refactor(tvix/eval): Carry lambda & upvalues directly in CallFrame

CallFrame has to work for both thunks & closures (as a thunk is
basically a "weird 0-argument closure").

We opt to store the common, relevant fields directly in the frame to
avoid having to dereference through the nested structures
constantly (which would be especially annoying in the case of thunks).

Change-Id: I47781597b84ec5cd55502dba1713e92cf2592af3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6348
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/vm.rs

@@ -1,12 +1,12 @@
 //! This module implements the virtual (or abstract) machine that runs
 //! Tvix bytecode.
 
-use std::{cell::Ref, rc::Rc};
+use std::rc::Rc;
 
 use crate::{
     chunk::Chunk,
     errors::{Error, ErrorKind, EvalResult},
-    opcode::{ConstantIdx, Count, JumpOffset, OpCode, StackIdx},
+    opcode::{ConstantIdx, Count, JumpOffset, OpCode, StackIdx, UpvalueIdx},
     upvalues::UpvalueCarrier,
     value::{Closure, Lambda, NixAttrs, NixList, Value},
 };
@@ -15,11 +15,19 @@ use crate::{
 use crate::disassembler::Tracer;
 
 struct CallFrame {
-    closure: Closure,
+    lambda: Rc<Lambda>,
+    upvalues: Vec<Value>,
     ip: usize,
     stack_offset: usize,
 }
 
+impl CallFrame {
+    /// Retrieve an upvalue from this frame at the given index.
+    fn upvalue(&self, idx: UpvalueIdx) -> &Value {
+        &self.upvalues[idx.0]
+    }
+}
+
 pub struct VM {
     frames: Vec<CallFrame>,
     stack: Vec<Value>,
@@ -86,8 +94,8 @@ impl VM {
         &self.frames[self.frames.len() - 1]
     }
 
-    fn chunk(&self) -> Ref<'_, Chunk> {
-        self.frame().closure.chunk()
+    fn chunk(&self) -> &Chunk {
+        &self.frame().lambda.chunk
     }
 
     fn frame_mut(&mut self) -> &mut CallFrame {
@@ -117,9 +125,10 @@ impl VM {
         &self.stack[self.stack.len() - 1 - offset]
     }
 
-    fn call(&mut self, closure: Closure, arg_count: usize) {
+    pub fn call(&mut self, lambda: Rc<Lambda>, upvalues: Vec<Value>, arg_count: usize) {
         let frame = CallFrame {
-            closure,
+            lambda,
+            upvalues,
             ip: 0,
             stack_offset: self.stack.len() - arg_count,
         };
@@ -353,7 +362,7 @@ impl VM {
                             kind: ErrorKind::UnknownDynamicVariable(_),
                             ..
                         }) => {
-                            let value = self.frame().closure.upvalue(idx).clone();
+                            let value = self.frame().upvalue(idx).clone();
                             self.push(value);
                         }
 
@@ -370,7 +379,10 @@ impl VM {
                 OpCode::OpCall => {
                     let callable = self.pop();
                     match callable {
-                        Value::Closure(closure) => self.call(closure, 1),
+                        Value::Closure(closure) => {
+                            self.call(closure.lambda(), closure.upvalues().to_vec(), 1)
+                        }
+
                         Value::Builtin(builtin) => {
                             let arg = self.pop();
                             let result = builtin.apply(arg)?;
@@ -381,7 +393,7 @@ impl VM {
                 }
 
                 OpCode::OpGetUpvalue(upv_idx) => {
-                    let value = self.frame().closure.upvalue(upv_idx).clone();
+                    let value = self.frame().upvalue(upv_idx).clone();
                     if let Value::DynamicUpvalueMissing(name) = value {
                         return Err(
                             ErrorKind::UnknownDynamicVariable(name.as_str().to_string()).into()
@@ -419,7 +431,7 @@ impl VM {
                             }
 
                             OpCode::DataUpvalueIdx(upv_idx) => {
-                                closure.push_upvalue(self.frame().closure.upvalue(upv_idx).clone());
+                                closure.push_upvalue(self.frame().upvalue(upv_idx).clone());
                             }
 
                             OpCode::DataDynamicIdx(ident_idx) => {
@@ -532,7 +544,7 @@ impl VM {
                 kind: ErrorKind::UnknownDynamicVariable(_),
                 ..
             }) => match up {
-                Some(idx) => Ok(self.frame().closure.upvalue(idx).clone()),
+                Some(idx) => Ok(self.frame().upvalue(idx).clone()),
                 None => Ok(Value::DynamicUpvalueMissing(ident.into())),
             },
 
@@ -567,6 +579,6 @@ pub fn run_lambda(lambda: Lambda) -> EvalResult<Value> {
         with_stack: vec![],
     };
 
-    vm.call(Closure::new(Rc::new(lambda)), 0);
+    vm.call(Rc::new(lambda), vec![], 0);
     vm.run()
 }