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perf(vm): lazily clone parent upvalues on Closure 🪺 #156

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Merged
timfennis merged 1 commit into from
May 24, 2026
Merged

perf(vm): lazily clone parent upvalues on Closure 🪺 #156

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32 changes: 17 additions & 15 deletions ndc_vm/src/vm.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -498,28 +498,30 @@ impl Vm {
// and before `capture_upvalue` needs `&mut self`.
let idx = *idx;
let values = Rc::clone(values);
let frame = self.frames.last().expect("no frame");
let Value::Object(obj) = frame.closure.prototype.body.constant(idx) else {
panic!("Closure opcode: constant at index {idx} is not an Object");
};
let Object::Function(Function::Compiled(compiled)) = &**obj else {
panic!(
"Closure opcode: constant at index {idx} is not a Compiled function"
);
let (compiled, frame_pointer) = {
let frame = self.frames.last().expect("no frame");
let Value::Object(obj) = frame.closure.prototype.body.constant(idx) else {
panic!("Closure opcode: constant at index {idx} is not an Object");
};
let Object::Function(Function::Compiled(compiled)) = &**obj else {
panic!(
"Closure opcode: constant at index {idx} is not a Compiled function"
);
};
(Rc::clone(compiled), frame.frame_pointer)
};
let compiled = Rc::clone(compiled);
let frame_pointer = frame.frame_pointer;
// Pre-clone parent upvalue Rcs so we can drop the frame borrow
// before calling capture_upvalue (which needs &mut self).
let parent_upvalues: Vec<_> =
frame.closure.upvalues.iter().map(Rc::clone).collect();
// Resolve captures lazily: only clone the specific parent upvalue
// Rcs that this closure references, and capture locals on demand.
let upvalues = values
.iter()
.map(|c| match c {
CaptureSource::Local(slot) => {
self.capture_upvalue(frame_pointer + slot)
}
CaptureSource::Upvalue(slot) => Rc::clone(&parent_upvalues[*slot]),
CaptureSource::Upvalue(slot) => {
let frame = self.frames.last().expect("no frame");
Rc::clone(&frame.closure.upvalues[*slot])
}
})
.collect();
let closure = Value::function(Function::Closure(ClosureFunction {
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