//! This module implements the virtual (or abstract) machine that runs
//! Tvix bytecode.
use std::{cell::RefMut, rc::Rc};
use crate::{
chunk::Chunk,
errors::{Error, ErrorKind, EvalResult},
observer::Observer,
opcode::{CodeIdx, Count, JumpOffset, OpCode, StackIdx, UpvalueIdx},
upvalues::{UpvalueCarrier, Upvalues},
value::{Builtin, Closure, Lambda, NixAttrs, NixList, Thunk, Value},
};
struct CallFrame {
lambda: Rc<Lambda>,
upvalues: Upvalues,
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]
}
}
pub struct VM<'o> {
frames: Vec<CallFrame>,
stack: Vec<Value>,
/// Stack indices of attribute sets from which variables should be
/// dynamically resolved (`with`).
with_stack: Vec<usize>,
observer: &'o mut dyn Observer,
}
/// This macro wraps a computation that returns an ErrorKind or a
/// result, and wraps the ErrorKind in an Error struct if present.
///
/// The reason for this macro's existence is that calculating spans is
/// potentially expensive, so it should be avoided to the last moment
/// (i.e. definite instantiation of a runtime error) if possible.
macro_rules! fallible {
( $self:ident, $body:expr) => {
match $body {
Ok(result) => result,
Err(kind) => {
return Err(Error {
kind,
span: $self.current_span(),
})
}
}
};
}
#[macro_export]
macro_rules! arithmetic_op {
( $self:ident, $op:tt ) => {{
let b = $self.pop();
let a = $self.pop();
let result = fallible!($self, arithmetic_op!(a, b, $op));
$self.push(result);
}};
( $a:expr, $b:expr, $op:tt ) => {{
match ($a, $b) {
(Value::Integer(i1), Value::Integer(i2)) => Ok(Value::Integer(i1 $op i2)),
(Value::Float(f1), Value::Float(f2)) => Ok(Value::Float(f1 $op f2)),
(Value::Integer(i1), Value::Float(f2)) => Ok(Value::Float(i1 as f64 $op f2)),
(Value::Float(f1), Value::Integer(i2)) => Ok(Value::Float(f1 $op i2 as f64)),
(v1, v2) => Err(ErrorKind::TypeError {
expected: "number (either int or float)",
actual: if v1.is_number() {
v2.type_of()
} else {
v1.type_of()
},
}),
}
}};
}
macro_rules! cmp_op {
( $self:ident, $op:tt ) => {{
let b = $self.pop();
let a = $self.pop();
// Comparable (in terms of ordering) values are numbers and
// strings. Numbers need to be coerced similarly to arithmetic
// ops if mixed types are encountered.
let result = match (a, b) {
(Value::Integer(i1), Value::Integer(i2)) => i1 $op i2,
(Value::Float(f1), Value::Float(f2)) => f1 $op f2,
(Value::Integer(i1), Value::Float(f2)) => (i1 as f64) $op f2,
(Value::Float(f1), Value::Integer(i2)) => f1 $op (i2 as f64),
(Value::String(s1), Value::String(s2)) => s1 $op s2,
(lhs, rhs) => return Err($self.error(ErrorKind::Incomparable {
lhs: lhs.type_of(),
rhs: rhs.type_of(),
})),
};
$self.push(Value::Bool(result));
}};
}
impl<'o> VM<'o> {
pub fn new(observer: &'o mut dyn Observer) -> Self {
Self {
observer,
frames: vec![],
stack: vec![],
with_stack: vec![],
}
}
fn frame(&self) -> &CallFrame {
&self.frames[self.frames.len() - 1]
}
fn chunk(&self) -> &Chunk {
&self.frame().lambda.chunk
}
fn frame_mut(&mut self) -> &mut CallFrame {
let idx = self.frames.len() - 1;
&mut self.frames[idx]
}
fn inc_ip(&mut self) -> OpCode {
let op = self.chunk().code[self.frame().ip];
self.frame_mut().ip += 1;
op
}
fn pop(&mut self) -> Value {
self.stack.pop().expect("runtime stack empty")
}
fn push(&mut self, value: Value) {
self.stack.push(value)
}
fn peek(&self, offset: usize) -> &Value {
&self.stack[self.stack.len() - 1 - offset]
}
/// Returns the source span of the instruction currently being
/// executed.
fn current_span(&self) -> codemap::Span {
self.chunk().get_span(CodeIdx(self.frame().ip - 1))
}
/// Construct an error from the given ErrorKind and the source
/// span of the current instruction.
fn error(&self, kind: ErrorKind) -> Error {
Error {
kind,
span: self.current_span(),
}
}
#[allow(clippy::let_and_return)] // due to disassembler
/// Execute the given lambda in this VM's context, returning its
/// value after its stack frame completes.
pub fn call(
&mut self,
lambda: Rc<Lambda>,
upvalues: Upvalues,
arg_count: usize,
) -> EvalResult<Value> {
self.observer
.observe_enter_frame(arg_count, &lambda, self.frames.len() + 1);
let frame = CallFrame {
lambda,
upvalues,
ip: 0,
stack_offset: self.stack.len() - arg_count,
};
self.frames.push(frame);
let result = self.run();
self.observer.observe_exit_frame(self.frames.len() + 1);
result
}
/// Run the VM's current stack frame to completion and return the
/// value.
fn run(&mut self) -> EvalResult<Value> {
loop {
// Break the loop if this call frame has already run to
// completion, pop it off, and return the value to the
// caller.
if self.frame().ip == self.chunk().code.len() {
self.frames.pop();
return Ok(self.pop());
}
let op = self.inc_ip();
self.observer
.observe_execute_op(self.frame().ip, &op, &self.stack);
match op {
OpCode::OpConstant(idx) => {
let c = self.chunk()[idx].clone();
self.push(c);
}
OpCode::OpPop => {
self.pop();
}
OpCode::OpAdd => {
let b = self.pop();
let a = self.pop();
let result = if let (Value::String(s1), Value::String(s2)) = (&a, &b) {
Value::String(s1.concat(s2))
} else {
fallible!(self, arithmetic_op!(a, b, +))
};
self.push(result)
}
OpCode::OpSub => arithmetic_op!(self, -),
OpCode::OpMul => arithmetic_op!(self, *),
OpCode::OpDiv => arithmetic_op!(self, /),
OpCode::OpInvert => {
let v = fallible!(self, self.pop().as_bool());
self.push(Value::Bool(!v));
}
OpCode::OpNegate => match self.pop() {
Value::Integer(i) => self.push(Value::Integer(-i)),
Value::Float(f) => self.push(Value::Float(-f)),
v => {
return Err(self.error(ErrorKind::TypeError {
expected: "number (either int or float)",
actual: v.type_of(),
}));
}
},
OpCode::OpEqual => {
let v2 = self.pop();
let v1 = self.pop();
self.push(Value::Bool(v1 == v2))
}
OpCode::OpLess => cmp_op!(self, <),
OpCode::OpLessOrEq => cmp_op!(self, <=),
OpCode::OpMore => cmp_op!(self, >),
OpCode::OpMoreOrEq => cmp_op!(self, >=),
OpCode::OpNull => self.push(Value::Null),
OpCode::OpTrue => self.push(Value::Bool(true)),
OpCode::OpFalse => self.push(Value::Bool(false)),
OpCode::OpAttrs(Count(count)) => self.run_attrset(count)?,
OpCode::OpAttrPath(Count(count)) => self.run_attr_path(count)?,
OpCode::OpAttrsUpdate => {
let rhs = unwrap_or_clone_rc(fallible!(self, self.pop().to_attrs()));
let lhs = unwrap_or_clone_rc(fallible!(self, self.pop().to_attrs()));
self.push(Value::Attrs(Rc::new(lhs.update(rhs))))
}
OpCode::OpAttrsSelect => {
let key = fallible!(self, self.pop().to_str());
let attrs = fallible!(self, self.pop().to_attrs());
match attrs.select(key.as_str()) {
Some(value) => self.push(value.clone()),
None => {
return Err(self.error(ErrorKind::AttributeNotFound {
name: key.as_str().to_string(),
}))
}
}
}
OpCode::OpAttrsTrySelect => {
let key = fallible!(self, self.pop().to_str());
let value = match self.pop() {
Value::Attrs(attrs) => match attrs.select(key.as_str()) {
Some(value) => value.clone(),
None => Value::AttrNotFound,
},
_ => Value::AttrNotFound,
};
self.push(value);
}
OpCode::OpAttrsIsSet => {
let key = fallible!(self, self.pop().to_str());
let result = match self.pop() {
Value::Attrs(attrs) => attrs.contains(key.as_str()),
// Nix allows use of `?` on non-set types, but
// always returns false in those cases.
_ => false,
};
self.push(Value::Bool(result));
}
OpCode::OpList(Count(count)) => {
let list =
NixList::construct(count, self.stack.split_off(self.stack.len() - count));
self.push(Value::List(list));
}
OpCode::OpConcat => {
let rhs = fallible!(self, self.pop().to_list());
let lhs = fallible!(self, self.pop().to_list());
self.push(Value::List(lhs.concat(&rhs)))
}
OpCode::OpInterpolate(Count(count)) => self.run_interpolate(count)?,
OpCode::OpJump(JumpOffset(offset)) => {
debug_assert!(offset != 0);
self.frame_mut().ip += offset;
}
OpCode::OpJumpIfTrue(JumpOffset(offset)) => {
debug_assert!(offset != 0);
if fallible!(self, self.peek(0).as_bool()) {
self.frame_mut().ip += offset;
}
}
OpCode::OpJumpIfFalse(JumpOffset(offset)) => {
debug_assert!(offset != 0);
if !fallible!(self, self.peek(0).as_bool()) {
self.frame_mut().ip += offset;
}
}
OpCode::OpJumpIfNotFound(JumpOffset(offset)) => {
debug_assert!(offset != 0);
if matches!(self.peek(0), Value::AttrNotFound) {
self.pop();
self.frame_mut().ip += offset;
}
}
// These assertion operations error out if the stack
// top is not of the expected type. This is necessary
// to implement some specific behaviours of Nix
// exactly.
OpCode::OpAssertBool => {
let val = self.peek(0);
if !val.is_bool() {
return Err(self.error(ErrorKind::TypeError {
expected: "bool",
actual: val.type_of(),
}));
}
}
// Remove the given number of elements from the stack,
// but retain the top value.
OpCode::OpCloseScope(Count(count)) => {
// Immediately move the top value into the right
// position.
let target_idx = self.stack.len() - 1 - count;
self.stack[target_idx] = self.pop();
// Then drop the remaining values.
for _ in 0..(count - 1) {
self.pop();
}
}
OpCode::OpGetLocal(StackIdx(local_idx)) => {
let idx = self.frame().stack_offset + local_idx;
self.push(self.stack[idx].clone());
}
OpCode::OpPushWith(StackIdx(idx)) => {
self.with_stack.push(self.frame().stack_offset + idx)
}
OpCode::OpPopWith => {
self.with_stack.pop();
}
OpCode::OpResolveWith => {
let ident = fallible!(self, self.pop().to_str());
let value = self.resolve_with(ident.as_str())?;
self.push(value)
}
OpCode::OpResolveWithOrUpvalue(idx) => {
let ident = fallible!(self, self.pop().to_str());
match self.resolve_with(ident.as_str()) {
// Variable found in local `with`-stack.
Ok(value) => self.push(value),
// Variable not found => check upvalues.
Err(Error {
kind: ErrorKind::UnknownDynamicVariable(_),
..
}) => {
let value = self.frame().upvalue(idx).clone();
self.push(value);
}
Err(err) => return Err(err),
}
}
OpCode::OpAssert => {
if !fallible!(self, self.pop().as_bool()) {
return Err(self.error(ErrorKind::AssertionFailed));
}
}
OpCode::OpCall => {
let callable = self.pop();
match callable {
Value::Closure(closure) => {
let result =
self.call(closure.lambda(), closure.upvalues().clone(), 1)?;
self.push(result)
}
Value::Builtin(builtin) => self.call_builtin(builtin)?,
_ => return Err(self.error(ErrorKind::NotCallable)),
};
}
OpCode::OpTailCall => {
let callable = self.pop();
match callable {
Value::Builtin(builtin) => self.call_builtin(builtin)?,
Value::Closure(closure) => {
let lambda = closure.lambda();
self.observer.observe_tail_call(self.frames.len(), &lambda);
// Replace the current call frames
// internals with that of the tail-called
// closure.
let mut frame = self.frame_mut();
frame.lambda = lambda;
frame.upvalues = closure.upvalues().clone();
frame.ip = 0; // reset instruction pointer to beginning
}
_ => return Err(self.error(ErrorKind::NotCallable)),
}
}
OpCode::OpGetUpvalue(upv_idx) => {
let value = self.frame().upvalue(upv_idx).clone();
self.push(value);
}
OpCode::OpClosure(idx) => {
let blueprint = match &self.chunk()[idx] {
Value::Blueprint(lambda) => lambda.clone(),
_ => panic!("compiler bug: non-blueprint in blueprint slot"),
};
let upvalue_count = blueprint.upvalue_count;
debug_assert!(
upvalue_count > 0,
"OpClosure should not be called for plain lambdas"
);
let closure = Closure::new(blueprint);
let upvalues = closure.upvalues_mut();
self.push(Value::Closure(closure.clone()));
// From this point on we internally mutate the
// closure object's upvalues. The closure is
// already in its stack slot, which means that it
// can capture itself as an upvalue for
// self-recursion.
self.populate_upvalues(upvalue_count, upvalues)?;
}
OpCode::OpThunk(idx) => {
let blueprint = match &self.chunk()[idx] {
Value::Blueprint(lambda) => lambda.clone(),
_ => panic!("compiler bug: non-blueprint in blueprint slot"),
};
let upvalue_count = blueprint.upvalue_count;
let thunk = Thunk::new(blueprint);
let upvalues = thunk.upvalues_mut();
self.push(Value::Thunk(thunk.clone()));
self.populate_upvalues(upvalue_count, upvalues)?;
}
OpCode::OpForce => {
let mut value = self.pop();
if let Value::Thunk(thunk) = value {
fallible!(self, thunk.force(self));
value = thunk.value().clone();
}
self.push(value);
}
OpCode::OpFinalise(StackIdx(idx)) => {
match &self.stack[self.frame().stack_offset + idx] {
Value::Closure(closure) => closure
.resolve_deferred_upvalues(&self.stack[self.frame().stack_offset..]),
Value::Thunk(thunk) => thunk
.resolve_deferred_upvalues(&self.stack[self.frame().stack_offset..]),
v => panic!("compiler error: invalid finaliser value: {}", v),
}
}
// Data-carrying operands should never be executed,
// that is a critical error in the VM.
OpCode::DataLocalIdx(_)
| OpCode::DataDeferredLocal(_)
| OpCode::DataUpvalueIdx(_)
| OpCode::DataCaptureWith => {
panic!("VM bug: attempted to execute data-carrying operand")
}
}
}
}
/// Construct runtime representation of an attr path (essentially
/// just a list of strings).
///
/// The difference to the list construction operation is that this
/// forces all elements into strings, as attribute set keys are
/// required to be strict in Nix.
fn run_attr_path(&mut self, count: usize) -> EvalResult<()> {
debug_assert!(count > 1, "AttrPath needs at least two fragments");
let mut path = Vec::with_capacity(count);
for _ in 0..count {
path.push(fallible!(self, self.pop().to_str()));
}
self.push(Value::AttrPath(path));
Ok(())
}
fn run_attrset(&mut self, count: usize) -> EvalResult<()> {
let attrs = fallible!(
self,
NixAttrs::construct(count, self.stack.split_off(self.stack.len() - count * 2))
);
self.push(Value::Attrs(Rc::new(attrs)));
Ok(())
}
/// 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, count: usize) -> EvalResult<()> {
let mut out = String::new();
for _ in 0..count {
out.push_str(fallible!(self, self.pop().to_str()).as_str());
}
self.push(Value::String(out.into()));
Ok(())
}
/// Resolve a dynamic identifier through the with-stack at runtime.
fn resolve_with(&mut self, ident: &str) -> EvalResult<Value> {
// Iterate over the with_stack manually to avoid borrowing
// self, which is required for forcing the set.
for with_stack_idx in (0..self.with_stack.len()).rev() {
let with = self.stack[self.with_stack[with_stack_idx]].clone();
if let Value::Thunk(thunk) = &with {
fallible!(self, thunk.force(self));
}
match fallible!(self, with.to_attrs()).select(ident) {
None => continue,
Some(val) => return Ok(val.clone()),
}
}
// Iterate over the captured with stack if one exists. This is
// extra tricky to do without a lot of cloning.
for idx in (0..self.frame().upvalues.with_stack_len()).rev() {
// This is safe because having an index here guarantees
// that the stack is present.
let with =
unsafe { self.frame().upvalues.with_stack().unwrap_unchecked()[idx].clone() };
if let Value::Thunk(thunk) = &with {
fallible!(self, thunk.force(self));
}
match fallible!(self, with.to_attrs()).select(ident) {
None => continue,
Some(val) => return Ok(val.clone()),
}
}
Err(self.error(ErrorKind::UnknownDynamicVariable(ident.to_string())))
}
/// Populate the upvalue fields of a thunk or closure under construction.
fn populate_upvalues(
&mut self,
count: usize,
mut upvalues: RefMut<'_, Upvalues>,
) -> EvalResult<()> {
for _ in 0..count {
match self.inc_ip() {
OpCode::DataLocalIdx(StackIdx(local_idx)) => {
let idx = self.frame().stack_offset + local_idx;
upvalues.push(self.stack[idx].clone());
}
OpCode::DataUpvalueIdx(upv_idx) => {
upvalues.push(self.frame().upvalue(upv_idx).clone());
}
OpCode::DataDeferredLocal(idx) => {
upvalues.push(Value::DeferredUpvalue(idx));
}
OpCode::DataCaptureWith => {
// Start the captured with_stack off of the
// current call frame's captured with_stack, ...
let mut captured_with_stack = self
.frame()
.upvalues
.with_stack()
.map(Clone::clone)
// ... 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.set_with_stack(captured_with_stack);
}
_ => panic!("compiler error: missing closure operand"),
}
}
Ok(())
}
/// Strictly evaluate the supplied value for outputting it. This
/// will ensure that lists and attribute sets do not contain
/// chunks which, for users, are displayed in a strange and often
/// unexpected way.
fn force_for_output(&mut self, value: &Value) -> EvalResult<()> {
match value {
Value::Attrs(attrs) => {
for (_, value) in attrs.iter() {
self.force_for_output(value)?;
}
Ok(())
}
Value::List(list) => list.iter().try_for_each(|elem| self.force_for_output(elem)),
Value::Thunk(thunk) => {
fallible!(self, thunk.force(self));
self.force_for_output(&thunk.value())
}
// If any of these internal values are encountered here a
// critical error has happened (likely a compiler bug).
Value::AttrPath(_)
| Value::AttrNotFound
| Value::DynamicUpvalueMissing(_)
| Value::Blueprint(_)
| Value::DeferredUpvalue(_) => {
panic!("tvix bug: internal value left on stack: {:?}", value)
}
_ => Ok(()),
}
}
fn call_builtin(&mut self, builtin: Builtin) -> EvalResult<()> {
let builtin_name = builtin.name();
self.observer.observe_enter_builtin(builtin_name);
let arg = self.pop();
let result = fallible!(self, builtin.apply(self, arg));
self.observer.observe_exit_builtin(builtin_name);
self.push(result);
Ok(())
}
}
// TODO: use Rc::unwrap_or_clone once it is stabilised.
// https://doc.rust-lang.org/std/rc/struct.Rc.html#method.unwrap_or_clone
fn unwrap_or_clone_rc<T: Clone>(rc: Rc<T>) -> T {
Rc::try_unwrap(rc).unwrap_or_else(|rc| (*rc).clone())
}
pub fn run_lambda(observer: &mut dyn Observer, lambda: Rc<Lambda>) -> EvalResult<Value> {
let mut vm = VM::new(observer);
let value = vm.call(lambda, Upvalues::with_capacity(0), 0)?;
vm.force_for_output(&value)?;
Ok(value)
}
