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refactor(tvix/eval): capture entire with_stack in upvalues

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This completely rewrites the handling of "dynamic upvalues" to,
instead of resolving them at thunk/closure instantiation time (which
forces some values too early), capture the entire with stack of parent
contexts if it exists.

There are a couple of things in here that could be written more
efficiently, but I'm first working through this to get to a bug
related to with + recursion and the code complexity of some of the
optimisations is distracting.

Change-Id: Ia538e06c9146e3bf8decb9adf02dd726d2c651cf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6486
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
This commit is contained in:
Vincent Ambo 2022-09-06 23:13:48 +03:00 • committed by tazjin
parent d75b207a63
commit 07ea30370e
5 changed files with 122 additions and 148 deletions
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149
tvix/eval/src/compiler/mod.rs
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@ -18,7 +18,6 @@ mod scope;
use path_clean::PathClean;
use rnix::ast::{self, AstToken, HasEntry};
use rowan::ast::{AstChildren, AstNode};
use smol_str::SmolStr;
use std::collections::HashMap;
use std::path::{Path, PathBuf};
use std::rc::Rc;
@ -45,6 +44,7 @@ pub struct CompilationOutput {
struct LambdaCtx {
lambda: Lambda,
scope: Scope,
captures_with_stack: bool,
}
impl LambdaCtx {
@ -52,6 +52,7 @@ impl LambdaCtx {
LambdaCtx {
lambda: Lambda::new_anonymous(),
scope: Default::default(),
captures_with_stack: false,
}
}
@ -59,6 +60,7 @@ impl LambdaCtx {
LambdaCtx {
lambda: Lambda::new_anonymous(),
scope: self.scope.inherit(),
captures_with_stack: false,
}
}
}
@ -869,32 +871,18 @@ impl Compiler<'_, '_> {
return;
}
// Even worse - are we dealing with a dynamic upvalue?
if let Some(idx) =
self.resolve_dynamic_upvalue(self.contexts.len() - 1, ident.text(), &node)
{
// Edge case: Current scope *also* has a non-empty
// `with`-stack. This means we need to resolve
// both in this scope, and in the upvalues.
if self.scope().has_with() {
self.emit_literal_ident(&node);
self.push_op(OpCode::OpResolveWithOrUpvalue(idx), &node);
return;
}
self.push_op(OpCode::OpGetUpvalue(idx), &node);
// If there is a non-empty `with`-stack (or a parent
// context with one), emit a runtime dynamic
// resolution instruction.
if self.has_dynamic_ancestor() {
self.emit_literal_ident(&node);
self.push_op(OpCode::OpResolveWith, &node);
return;
}
if !self.scope().has_with() {
self.emit_error(self.span_for(&node), ErrorKind::UnknownStaticVariable);
return;
}
// Variable needs to be dynamically resolved at
// runtime.
self.emit_literal_ident(&node);
self.push_op(OpCode::OpResolveWith, &node);
// Otherwise, this variable is missing.
self.emit_error(self.span_for(&node), ErrorKind::UnknownStaticVariable);
return;
}
LocalPosition::Known(idx) => {
@ -1078,6 +1066,12 @@ impl Compiler<'_, '_> {
// Check if tail-call optimisation is possible and perform it.
optimise_tail_call(&mut compiled.lambda.chunk);
// 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;
}
let lambda = Rc::new(compiled.lambda);
self.observer.observe_compiled_lambda(&lambda);
@ -1095,7 +1089,12 @@ impl Compiler<'_, '_> {
let blueprint_idx = self.chunk().push_constant(Value::Blueprint(lambda));
self.push_op(OpCode::OpClosure(blueprint_idx), &node);
self.emit_upvalue_data(outer_slot, compiled.scope.upvalues);
self.emit_upvalue_data(
outer_slot,
&node,
compiled.scope.upvalues,
compiled.captures_with_stack,
);
}
fn compile_apply(&mut self, slot: LocalIdx, node: ast::Apply) {
@ -1126,6 +1125,13 @@ impl Compiler<'_, '_> {
let mut thunk = self.contexts.pop().unwrap();
optimise_tail_call(&mut thunk.lambda.chunk);
// Capturing the with stack counts as an upvalue, as it is
// emitted as an upvalue data instruction.
if thunk.captures_with_stack {
thunk.lambda.upvalue_count += 1;
}
let lambda = Rc::new(thunk.lambda);
self.observer.observe_compiled_thunk(&lambda);
@ -1140,12 +1146,23 @@ impl Compiler<'_, '_> {
let blueprint_idx = self.chunk().push_constant(Value::Blueprint(lambda));
self.push_op(OpCode::OpThunk(blueprint_idx), node);
self.emit_upvalue_data(outer_slot, thunk.scope.upvalues);
self.emit_upvalue_data(
outer_slot,
node,
thunk.scope.upvalues,
thunk.captures_with_stack,
);
}
/// Emit the data instructions that the runtime needs to correctly
/// assemble the provided upvalues array.
fn emit_upvalue_data(&mut self, slot: LocalIdx, upvalues: Vec<Upvalue>) {
/// assemble the upvalues struct.
fn emit_upvalue_data<T: AstNode>(
&mut self,
slot: LocalIdx,
node: &T,
upvalues: Vec<Upvalue>,
capture_with: bool,
) {
let this_depth = self.scope()[slot].depth;
let this_stack_slot = self.scope().stack_index(slot);
@ -1170,16 +1187,13 @@ impl Compiler<'_, '_> {
UpvalueKind::Upvalue(idx) => {
self.push_op(OpCode::DataUpvalueIdx(idx), &upvalue.node);
}
UpvalueKind::Dynamic { name, up } => {
let idx = self.chunk().push_constant(Value::String(name.into()));
self.push_op(OpCode::DataDynamicIdx(idx), &upvalue.node);
if let Some(up) = up {
self.push_op(OpCode::DataDynamicAncestor(up), &upvalue.node);
}
}
};
}
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
@ -1322,58 +1336,25 @@ impl Compiler<'_, '_> {
None
}
/// If no static resolution for a potential upvalue was found,
/// finds the lowest lambda context that has a `with`-stack and
/// thread dynamic upvalues all the way through.
///
/// At runtime, as closures are being constructed they either
/// capture a dynamically available upvalue, take an upvalue from
/// their "ancestor" or leave a sentinel value on the stack.
///
/// As such an upvalue is actually accessed, an error is produced
/// when the sentinel is found. See the runtime's handling of
/// dynamic upvalues for details.
fn resolve_dynamic_upvalue(
&mut self,
at: usize,
name: &str,
node: &rnix::ast::Ident,
) -> Option<UpvalueIdx> {
if at == 0 {
// There can not be any upvalue at the outermost context.
return None;
}
/// Determine whether the current lambda context has any ancestors
/// that use dynamic scope resolution, and mark contexts as
/// needing to capture their enclosing `with`-stack in their
/// upvalues.
fn has_dynamic_ancestor(&mut self) -> bool {
let mut ancestor_has_with = false;
if let Some((lowest_idx, _)) = self
.contexts
.iter()
.enumerate()
.find(|(_, c)| c.scope.has_with())
{
// An enclosing lambda context has dynamic values. Each
// context in the chain from that point on now needs to
// capture dynamic upvalues because we can not statically
// know at which level the correct one is located.
let name = SmolStr::new(name);
let mut upvalue_idx = None;
for idx in lowest_idx..=at {
upvalue_idx = Some(self.add_upvalue(
idx,
node,
UpvalueKind::Dynamic {
name: name.clone(),
up: upvalue_idx,
},
));
for ctx in self.contexts.iter_mut() {
if ancestor_has_with {
// If the ancestor has an active with stack, mark this
// lambda context as needing to capture it.
ctx.captures_with_stack = true;
} else {
// otherwise, check this context and move on
ancestor_has_with = ctx.scope.has_with();
}
// Return the outermost upvalue index (i.e. the one of the
// current context).
return upvalue_idx;
}
None
ancestor_has_with
}
fn add_upvalue(