
 
@@ -589,16 +616,18 @@ impl Compiler<'_, '_> {
         self.compile(slot, node.lhs().unwrap());
         self.emit_force(&node.lhs().unwrap());
 
-        let throw_idx = self.push_op(OpCode::OpJumpIfCatchable(JumpOffset(0)), node);
+        let throw_idx = self.push_op(Op::JumpIfCatchable, node);
+        self.push_u16(0);
         // Opposite of above: If this value is **true**, we can
         // short-circuit the right-hand side.
-        let end_idx = self.push_op(OpCode::OpJumpIfTrue(JumpOffset(0)), node);
-        self.push_op(OpCode::OpPop, node);
+        let end_idx = self.push_op(Op::JumpIfTrue, node);
+        self.push_u16(0);
+        self.push_op(Op::Pop, node);
         self.compile(slot, node.rhs().unwrap());
         self.emit_force(&node.rhs().unwrap());
 
         self.patch_jump(end_idx);
-        self.push_op(OpCode::OpAssertBool, node);
+        self.push_op(Op::AssertBool, node);
         self.patch_jump(throw_idx);
     }
 
@@ -612,17 +641,20 @@ impl Compiler<'_, '_> {
         // Leave left-hand side value on the stack and invert it.
         self.compile(slot, node.lhs().unwrap());
         self.emit_force(&node.lhs().unwrap());
-        let throw_idx = self.push_op(OpCode::OpJumpIfCatchable(JumpOffset(0)), node);
-        self.push_op(OpCode::OpInvert, node);
+        let throw_idx = self.push_op(Op::JumpIfCatchable, node);
+        self.push_u16(0);
+        self.push_op(Op::Invert, node);
 
         // Exactly as `||` (because `a -> b` = `!a || b`).
-        let end_idx = self.push_op(OpCode::OpJumpIfTrue(JumpOffset(0)), node);
-        self.push_op(OpCode::OpPop, node);
+        let end_idx = self.push_op(Op::JumpIfTrue, node);
+        self.push_u16(0);
+
+        self.push_op(Op::Pop, node);
         self.compile(slot, node.rhs().unwrap());
         self.emit_force(&node.rhs().unwrap());
 
         self.patch_jump(end_idx);
-        self.push_op(OpCode::OpAssertBool, node);
+        self.push_op(Op::AssertBool, node);
         self.patch_jump(throw_idx);
     }
 
@@ -657,11 +689,8 @@ impl Compiler<'_, '_> {
             self.scope_mut().mark_initialised(item_slot);
         }
 
-        if count == 0 {
-            self.unthunk();
-        }
-
-        self.push_op(OpCode::OpList(Count(count)), node);
+        self.push_op(Op::List, node);
+        self.push_uvarint(count as u64);
         self.scope_mut().end_scope();
     }
 
@@ -690,7 +719,7 @@ impl Compiler<'_, '_> {
         // next nested element, for all fragments except the last one.
         for (count, fragment) in node.attrpath().unwrap().attrs().enumerate() {
             if count > 0 {
-                self.push_op(OpCode::OpAttrsTrySelect, &fragment);
+                self.push_op(Op::AttrsTrySelect, &fragment);
                 self.emit_force(&fragment);
             }
 
@@ -699,7 +728,7 @@ impl Compiler<'_, '_> {
 
         // After the last fragment, emit the actual instruction that
         // leaves a boolean on the stack.
-        self.push_op(OpCode::OpHasAttr, node);
+        self.push_op(Op::HasAttr, node);
     }
 
     /// When compiling select or select_or expressions, an optimisation is
@@ -723,8 +752,9 @@ impl Compiler<'_, '_> {
         // set that is lacking a key, because that thunk is never
         // evaluated). If anything is missing, just move on. We may
         // want to emit warnings here in the future.
-        if let Some(OpCode::OpConstant(ConstantIdx(idx))) = self.chunk().code.last().cloned() {
-            let constant = &mut self.chunk().constants[idx];
+        if let Some((Op::Constant, op_idx)) = self.chunk().last_op() {
+            let (idx, _) = self.chunk().read_uvarint(op_idx + 1);
+            let constant = &mut self.chunk().constants[idx as usize];
             if let Value::Attrs(attrs) = constant {
                 let mut path_iter = path.attrs();
 
@@ -736,10 +766,6 @@ impl Compiler<'_, '_> {
                     if let Some(ident) = expr_static_attr_str(&attr) {
                         if let Some(selected_value) = attrs.select(ident.as_bytes()) {
                             *constant = selected_value.clone();
-
-                            // If this worked, we can unthunk the current thunk.
-                            self.unthunk();
-
                             return true;
                         }
                     }
@@ -773,7 +799,7 @@ impl Compiler<'_, '_> {
             self.emit_force(&set);
 
             self.compile_attr(slot, &fragment);
-            self.push_op(OpCode::OpAttrsSelect, &fragment);
+            self.push_op(Op::AttrsSelect, &fragment);
         }
     }
 
@@ -823,11 +849,13 @@ impl Compiler<'_, '_> {
         for fragment in path.attrs() {
             self.emit_force(&fragment);
             self.compile_attr(slot, &fragment.clone());
-            self.push_op(OpCode::OpAttrsTrySelect, &fragment);
-            jumps.push(self.push_op(OpCode::OpJumpIfNotFound(JumpOffset(0)), &fragment));
+            self.push_op(Op::AttrsTrySelect, &fragment);
+            jumps.push(self.push_op(Op::JumpIfNotFound, &fragment));
+            self.push_u16(0);
         }
 
-        let final_jump = self.push_op(OpCode::OpJump(JumpOffset(0)), &path);
+        let final_jump = self.push_op(Op::Jump, &path);
+        self.push_u16(0);
 
         for jump in jumps {
             self.patch_jump(jump);
@@ -855,17 +883,22 @@ impl Compiler<'_, '_> {
         // Compile the assertion condition to leave its value on the stack.
         self.compile(slot, node.condition().unwrap());
         self.emit_force(&node.condition().unwrap());
-        let throw_idx = self.push_op(OpCode::OpJumpIfCatchable(JumpOffset(0)), node);
-        let then_idx = self.push_op(OpCode::OpJumpIfFalse(JumpOffset(0)), node);
 
-        self.push_op(OpCode::OpPop, node);
+        let throw_idx = self.push_op(Op::JumpIfCatchable, node);
+        self.push_u16(0);
+
+        let then_idx = self.push_op(Op::JumpIfFalse, node);
+        self.push_u16(0);
+
+        self.push_op(Op::Pop, node);
         self.compile(slot, node.body().unwrap());
 
-        let else_idx = self.push_op(OpCode::OpJump(JumpOffset(0)), node);
+        let else_idx = self.push_op(Op::Jump, node);
+        self.push_u16(0);
 
         self.patch_jump(then_idx);
-        self.push_op(OpCode::OpPop, node);
-        self.push_op(OpCode::OpAssertFail, &node.condition().unwrap());
+        self.push_op(Op::Pop, node);
+        self.push_op(Op::AssertFail, &node.condition().unwrap());
 
         self.patch_jump(else_idx);
         self.patch_jump(throw_idx);
@@ -887,22 +920,20 @@ impl Compiler<'_, '_> {
         self.compile(slot, node.condition().unwrap());
         self.emit_force(&node.condition().unwrap());
 
-        let throw_idx = self.push_op(
-            OpCode::OpJumpIfCatchable(JumpOffset(0)),
-            &node.condition().unwrap(),
-        );
-        let then_idx = self.push_op(
-            OpCode::OpJumpIfFalse(JumpOffset(0)),
-            &node.condition().unwrap(),
-        );
+        let throw_idx = self.push_op(Op::JumpIfCatchable, &node.condition().unwrap());
+        self.push_u16(0);
+
+        let then_idx = self.push_op(Op::JumpIfFalse, &node.condition().unwrap());
+        self.push_u16(0);
 
-        self.push_op(OpCode::OpPop, node); // discard condition value
+        self.push_op(Op::Pop, node); // discard condition value
         self.compile(slot, node.body().unwrap());
 
-        let else_idx = self.push_op(OpCode::OpJump(JumpOffset(0)), node);
+        let else_idx = self.push_op(Op::Jump, node);
+        self.push_u16(0);
 
         self.patch_jump(then_idx); // patch jump *to* else_body
-        self.push_op(OpCode::OpPop, node); // discard condition value
+        self.push_op(Op::Pop, node); // discard condition value
         self.compile(slot, node.else_body().unwrap());
 
         self.patch_jump(else_idx); // patch jump *over* else body
@@ -931,11 +962,12 @@ impl Compiler<'_, '_> {
 
         self.scope_mut().push_with();
 
-        self.push_op(OpCode::OpPushWith(with_idx), &node.namespace().unwrap());
+        self.push_op(Op::PushWith, &node.namespace().unwrap());
+        self.push_uvarint(with_idx.0 as u64);
 
         self.compile(slot, node.body().unwrap());
 
-        self.push_op(OpCode::OpPopWith, node);
+        self.push_op(Op::PopWith, node);
         self.scope_mut().pop_with();
         self.cleanup_scope(node);
     }
@@ -995,13 +1027,15 @@ impl Compiler<'_, '_> {
         // At call time, the attribute set is already at the top of the stack.
         self.scope_mut().mark_initialised(set_idx);
         self.emit_force(pattern);
-        let throw_idx = self.push_op(OpCode::OpJumpIfCatchable(JumpOffset(0)), pattern);
+        let throw_idx = self.push_op(Op::JumpIfCatchable, pattern);
+        self.push_u16(0);
+
         // Evaluation fails on a type error, even if the argument(s) are unused.
-        self.push_op(OpCode::OpAssertAttrs, pattern);
+        self.push_op(Op::AssertAttrs, pattern);
 
         let ellipsis = pattern.ellipsis_token().is_some();
         if !ellipsis {
-            self.push_op(OpCode::OpValidateClosedFormals, pattern);
+            self.push_op(Op::ValidateClosedFormals, pattern);
         }
 
         // Similar to `let ... in ...`, we now do multiple passes over
@@ -1041,7 +1075,8 @@ impl Compiler<'_, '_> {
         // attempt to select from it.
         let stack_idx = self.scope().stack_index(set_idx);
         for tracked_formal in entries.iter() {
-            self.push_op(OpCode::OpGetLocal(stack_idx), pattern);
+            self.push_op(Op::GetLocal, pattern);
+            self.push_uvarint(stack_idx.0 as u64);
             self.emit_literal_ident(&tracked_formal.pattern_entry().ident().unwrap());
 
             let idx = tracked_formal.local_idx();
@@ -1070,14 +1105,14 @@ impl Compiler<'_, '_> {
                     // we only know better after compiling the default expression, so
                     // avoiding unnecessary locals would mean we'd need to modify the chunk
                     // after the fact.
-                    self.push_op(OpCode::OpAttrsTrySelect, &pattern_entry.ident().unwrap());
-                    let jump_to_default =
-                        self.push_op(OpCode::OpJumpIfNotFound(JumpOffset(0)), default_expr);
+                    self.push_op(Op::AttrsTrySelect, &pattern_entry.ident().unwrap());
+                    let jump_to_default = self.push_op(Op::JumpIfNotFound, default_expr);
+                    self.push_u16(0);
 
                     self.emit_constant(Value::FinaliseRequest(false), default_expr);
 
-                    let jump_over_default =
-                        self.push_op(OpCode::OpJump(JumpOffset(0)), default_expr);
+                    let jump_over_default = self.push_op(Op::Jump, default_expr);
+                    self.push_u16(0);
 
                     self.patch_jump(jump_to_default);
 
@@ -1089,7 +1124,7 @@ impl Compiler<'_, '_> {
                     self.patch_jump(jump_over_default);
                 }
                 TrackedFormal::NoDefault { pattern_entry, .. } => {
-                    self.push_op(OpCode::OpAttrsSelect, &pattern_entry.ident().unwrap());
+                    self.push_op(Op::AttrsSelect, &pattern_entry.ident().unwrap());
                 }
             }
 
@@ -1113,23 +1148,16 @@ impl Compiler<'_, '_> {
                         let finalise_request_stack_idx = self.scope().stack_index(*finalise_request_idx);
 
                         // TODO(sterni): better spans
-                        self.push_op(
-                            OpCode::OpGetLocal(finalise_request_stack_idx),
-                            pattern
-                        );
+                        self.push_op(Op::GetLocal, pattern);
+                        self.push_uvarint(finalise_request_stack_idx.0 as u64);
                         let jump_over_finalise =
-                            self.push_op(
-                                OpCode::OpJumpIfNoFinaliseRequest(
-                                    JumpOffset(0)),
-                                pattern
-                            );
-                        self.push_op(
-                            OpCode::OpFinalise(stack_idx),
-                            pattern,
-                        );
+                            self.push_op(Op::JumpIfNoFinaliseRequest, pattern);
+                        self.push_u16(0);
+                        self.push_op(Op::Finalise, pattern);
+                        self.push_uvarint(stack_idx.0 as u64);
                         self.patch_jump(jump_over_finalise);
                         // Get rid of finaliser request value on the stack
-                        self.push_op(OpCode::OpPop, pattern);
+                        self.push_op(Op::Pop, pattern);
                     }
                 }
             }
@@ -1188,12 +1216,6 @@ impl Compiler<'_, '_> {
         })
     }
 
-    /// Mark the current thunk as redundant, i.e. possible to merge directly
-    /// into its parent lambda context without affecting runtime behaviour.
-    fn unthunk(&mut self) {
-        self.context_mut().unthunk = true;
-    }
-
     /// Compile an expression into a runtime closure or thunk
     fn compile_lambda_or_thunk<N, F>(
         &mut self,
@@ -1222,31 +1244,15 @@ impl Compiler<'_, '_> {
             self.patch_jump(throw_idx);
         }
 
-        // TODO: determine and insert enclosing name, if available.
-
         // Pop the lambda context back off, and emit the finished
         // lambda as a constant.
         let mut compiled = self.contexts.pop().unwrap();
 
-        // The compiler might have decided to unthunk, i.e. raise the compiled
-        // code to the parent context. In that case we do so and return right
-        // away.
-        if compiled.unthunk && is_suspended_thunk {
-            self.chunk().extend(compiled.lambda.chunk);
-            return;
-        }
-
         // Emit an instruction to inform the VM that the chunk has ended.
         compiled
             .lambda
             .chunk
-            .push_op(OpCode::OpReturn, self.span_for(node));
-
-        // Capturing the with stack counts as an upvalue, as it is
-        // emitted as an upvalue data instruction.
-        if compiled.captures_with_stack {
-            compiled.lambda.upvalue_count += 1;
-        }
+            .push_op(Op::Return, self.span_for(node));
 
         let lambda = Rc::new(compiled.lambda);
         if is_suspended_thunk {
@@ -1256,7 +1262,7 @@ impl Compiler<'_, '_> {
         }
 
         // If no upvalues are captured, emit directly and move on.
-        if lambda.upvalue_count == 0 {
+        if lambda.upvalue_count == 0 && !compiled.captures_with_stack {
             self.emit_constant(
                 if is_suspended_thunk {
                     Value::Thunk(Thunk::new_suspended(lambda, span))
@@ -1276,12 +1282,13 @@ impl Compiler<'_, '_> {
 
         let code_idx = self.push_op(
             if is_suspended_thunk {
-                OpCode::OpThunkSuspended(blueprint_idx)
+                Op::ThunkSuspended
             } else {
-                OpCode::OpThunkClosure(blueprint_idx)
+                Op::ThunkClosure
             },
             node,
         );
+        self.push_uvarint(blueprint_idx.0 as u64);
 
         self.emit_upvalue_data(
             outer_slot,
@@ -1292,18 +1299,21 @@ impl Compiler<'_, '_> {
 
         if !is_suspended_thunk && !self.scope()[outer_slot].needs_finaliser {
             if !self.scope()[outer_slot].must_thunk {
-                // The closure has upvalues, but is not recursive.  Therefore no thunk is required,
-                // which saves us the overhead of Rc<RefCell<>>
-                self.chunk()[code_idx] = OpCode::OpClosure(blueprint_idx);
+                // The closure has upvalues, but is not recursive. Therefore no
+                // thunk is required, which saves us the overhead of
+                // Rc<RefCell<>>
+                self.chunk().code[code_idx.0] = Op::Closure as u8;
             } else {
-                // This case occurs when a closure has upvalue-references to itself but does not need a
-                // finaliser.  Since no OpFinalise will be emitted later on we synthesize one here.
-                // It is needed here only to set [`Closure::is_finalised`] which is used for sanity checks.
+                // This case occurs when a closure has upvalue-references to
+                // itself but does not need a finaliser. Since no OpFinalise
+                // will be emitted later on we synthesize one here. It is needed
+                // here only to set [`Closure::is_finalised`] which is used for
+                // sanity checks.
                 #[cfg(debug_assertions)]
-                self.push_op(
-                    OpCode::OpFinalise(self.scope().stack_index(outer_slot)),
-                    &self.span_for(node),
-                );
+                {
+                    self.push_op(Op::Finalise, &self.span_for(node));
+                    self.push_uvarint(self.scope().stack_index(outer_slot).0 as u64);
+                }
             }
         }
     }
@@ -1316,7 +1326,7 @@ impl Compiler<'_, '_> {
         self.compile(slot, node.argument().unwrap());
         self.compile(slot, node.lambda().unwrap());
         self.emit_force(&node.lambda().unwrap());
-        self.push_op(OpCode::OpCall, node);
+        self.push_op(Op::Call, node);
     }
 
     /// Emit the data instructions that the runtime needs to correctly
@@ -1324,10 +1334,18 @@ impl Compiler<'_, '_> {
     fn emit_upvalue_data<T: ToSpan>(
         &mut self,
         slot: LocalIdx,
-        node: &T,
+        _: &T, // TODO
         upvalues: Vec<Upvalue>,
         capture_with: bool,
     ) {
+        // Push the count of arguments to be expected, with one bit set to
+        // indicate whether the with stack needs to be captured.
+        let mut count = (upvalues.len() as u64) << 1;
+        if capture_with {
+            count |= 1;
+        }
+        self.push_uvarint(count);
+
         for upvalue in upvalues {
             match upvalue.kind {
                 UpvalueKind::Local(idx) => {
@@ -1337,27 +1355,22 @@ impl Compiler<'_, '_> {
                     // If the target is not yet initialised, we need to defer
                     // the local access
                     if !target.initialised {
-                        self.push_op(OpCode::DataDeferredLocal(stack_idx), &upvalue.span);
+                        self.push_uvarint(Position::deferred_local(stack_idx).0);
                         self.scope_mut().mark_needs_finaliser(slot);
                     } else {
                         // a self-reference
                         if slot == idx {
                             self.scope_mut().mark_must_thunk(slot);
                         }
-                        self.push_op(OpCode::DataStackIdx(stack_idx), &upvalue.span);
+                        self.push_uvarint(Position::stack_index(stack_idx).0);
                     }
                 }
 
                 UpvalueKind::Upvalue(idx) => {
-                    self.push_op(OpCode::DataUpvalueIdx(idx), &upvalue.span);
+                    self.push_uvarint(Position::upvalue_index(idx).0);
                 }
             };
         }
-
-        if capture_with {
-            // TODO(tazjin): probably better to emit span for the ident that caused this
-            self.push_op(OpCode::DataCaptureWith, node);
-        }
     }
 
     /// Emit the literal string value of an identifier. Required for
@@ -1374,20 +1387,7 @@ impl Compiler<'_, '_> {
     /// not known at the time when the jump operation itself is
     /// emitted.
     fn patch_jump(&mut self, idx: CodeIdx) {
-        let offset = JumpOffset(self.chunk().code.len() - 1 - idx.0);
-
-        match &mut self.chunk().code[idx.0] {
-            OpCode::OpJump(n)
-            | OpCode::OpJumpIfFalse(n)
-            | OpCode::OpJumpIfTrue(n)
-            | OpCode::OpJumpIfCatchable(n)
-            | OpCode::OpJumpIfNotFound(n)
-            | OpCode::OpJumpIfNoFinaliseRequest(n) => {
-                *n = offset;
-            }
-
-            op => panic!("attempted to patch unsupported op: {:?}", op),
-        }
+        self.chunk().patch_jump(idx.0);
     }
 
     /// Decrease scope depth of the current function and emit
@@ -1403,7 +1403,8 @@ impl Compiler<'_, '_> {
         }
 
         if popcount > 0 {
-            self.push_op(OpCode::OpCloseScope(Count(popcount)), node);
+            self.push_op(Op::CloseScope, node);
+            self.push_uvarint(popcount as u64);
         }
     }
 
@@ -1461,16 +1462,7 @@ impl Compiler<'_, '_> {
     }
 
     fn emit_force<N: ToSpan>(&mut self, node: &N) {
-        if let Some(&OpCode::OpConstant(c)) = self.chunk().last_op() {
-            if !self.chunk().get_constant(c).unwrap().is_thunk() {
-                // Optimization: Don't emit a force op for non-thunk constants, since they don't
-                // need one!
-                // TODO: this is probably doable for more ops (?)
-                return;
-            }
-        }
-
-        self.push_op(OpCode::OpForce, node);
+        self.push_op(Op::Force, node);
     }
 
     fn emit_warning<N: ToSpan>(&mut self, node: &N, kind: WarningKind) {
@@ -1673,10 +1665,11 @@ pub fn compile(
     c.emit_force(expr);
     if let Some(env) = env {
         if !env.is_empty() {
-            c.push_op(OpCode::OpCloseScope(Count(env.len())), &root_span);
+            c.push_op(Op::CloseScope, &root_span);
+            c.push_uvarint(env.len() as u64);
         }
     }
-    c.push_op(OpCode::OpReturn, &root_span);
+    c.push_op(Op::Return, &root_span);
 
     let lambda = Rc::new(c.contexts.pop().unwrap().lambda);
     c.observer.observe_compiled_toplevel(&lambda);
diff --git a/tvix/eval/src/compiler/scope.rs b/tvix/eval/src/compiler/scope.rs
index 1087e0153e42..bb1784e67b74 100644
--- a/tvix/eval/src/compiler/scope.rs
+++ b/tvix/eval/src/compiler/scope.rs
@@ -105,7 +105,6 @@ pub enum UpvalueKind {
 #[derive(Clone, Debug)]
 pub struct Upvalue {
     pub kind: UpvalueKind,
-    pub span: codemap::Span,
 }
 
 /// The index of a local in the scope's local array at compile time.
diff --git a/tvix/eval/src/observer.rs b/tvix/eval/src/observer.rs
index f5de399315c7..5e6526418b3b 100644
--- a/tvix/eval/src/observer.rs
+++ b/tvix/eval/src/observer.rs
@@ -13,7 +13,7 @@ use tabwriter::TabWriter;
 
 use crate::chunk::Chunk;
 use crate::generators::VMRequest;
-use crate::opcode::{CodeIdx, OpCode};
+use crate::opcode::{CodeIdx, Op};
 use crate::value::Lambda;
 use crate::SourceCode;
 use crate::Value;
@@ -73,7 +73,7 @@ pub trait RuntimeObserver {
 
     /// 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: CodeIdx, _: &OpCode, _: &[Value]) {}
+    fn observe_execute_op(&mut self, _ip: CodeIdx, _: &Op, _: &[Value]) {}
 }
 
 #[derive(Default)]
@@ -112,8 +112,12 @@ impl<W: Write> DisassemblingObserver<W> {
         // 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.source, width, CodeIdx(idx));
+        let mut idx = 0;
+        while idx < chunk.code.len() {
+            let size = chunk
+                .disassemble_op(&mut self.writer, &self.source, width, CodeIdx(idx))
+                .expect("writing debug output should work");
+            idx += size;
         }
     }
 }
@@ -304,7 +308,7 @@ impl<W: Write> RuntimeObserver for TracingObserver<W> {
         );
     }
 
-    fn observe_execute_op(&mut self, ip: CodeIdx, op: &OpCode, stack: &[Value]) {
+    fn observe_execute_op(&mut self, ip: CodeIdx, op: &Op, stack: &[Value]) {
         self.maybe_write_time();
         let _ = write!(&mut self.writer, "{:04} {:?}\t", ip.0, op);
         self.write_stack(stack);
diff --git a/tvix/eval/src/opcode.rs b/tvix/eval/src/opcode.rs
index f89c1c12e7fd..ddf1304b3aea 100644
--- a/tvix/eval/src/opcode.rs
+++ b/tvix/eval/src/opcode.rs
@@ -52,8 +52,7 @@ pub struct JumpOffset(pub usize);
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct Count(pub usize);
 
-/// All variants of this enum carry a bounded amount of data to
-/// ensure that no heap allocations are needed for an Opcode.
+/// Op represents all instructions in the Tvix abstract machine.
 ///
 /// In documentation comments, stack positions are referred to by
 /// indices written in `{}` as such, where required:
@@ -70,187 +69,182 @@ pub struct Count(pub usize);
 ///
 /// Unless otherwise specified, operations leave their result at the
 /// top of the stack.
-#[warn(variant_size_differences)]
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
-pub enum OpCode {
+#[repr(u8)]
+#[derive(Debug, PartialEq, Eq)]
+pub enum Op {
     /// Push a constant onto the stack.
-    OpConstant(ConstantIdx),
+    Constant,
 
-    // Unary operators
-    /// Discard a value from the stack.
-    OpPop,
+    /// Discard the value on top of the stack.
+    Pop,
 
     /// Invert the boolean at the top of the stack.
-    OpInvert,
+    Invert,
 
-    // Binary operators
     /// Invert the sign of the number at the top of the stack.
-    OpNegate,
+    Negate,
 
     /// Sum up the two numbers at the top of the stack.
-    OpAdd,
+    Add,
 
     /// Subtract the number at {1} from the number at {2}.
-    OpSub,
+    Sub,
 
     /// Multiply the two numbers at the top of the stack.
-    OpMul,
+    Mul,
 
     /// Divide the two numbers at the top of the stack.
-    OpDiv,
+    Div,
 
-    // Comparison operators
     /// Check the two values at the top of the stack for Nix-equality.
-    OpEqual,
+    Equal,
 
     /// Check whether the value at {2} is less than {1}.
-    OpLess,
+    Less,
 
     /// Check whether the value at {2} is less than or equal to {1}.
-    OpLessOrEq,
+    LessOrEq,
 
     /// Check whether the value at {2} is greater than {1}.
-    OpMore,
+    More,
 
     /// Check whether the value at {2} is greater than or equal to {1}.
-    OpMoreOrEq,
+    MoreOrEq,
 
-    // Logical operators & generic jumps
     /// Jump forward in the bytecode specified by the number of
     /// instructions in its usize operand.
-    OpJump(JumpOffset),
+    Jump,
 
     /// Jump forward in the bytecode specified by the number of
     /// instructions in its usize operand, *if* the value at the top
     /// of the stack is `true`.
-    OpJumpIfTrue(JumpOffset),
+    JumpIfTrue,
 
     /// Jump forward in the bytecode specified by the number of
     /// instructions in its usize operand, *if* the value at the top
     /// of the stack is `false`.
-    OpJumpIfFalse(JumpOffset),
+    JumpIfFalse,
 
     /// Pop one stack item and jump forward in the bytecode
     /// specified by the number of instructions in its usize
     /// operand, *if* the value at the top of the stack is a
     /// Value::Catchable.
-    OpJumpIfCatchable(JumpOffset),
+    JumpIfCatchable,
 
     /// Jump forward in the bytecode specified by the number of
     /// instructions in its usize operand, *if* the value at the top
     /// of the stack is the internal value representing a missing
     /// attribute set key.
-    OpJumpIfNotFound(JumpOffset),
+    JumpIfNotFound,
 
     /// Jump forward in the bytecode specified by the number of
     /// instructions in its usize operand, *if* the value at the top
     /// of the stack is *not* the internal value requesting a
     /// stack value finalisation.
-    OpJumpIfNoFinaliseRequest(JumpOffset),
+    JumpIfNoFinaliseRequest,
+
+    /// Construct an attribute set from the given number of key-value pairs on
+    /// the top of the stack. The operand gives the count of *pairs*, not the
+    /// number of *stack values* - the actual number of values popped off the
+    /// stack will be twice the argument to this op.
+    Attrs,
 
-    // Attribute sets
-    /// Construct an attribute set from the given number of key-value pairs on the top of the stack
-    ///
-    /// Note that this takes the count of *pairs*, not the number of *stack values* - the actual
-    /// number of values popped off the stack will be twice the argument to this op
-    OpAttrs(Count),
     /// Merge the attribute set at {2} into the attribute set at {1},
     /// and leave the new set at the top of the stack.
-    OpAttrsUpdate,
+    AttrsUpdate,
 
     /// Select the attribute with the name at {1} from the set at {2}.
-    OpAttrsSelect,
+    AttrsSelect,
 
     /// Select the attribute with the name at {1} from the set at {2}, but leave
     /// a `Value::AttrNotFound` in the stack instead of failing if it is
     /// missing.
-    OpAttrsTrySelect,
+    AttrsTrySelect,
 
     /// Check for the presence of the attribute with the name at {1} in the set
     /// at {2}.
-    OpHasAttr,
+    HasAttr,
 
     /// Throw an error if the attribute set at the top of the stack has any attributes
     /// other than those listed in the formals of the current lambda
     ///
     /// Panics if the current frame is not a lambda with formals
-    OpValidateClosedFormals,
+    ValidateClosedFormals,
 
-    // `with`-handling
     /// Push a value onto the runtime `with`-stack to enable dynamic identifier
     /// resolution. The absolute stack index of the value is supplied as a usize
     /// operand.
-    OpPushWith(StackIdx),
+    PushWith,
 
     /// Pop the last runtime `with`-stack element.
-    OpPopWith,
+    PopWith,
 
     /// Dynamically resolve an identifier with the name at {1} from the runtime
     /// `with`-stack.
-    OpResolveWith,
+    ResolveWith,
 
     // Lists
     /// Construct a list from the given number of values at the top of the
     /// stack.
-    OpList(Count),
+    List,
 
     /// Concatenate the lists at {2} and {1}.
-    OpConcat,
+    Concat,
 
     // Strings
     /// Interpolate the given number of string fragments into a single string.
-    OpInterpolate(Count),
+    Interpolate,
 
     /// Force the Value on the stack and coerce it to a string
-    OpCoerceToString(crate::CoercionKind),
+    CoerceToString,
 
     // Paths
     /// Attempt to resolve the Value on the stack using the configured [`NixSearchPath`][]
     ///
     /// [`NixSearchPath`]: crate::nix_search_path::NixSearchPath
-    OpFindFile,
+    FindFile,
 
     /// Attempt to resolve a path literal relative to the home dir
-    OpResolveHomePath,
+    ResolveHomePath,
 
     // Type assertion operators
     /// Assert that the value at {1} is a boolean, and fail with a runtime error
     /// otherwise.
-    OpAssertBool,
-    OpAssertAttrs,
+    AssertBool,
+    AssertAttrs,
 
     /// Access local identifiers with statically known positions.
-    OpGetLocal(StackIdx),
+    GetLocal,
 
     /// Close scopes while leaving their expression value around.
-    OpCloseScope(Count), // number of locals to pop
+    CloseScope,
 
     /// Return an error indicating that an `assert` failed
-    OpAssertFail,
+    AssertFail,
 
     // Lambdas & closures
     /// Call the value at {1} in a new VM callframe
-    OpCall,
+    Call,
 
     /// Retrieve the upvalue at the given index from the closure or thunk
     /// currently under evaluation.
-    OpGetUpvalue(UpvalueIdx),
+    GetUpvalue,
 
     /// Construct a closure which has upvalues but no self-references
-    OpClosure(ConstantIdx),
+    Closure,
 
     /// Construct a closure which has self-references (direct or via upvalues)
-    OpThunkClosure(ConstantIdx),
+    ThunkClosure,
 
     /// Construct a suspended thunk, used to delay a computation for laziness.
-    OpThunkSuspended(ConstantIdx),
+    ThunkSuspended,
 
     /// Force the value at {1} until it is a `Thunk::Evaluated`.
-    OpForce,
+    Force,
 
     /// Finalise initialisation of the upvalues of the value in the given stack
     /// index (which must be a Value::Thunk) after the scope is fully bound.
-    OpFinalise(StackIdx),
+    Finalise,
 
     /// Final instruction emitted in a chunk. Does not have an
     /// inherent effect, but can simplify VM logic as a marker in some
@@ -258,27 +252,140 @@ pub enum OpCode {
     ///
     /// Can be thought of as "returning" the value to the parent
     /// frame, hence the name.
-    OpReturn,
-
-    // [`OpClosure`], [`OpThunkSuspended`], and [`OpThunkClosure`] have a
-    // variable number of arguments to the instruction, which is
-    // represented here by making their data part of the opcodes.
-    // Each of these two opcodes has a `ConstantIdx`, which must
-    // reference a `Value::Blueprint(Lambda)`.  The `upvalue_count`
-    // field in that `Lambda` indicates the number of arguments it
-    // takes, and the opcode must be followed by exactly this number
-    // of `Data*` opcodes.  The VM skips over these by advancing the
-    // instruction pointer.
-    //
-    // It is illegal for a `Data*` opcode to appear anywhere else.
-    /// Populate a static upvalue by copying from the stack immediately.
-    DataStackIdx(StackIdx),
-    /// Populate a static upvalue of a thunk by copying it the stack, but do
-    /// when the thunk is finalised (by OpFinalise) rather than immediately.
-    DataDeferredLocal(StackIdx),
-    /// Populate a static upvalue by copying it from the upvalues of an
-    /// enclosing scope.
-    DataUpvalueIdx(UpvalueIdx),
-    /// Populate dynamic upvalues by saving a copy of the with-stack.
-    DataCaptureWith,
+    Return,
+
+    /// Sentinel value to signal invalid bytecode. This MUST always be the last
+    /// value in the enum. Do not move it!
+    Invalid,
+}
+
+const _ASSERT_SMALL_OP: () = assert!(std::mem::size_of::<Op>() == 1);
+
+impl From<u8> for Op {
+    fn from(num: u8) -> Self {
+        if num >= Self::Invalid as u8 {
+            return Self::Invalid;
+        }
+
+        // SAFETY: As long as `Invalid` remains the last variant of the enum,
+        // and as long as variant values are not specified manually, this
+        // conversion is safe.
+        unsafe { std::mem::transmute(num) }
+    }
+}
+
+pub enum OpArg {
+    None,
+    Uvarint,
+    Fixed,
+    Custom,
+}
+
+impl Op {
+    pub fn arg_type(&self) -> OpArg {
+        match self {
+            Op::Constant
+            | Op::Attrs
+            | Op::PushWith
+            | Op::List
+            | Op::Interpolate
+            | Op::GetLocal
+            | Op::CloseScope
+            | Op::GetUpvalue
+            | Op::Finalise => OpArg::Uvarint,
+
+            Op::Jump
+            | Op::JumpIfTrue
+            | Op::JumpIfFalse
+            | Op::JumpIfCatchable
+            | Op::JumpIfNotFound
+            | Op::JumpIfNoFinaliseRequest => OpArg::Fixed,
+
+            Op::CoerceToString | Op::Closure | Op::ThunkClosure | Op::ThunkSuspended => {
+                OpArg::Custom
+            }
+            _ => OpArg::None,
+        }
+    }
+}
+
+/// Position is used to represent where to capture an upvalue from.
+#[derive(Clone, Copy)]
+pub struct Position(pub u64);
+
+impl Position {
+    pub fn stack_index(idx: StackIdx) -> Self {
+        Position((idx.0 as u64) << 2)
+    }
+
+    pub fn deferred_local(idx: StackIdx) -> Self {
+        Position(((idx.0 as u64) << 2) | 1)
+    }
+
+    pub fn upvalue_index(idx: UpvalueIdx) -> Self {
+        Position(((idx.0 as u64) << 2) | 2)
+    }
+
+    pub fn runtime_stack_index(&self) -> Option<StackIdx> {
+        if (self.0 & 0b11) == 0 {
+            return Some(StackIdx((self.0 >> 2) as usize));
+        }
+
+        None
+    }
+
+    pub fn runtime_deferred_local(&self) -> Option<StackIdx> {
+        if (self.0 & 0b11) == 1 {
+            return Some(StackIdx((self.0 >> 2) as usize));
+        }
+
+        None
+    }
+
+    pub fn runtime_upvalue_index(&self) -> Option<UpvalueIdx> {
+        if (self.0 & 0b11) == 2 {
+            return Some(UpvalueIdx((self.0 >> 2) as usize));
+        }
+
+        None
+    }
+}
+
+#[cfg(test)]
+mod position_tests {
+    use super::Position; // he-he
+    use super::{StackIdx, UpvalueIdx};
+
+    #[test]
+    fn test_stack_index_position() {
+        let idx = StackIdx(42);
+        let pos = Position::stack_index(idx);
+        let result = pos.runtime_stack_index();
+
+        assert_eq!(result, Some(idx));
+        assert_eq!(pos.runtime_deferred_local(), None);
+        assert_eq!(pos.runtime_upvalue_index(), None);
+    }
+
+    #[test]
+    fn test_deferred_local_position() {
+        let idx = StackIdx(42);
+        let pos = Position::deferred_local(idx);
+        let result = pos.runtime_deferred_local();
+
+        assert_eq!(result, Some(idx));
+        assert_eq!(pos.runtime_stack_index(), None);
+        assert_eq!(pos.runtime_upvalue_index(), None);
+    }
+
+    #[test]
+    fn test_upvalue_index_position() {
+        let idx = UpvalueIdx(42);
+        let pos = Position::upvalue_index(idx);
+        let result = pos.runtime_upvalue_index();
+
+        assert_eq!(result, Some(idx));
+        assert_eq!(pos.runtime_stack_index(), None);
+        assert_eq!(pos.runtime_deferred_local(), None);
+    }
 }
diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index 27c198ca9483..6109a0da2385 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -186,6 +186,21 @@ pub struct CoercionKind {
     pub import_paths: bool,
 }
 
+impl From<CoercionKind> for u8 {
+    fn from(k: CoercionKind) -> u8 {
+        k.strong as u8 | (k.import_paths as u8) << 1
+    }
+}
+
+impl From<u8> for CoercionKind {
+    fn from(byte: u8) -> Self {
+        CoercionKind {
+            strong: byte & 0x01 != 0,
+            import_paths: byte & 0x02 != 0,
+        }
+    }
+}
+
 impl<T> From<T> for Value
 where
     T: Into<NixString>,
diff --git a/tvix/eval/src/value/thunk.rs b/tvix/eval/src/value/thunk.rs
index 2019e8ab3239..4b915019d47c 100644
--- a/tvix/eval/src/value/thunk.rs
+++ b/tvix/eval/src/value/thunk.rs
@@ -27,7 +27,7 @@ use std::{
 
 use crate::{
     errors::ErrorKind,
-    opcode::OpCode,
+    opcode::Op,
     upvalues::Upvalues,
     value::Closure,
     vm::generators::{self, GenCo},
@@ -170,13 +170,15 @@ impl Thunk {
         // This is basically a recreation of compile_apply():
         // We need to push the argument onto the stack and then the function.
         // The function (not the argument) needs to be forced before calling.
-        lambda.chunk.push_op(OpCode::OpConstant(arg_idx), span);
-        lambda.chunk().push_op(OpCode::OpConstant(f_idx), span);
-        lambda.chunk.push_op(OpCode::OpForce, span);
-        lambda.chunk.push_op(OpCode::OpCall, span);
+        lambda.chunk.push_op(Op::Constant, span);
+        lambda.chunk.push_uvarint(arg_idx.0 as u64);
+        lambda.chunk.push_op(Op::Constant, span);
+        lambda.chunk.push_uvarint(f_idx.0 as u64);
+        lambda.chunk.push_op(Op::Force, span);
+        lambda.chunk.push_op(Op::Call, span);
 
         // Inform the VM that the chunk has ended
-        lambda.chunk.push_op(OpCode::OpReturn, span);
+        lambda.chunk.push_op(Op::Return, span);
 
         Thunk(Rc::new(RefCell::new(ThunkRepr::Suspended {
             upvalues: Rc::new(Upvalues::with_capacity(0)),
diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs
index a6d0941e8d7a..7ac6d493fa1f 100644
--- a/tvix/eval/src/vm/mod.rs
+++ b/tvix/eval/src/vm/mod.rs
@@ -28,7 +28,7 @@ use crate::{
     lifted_pop,
     nix_search_path::NixSearchPath,
     observer::RuntimeObserver,
-    opcode::{CodeIdx, Count, JumpOffset, OpCode, StackIdx, UpvalueIdx},
+    opcode::{CodeIdx, Op, Position, UpvalueIdx},
     upvalues::Upvalues,
     value::{
         Builtin, BuiltinResult, Closure, CoercionKind, Lambda, NixAttrs, NixContext, NixList,
@@ -146,10 +146,32 @@ impl CallFrame {
 
     /// Increment this frame's instruction pointer and return the operation that
     /// the pointer moved past.
-    fn inc_ip(&mut self) -> OpCode {
-        let op = self.chunk()[self.ip];
+    fn inc_ip(&mut self) -> Op {
+        debug_assert!(
+            self.ip.0 < self.chunk().code.len(),
+            "out of bounds code at IP {} in {:p}",
+            self.ip.0,
+            self.lambda
+        );
+
+        let op = self.chunk().code[self.ip.0];
         self.ip += 1;
-        op
+        op.into()
+    }
+
+    /// Read a varint-encoded operand and return it. The frame pointer is
+    /// incremented internally.
+    fn read_uvarint(&mut self) -> u64 {
+        let (arg, size) = self.chunk().read_uvarint(self.ip.0);
+        self.ip += size;
+        arg
+    }
+
+    /// Read a fixed-size u16 and increment the frame pointer.
+    fn read_u16(&mut self) -> u16 {
+        let arg = self.chunk().read_u16(self.ip.0);
+        self.ip += 2;
+        arg
     }
 
     /// Construct an error result from the given ErrorKind and the source span
@@ -443,17 +465,25 @@ where
             self.observer.observe_execute_op(frame.ip, &op, &self.stack);
 
             match op {
-                OpCode::OpThunkSuspended(idx) | OpCode::OpThunkClosure(idx) => {
-                    let blueprint = match &frame.chunk()[idx] {
+                Op::ThunkSuspended | Op::ThunkClosure => {
+                    let idx = frame.read_uvarint() as usize;
+
+                    let blueprint = match &frame.chunk().constants[idx] {
                         Value::Blueprint(lambda) => lambda.clone(),
                         _ => panic!("compiler bug: non-blueprint in blueprint slot"),
                     };
 
-                    let upvalue_count = blueprint.upvalue_count;
-                    let thunk = if matches!(op, OpCode::OpThunkClosure(_)) {
+                    let upvalue_count = frame.read_uvarint();
+
+                    debug_assert!(
+                        (upvalue_count >> 1) == blueprint.upvalue_count as u64,
+                        "TODO: new upvalue count not correct",
+                    );
+
+                    let thunk = if op == Op::ThunkClosure {
                         debug_assert!(
-                            upvalue_count > 0,
-                            "OpThunkClosure should not be called for plain lambdas"
+                            (((upvalue_count >> 1) > 0) || (upvalue_count & 0b1 == 1)),
+                            "OpThunkClosure should not be called for plain lambdas",
                         );
                         Thunk::new_closure(blueprint)
                     } else {
@@ -470,7 +500,7 @@ where
                     self.populate_upvalues(&mut frame, upvalue_count, upvalues)?;
                 }
 
-                OpCode::OpForce => {
+                Op::Force => {
                     if let Some(Value::Thunk(_)) = self.stack.last() {
                         let thunk = match self.stack_pop() {
                             Value::Thunk(t) => t,
@@ -488,25 +518,35 @@ where
                     }
                 }
 
-                OpCode::OpGetUpvalue(upv_idx) => {
-                    let value = frame.upvalue(upv_idx).clone();
+                Op::GetUpvalue => {
+                    let idx = UpvalueIdx(frame.read_uvarint() as usize);
+                    let value = frame.upvalue(idx).clone();
                     self.stack.push(value);
                 }
 
                 // Discard the current frame.
-                OpCode::OpReturn => {
+                Op::Return => {
                     // TODO(amjoseph): I think this should assert `==` rather
                     // than `<=` but it fails with the stricter condition.
                     debug_assert!(self.stack.len() - 1 <= frame.stack_offset);
                     return Ok(true);
                 }
 
-                OpCode::OpConstant(idx) => {
-                    let c = frame.chunk()[idx].clone();
+                Op::Constant => {
+                    let idx = frame.read_uvarint() as usize;
+
+                    debug_assert!(
+                        idx < frame.chunk().constants.len(),
+                        "out of bounds constant at IP {} in {:p}",
+                        frame.ip.0,
+                        frame.lambda
+                    );
+
+                    let c = frame.chunk().constants[idx].clone();
                     self.stack.push(c);
                 }
 
-                OpCode::OpCall => {
+                Op::Call => {
                     let callable = self.stack_pop();
                     self.call_value(frame.current_span(), Some((span, frame)), callable)?;
 
@@ -516,7 +556,8 @@ where
 
                 // Remove the given number of elements from the stack,
                 // but retain the top value.
-                OpCode::OpCloseScope(Count(count)) => {
+                Op::CloseScope => {
+                    let count = frame.read_uvarint() as usize;
                     // Immediately move the top value into the right
                     // position.
                     let target_idx = self.stack.len() - 1 - count;
@@ -528,15 +569,22 @@ where
                     }
                 }
 
-                OpCode::OpClosure(idx) => {
-                    let blueprint = match &frame.chunk()[idx] {
+                Op::Closure => {
+                    let idx = frame.read_uvarint() as usize;
+                    let blueprint = match &frame.chunk().constants[idx] {
                         Value::Blueprint(lambda) => lambda.clone(),
                         _ => panic!("compiler bug: non-blueprint in blueprint slot"),
                     };
 
-                    let upvalue_count = blueprint.upvalue_count;
+                    let upvalue_count = frame.read_uvarint();
+
+                    debug_assert!(
+                        (upvalue_count >> 1) == blueprint.upvalue_count as u64,
+                        "TODO: new upvalue count not correct in closure",
+                    );
+
                     debug_assert!(
-                        upvalue_count > 0,
+                        ((upvalue_count >> 1) > 0 || (upvalue_count & 0b1 == 1)),
                         "OpClosure should not be called for plain lambdas"
                     );
 
@@ -549,7 +597,7 @@ where
                         ))));
                 }
 
-                OpCode::OpAttrsSelect => lifted_pop! {
+                Op::AttrsSelect => lifted_pop! {
                     self(key, attrs) => {
                         let key = key.to_str().with_span(&frame, self)?;
                         let attrs = attrs.to_attrs().with_span(&frame, self)?;
@@ -569,21 +617,24 @@ where
                     }
                 },
 
-                OpCode::OpJumpIfFalse(JumpOffset(offset)) => {
+                Op::JumpIfFalse => {
+                    let offset = frame.read_u16() as usize;
                     debug_assert!(offset != 0);
                     if !self.stack_peek(0).as_bool().with_span(&frame, self)? {
                         frame.ip += offset;
                     }
                 }
 
-                OpCode::OpJumpIfCatchable(JumpOffset(offset)) => {
+                Op::JumpIfCatchable => {
+                    let offset = frame.read_u16() as usize;
                     debug_assert!(offset != 0);
                     if self.stack_peek(0).is_catchable() {
                         frame.ip += offset;
                     }
                 }
 
-                OpCode::OpJumpIfNoFinaliseRequest(JumpOffset(offset)) => {
+                Op::JumpIfNoFinaliseRequest => {
+                    let offset = frame.read_u16() as usize;
                     debug_assert!(offset != 0);
                     match self.stack_peek(0) {
                         Value::FinaliseRequest(finalise) => {
@@ -595,11 +646,11 @@ where
                     }
                 }
 
-                OpCode::OpPop => {
+                Op::Pop => {
                     self.stack.pop();
                 }
 
-                OpCode::OpAttrsTrySelect => {
+                Op::AttrsTrySelect => {
                     let key = self.stack_pop().to_str().with_span(&frame, self)?;
                     let value = match self.stack_pop() {
                         Value::Attrs(attrs) => match attrs.select(&key) {
@@ -613,12 +664,14 @@ where
                     self.stack.push(value);
                 }
 
-                OpCode::OpGetLocal(StackIdx(local_idx)) => {
+                Op::GetLocal => {
+                    let local_idx = frame.read_uvarint() as usize;
                     let idx = frame.stack_offset + local_idx;
                     self.stack.push(self.stack[idx].clone());
                 }
 
-                OpCode::OpJumpIfNotFound(JumpOffset(offset)) => {
+                Op::JumpIfNotFound => {
+                    let offset = frame.read_u16() as usize;
                     debug_assert!(offset != 0);
                     if matches!(self.stack_peek(0), Value::AttrNotFound) {
                         self.stack_pop();
@@ -626,12 +679,13 @@ where
                     }
                 }
 
-                OpCode::OpJump(JumpOffset(offset)) => {
+                Op::Jump => {
+                    let offset = frame.read_u16() as usize;
                     debug_assert!(offset != 0);
                     frame.ip += offset;
                 }
 
-                OpCode::OpEqual => lifted_pop! {
+                Op::Equal => lifted_pop! {
                     self(b, a) => {
                         let gen_span = frame.current_span();
                         self.push_call_frame(span, frame);
@@ -646,7 +700,7 @@ where
                 // top is not of the expected type. This is necessary
                 // to implement some specific behaviours of Nix
                 // exactly.
-                OpCode::OpAssertBool => {
+                Op::AssertBool => {
                     let val = self.stack_peek(0);
                     // TODO(edef): propagate this into is_bool, since bottom values *are* values of any type
                     if !val.is_catchable() && !val.is_bool() {
@@ -660,7 +714,7 @@ where
                     }
                 }
 
-                OpCode::OpAssertAttrs => {
+                Op::AssertAttrs => {
                     let val = self.stack_peek(0);
                     // TODO(edef): propagate this into is_attrs, since bottom values *are* values of any type
                     if !val.is_catchable() && !val.is_attrs() {
@@ -674,9 +728,9 @@ where
                     }
                 }
 
-                OpCode::OpAttrs(Count(count)) => self.run_attrset(&frame, count)?,
+                Op::Attrs => self.run_attrset(frame.read_uvarint() as usize, &frame)?,
 
-                OpCode::OpAttrsUpdate => lifted_pop! {
+                Op::AttrsUpdate => lifted_pop! {
                     self(rhs, lhs) => {
                         let rhs = rhs.to_attrs().with_span(&frame, self)?;
                         let lhs = lhs.to_attrs().with_span(&frame, self)?;
@@ -684,28 +738,30 @@ where
                     }
                 },
 
-                OpCode::OpInvert => lifted_pop! {
+                Op::Invert => lifted_pop! {
                     self(v) => {
                         let v = v.as_bool().with_span(&frame, self)?;
                         self.stack.push(Value::Bool(!v));
                     }
                 },
 
-                OpCode::OpList(Count(count)) => {
+                Op::List => {
+                    let count = frame.read_uvarint() as usize;
                     let list =
                         NixList::construct(count, self.stack.split_off(self.stack.len() - count));
 
                     self.stack.push(Value::List(list));
                 }
 
-                OpCode::OpJumpIfTrue(JumpOffset(offset)) => {
+                Op::JumpIfTrue => {
+                    let offset = frame.read_u16() as usize;
                     debug_assert!(offset != 0);
                     if self.stack_peek(0).as_bool().with_span(&frame, self)? {
                         frame.ip += offset;
                     }
                 }
 
-                OpCode::OpHasAttr => lifted_pop! {
+                Op::HasAttr => lifted_pop! {
                     self(key, attrs) => {
                         let key = key.to_str().with_span(&frame, self)?;
                         let result = match attrs {
@@ -720,7 +776,7 @@ where
                     }
                 },
 
-                OpCode::OpConcat => lifted_pop! {
+                Op::Concat => lifted_pop! {
                     self(rhs, lhs) => {
                         let rhs = rhs.to_list().with_span(&frame, self)?.into_inner();
                         let lhs = lhs.to_list().with_span(&frame, self)?.into_inner();
@@ -728,7 +784,7 @@ where
                     }
                 },
 
-                OpCode::OpResolveWith => {
+                Op::ResolveWith => {
                     let ident = self.stack_pop().to_str().with_span(&frame, self)?;
 
                     // Re-enqueue this frame.
@@ -755,13 +811,19 @@ where
                     return Ok(false);
                 }
 
-                OpCode::OpFinalise(StackIdx(idx)) => match &self.stack[frame.stack_offset + idx] {
-                    Value::Closure(_) => panic!("attempted to finalise a closure"),
-                    Value::Thunk(thunk) => thunk.finalise(&self.stack[frame.stack_offset..]),
-                    _ => panic!("attempted to finalise a non-thunk"),
-                },
+                Op::Finalise => {
+                    let idx = frame.read_uvarint() as usize;
+                    match &self.stack[frame.stack_offset + idx] {
+                        Value::Closure(_) => panic!("attempted to finalise a closure"),
+                        Value::Thunk(thunk) => thunk.finalise(&self.stack[frame.stack_offset..]),
+                        _ => panic!("attempted to finalise a non-thunk"),
+                    }
+                }
+
+                Op::CoerceToString => {
+                    let kind: CoercionKind = frame.chunk().code[frame.ip.0].into();
+                    frame.ip.0 += 1;
 
-                OpCode::OpCoerceToString(kind) => {
                     let value = self.stack_pop();
                     let gen_span = frame.current_span();
                     self.push_call_frame(span, frame);
@@ -773,9 +835,9 @@ where
                     return Ok(false);
                 }
 
-                OpCode::OpInterpolate(Count(count)) => self.run_interpolate(&frame, count)?,
+                Op::Interpolate => self.run_interpolate(frame.read_uvarint(), &frame)?,
 
-                OpCode::OpValidateClosedFormals => {
+                Op::ValidateClosedFormals => {
                     let formals = frame.lambda.formals.as_ref().expect(
                         "OpValidateClosedFormals called within the frame of a lambda without formals",
                     );
@@ -799,7 +861,7 @@ where
                     }
                 }
 
-                OpCode::OpAdd => lifted_pop! {
+                Op::Add => lifted_pop! {
                     self(b, a) => {
                         let gen_span = frame.current_span();
                         self.push_call_frame(span, frame);
@@ -812,21 +874,21 @@ where
                     }
                 },
 
-                OpCode::OpSub => lifted_pop! {
+                Op::Sub => lifted_pop! {
                     self(b, a) => {
                         let result = arithmetic_op!(&a, &b, -).with_span(&frame, self)?;
                         self.stack.push(result);
                     }
                 },
 
-                OpCode::OpMul => lifted_pop! {
+                Op::Mul => lifted_pop! {
                     self(b, a) => {
                         let result = arithmetic_op!(&a, &b, *).with_span(&frame, self)?;
                         self.stack.push(result);
                     }
                 },
 
-                OpCode::OpDiv => lifted_pop! {
+                Op::Div => lifted_pop! {
                     self(b, a) => {
                         match b {
                             Value::Integer(0) => return frame.error(self, ErrorKind::DivisionByZero),
@@ -841,7 +903,7 @@ where
                     }
                 },
 
-                OpCode::OpNegate => match self.stack_pop() {
+                Op::Negate => match self.stack_pop() {
                     Value::Integer(i) => self.stack.push(Value::Integer(-i)),
                     Value::Float(f) => self.stack.push(Value::Float(-f)),
                     Value::Catchable(cex) => self.stack.push(Value::Catchable(cex)),
@@ -856,12 +918,12 @@ where
                     }
                 },
 
-                OpCode::OpLess => cmp_op!(self, frame, span, <),
-                OpCode::OpLessOrEq => cmp_op!(self, frame, span, <=),
-                OpCode::OpMore => cmp_op!(self, frame, span, >),
-                OpCode::OpMoreOrEq => cmp_op!(self, frame, span, >=),
+                Op::Less => cmp_op!(self, frame, span, <),
+                Op::LessOrEq => cmp_op!(self, frame, span, <=),
+                Op::More => cmp_op!(self, frame, span, >),
+                Op::MoreOrEq => cmp_op!(self, frame, span, >=),
 
-                OpCode::OpFindFile => match self.stack_pop() {
+                Op::FindFile => match self.stack_pop() {
                     Value::UnresolvedPath(path) => {
                         let resolved = self
                             .nix_search_path
@@ -873,7 +935,7 @@ where
                     _ => panic!("tvix compiler bug: OpFindFile called on non-UnresolvedPath"),
                 },
 
-                OpCode::OpResolveHomePath => match self.stack_pop() {
+                Op::ResolveHomePath => match self.stack_pop() {
                     Value::UnresolvedPath(path) => {
                         match dirs::home_dir() {
                             None => {
@@ -896,24 +958,23 @@ where
                     }
                 },
 
-                OpCode::OpPushWith(StackIdx(idx)) => self.with_stack.push(frame.stack_offset + idx),
+                Op::PushWith => self
+                    .with_stack
+                    .push(frame.stack_offset + frame.read_uvarint() as usize),
 
-                OpCode::OpPopWith => {
+                Op::PopWith => {
                     self.with_stack.pop();
                 }
 
-                OpCode::OpAssertFail => {
+                Op::AssertFail => {
                     self.stack
                         .push(Value::from(CatchableErrorKind::AssertionFailed));
                 }
 
-                // Data-carrying operands should never be executed,
-                // that is a critical error in the VM/compiler.
-                OpCode::DataStackIdx(_)
-                | OpCode::DataDeferredLocal(_)
-                | OpCode::DataUpvalueIdx(_)
-                | OpCode::DataCaptureWith => {
-                    panic!("Tvix bug: attempted to execute data-carrying operand")
+                // Encountering an invalid opcode is a critical error in the
+                // VM/compiler.
+                Op::Invalid => {
+                    panic!("Tvix bug: attempted to execute invalid opcode")
                 }
             }
         }
@@ -933,7 +994,7 @@ where
         &self.stack[self.stack.len() - 1 - offset]
     }
 
-    fn run_attrset(&mut self, frame: &CallFrame, count: usize) -> EvalResult<()> {
+    fn run_attrset(&mut self, count: usize, frame: &CallFrame) -> EvalResult<()> {
         let attrs = NixAttrs::construct(count, self.stack.split_off(self.stack.len() - count * 2))
             .with_span(frame, self)?
             .map(Value::attrs)
@@ -971,7 +1032,7 @@ where
     /// Interpolate string fragments by popping the specified number of
     /// fragments of the stack, evaluating them to strings, and pushing
     /// the concatenated result string back on the stack.
-    fn run_interpolate(&mut self, frame: &CallFrame, count: usize) -> EvalResult<()> {
+    fn run_interpolate(&mut self, count: u64, frame: &CallFrame) -> EvalResult<()> {
         let mut out = BString::default();
         // Interpolation propagates the context and union them.
         let mut context: NixContext = NixContext::new();
@@ -1090,64 +1151,74 @@ where
     }
 
     /// Populate the upvalue fields of a thunk or closure under construction.
+    ///
+    /// See the closely tied function `emit_upvalue_data` in the compiler
+    /// implementation for details on the argument processing.
     fn populate_upvalues(
         &mut self,
         frame: &mut CallFrame,
-        count: usize,
+        count: u64,
         mut upvalues: impl DerefMut<Target = Upvalues>,
     ) -> EvalResult<()> {
-        for _ in 0..count {
-            match frame.inc_ip() {
-                OpCode::DataStackIdx(StackIdx(stack_idx)) => {
-                    let idx = frame.stack_offset + stack_idx;
-
-                    let val = match self.stack.get(idx) {
-                        Some(val) => val.clone(),
-                        None => {
-                            return frame.error(
-                                self,
-                                ErrorKind::TvixBug {
-                                    msg: "upvalue to be captured was missing on stack",
-                                    metadata: Some(Rc::new(json!({
-                                        "ip": format!("{:#x}", frame.ip.0 - 1),
-                                        "stack_idx(relative)": stack_idx,
-                                        "stack_idx(absolute)": idx,
-                                    }))),
-                                },
-                            );
-                        }
-                    };
+        // Determine whether to capture the with stack, and then shift the
+        // actual count of upvalues back.
+        let capture_with = count & 0b1 == 1;
+        let count = count >> 1;
+        if capture_with {
+            // Start the captured with_stack off of the
+            // current call frame's captured with_stack, ...
+            let mut captured_with_stack = frame
+                .upvalues
+                .with_stack()
+                .cloned()
+                // ... or make an empty one if there isn't one already.
+                .unwrap_or_else(|| Vec::with_capacity(self.with_stack.len()));
+
+            for idx in &self.with_stack {
+                captured_with_stack.push(self.stack[*idx].clone());
+            }
 
-                    upvalues.deref_mut().push(val);
-                }
+            upvalues.deref_mut().set_with_stack(captured_with_stack);
+        }
 
-                OpCode::DataUpvalueIdx(upv_idx) => {
-                    upvalues.deref_mut().push(frame.upvalue(upv_idx).clone());
-                }
+        for _ in 0..count {
+            let pos = Position(frame.read_uvarint());
 
-                OpCode::DataDeferredLocal(idx) => {
-                    upvalues.deref_mut().push(Value::DeferredUpvalue(idx));
-                }
+            if let Some(stack_idx) = pos.runtime_stack_index() {
+                let idx = frame.stack_offset + stack_idx.0;
 
-                OpCode::DataCaptureWith => {
-                    // Start the captured with_stack off of the
-                    // current call frame's captured with_stack, ...
-                    let mut captured_with_stack = frame
-                        .upvalues
-                        .with_stack()
-                        .cloned()
-                        // ... or make an empty one if there isn't one already.
-                        .unwrap_or_else(|| Vec::with_capacity(self.with_stack.len()));
-
-                    for idx in &self.with_stack {
-                        captured_with_stack.push(self.stack[*idx].clone());
+                let val = match self.stack.get(idx) {
+                    Some(val) => val.clone(),
+                    None => {
+                        return frame.error(
+                            self,
+                            ErrorKind::TvixBug {
+                                msg: "upvalue to be captured was missing on stack",
+                                metadata: Some(Rc::new(json!({
+                                    "ip": format!("{:#x}", frame.ip.0 - 1),
+                                    "stack_idx(relative)": stack_idx.0,
+                                    "stack_idx(absolute)": idx,
+                                }))),
+                            },
+                        );
                     }
+                };
 
-                    upvalues.deref_mut().set_with_stack(captured_with_stack);
-                }
+                upvalues.deref_mut().push(val);
+                continue;
+            }
 
-                _ => panic!("compiler error: missing closure operand"),
+            if let Some(idx) = pos.runtime_deferred_local() {
+                upvalues.deref_mut().push(Value::DeferredUpvalue(idx));
+                continue;
             }
+
+            if let Some(idx) = pos.runtime_upvalue_index() {
+                upvalues.deref_mut().push(frame.upvalue(idx).clone());
+                continue;
+            }
+
+            panic!("Tvix bug: invalid capture position emitted")
         }
 
         Ok(())