diff options
Diffstat (limited to 'tvix/eval/src/compiler/mod.rs')
| -rw-r--r-- | tvix/eval/src/compiler/mod.rs | 199 |
1 files changed, 165 insertions, 34 deletions
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs index 436e504af2..b7b9192571 100644 --- a/tvix/eval/src/compiler/mod.rs +++ b/tvix/eval/src/compiler/mod.rs @@ -80,6 +80,47 @@ impl LambdaCtx { } } +/// When compiling functions with an argument attribute set destructuring pattern, +/// we need to do multiple passes over the declared formal arguments when setting +/// up their local bindings (similarly to `let … in` expressions and recursive +/// attribute sets. For this purpose, this struct is used to represent the two +/// kinds of formal arguments: +/// +/// - `TrackedFormal::NoDefault` is always required and causes an evaluation error +/// if the corresponding attribute is missing in a function call. +/// - `TrackedFormal::WithDefault` may be missing in the passed attribute set— +/// in which case a `default_expr` will be evaluated and placed in the formal +/// argument's local variable slot. +enum TrackedFormal { + NoDefault { + local_idx: LocalIdx, + pattern_entry: ast::PatEntry, + }, + WithDefault { + local_idx: LocalIdx, + /// Extra phantom local used for coordinating runtime dispatching not observable to + /// the language user. Detailed description in `compile_param_pattern()`. + finalise_request_idx: LocalIdx, + default_expr: ast::Expr, + pattern_entry: ast::PatEntry, + }, +} + +impl TrackedFormal { + fn pattern_entry(&self) -> &ast::PatEntry { + match self { + TrackedFormal::NoDefault { pattern_entry, .. } => pattern_entry, + TrackedFormal::WithDefault { pattern_entry, .. } => pattern_entry, + } + } + fn local_idx(&self) -> LocalIdx { + match self { + TrackedFormal::NoDefault { local_idx, .. } => *local_idx, + TrackedFormal::WithDefault { local_idx, .. } => *local_idx, + } + } +} + /// The map of globally available functions and other values that /// should implicitly be resolvable in the global scope. pub(crate) type GlobalsMap = HashMap<&'static str, Value>; @@ -894,10 +935,19 @@ impl Compiler<'_> { /// in <body> /// ``` /// - /// The only tricky bit being that bindings have to fail if too - /// many arguments are provided. This is done by emitting a - /// special instruction that checks the set of keys from a - /// constant containing the expected keys. + /// However, there are two properties of pattern function arguments that can + /// not be compiled by desugaring in this way: + /// + /// 1. Bindings have to fail if too many arguments are provided. This is + /// done by emitting a special instruction that checks the set of keys + /// from a constant containing the expected keys. + /// 2. Formal arguments with a default expression are (as an optimization and + /// because it is simpler) not wrapped in another thunk, instead compiled + /// and accessed separately. This means that the default expression may + /// never make it into the local's stack slot if the argument is provided + /// by the caller. We need to take this into account and skip any + /// operations specific to the expression like thunk finalisation in such + /// cases. fn compile_param_pattern(&mut self, pattern: &ast::Pattern) -> Formals { let span = self.span_for(pattern); let set_idx = match pattern.pat_bind() { @@ -905,10 +955,10 @@ impl Compiler<'_> { None => self.scope_mut().declare_phantom(span, true), }; - // At call time, the attribute set is already at the top of - // the stack. + // 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); + // Evaluation fails on a type error, even if the argument(s) are unused. self.push_op(OpCode::OpAssertAttrs, pattern); let ellipsis = pattern.ellipsis_token().is_some(); @@ -919,51 +969,131 @@ impl Compiler<'_> { // Similar to `let ... in ...`, we now do multiple passes over // the bindings to first declare them, then populate them, and // then finalise any necessary recursion into the scope. - let mut entries: Vec<(LocalIdx, ast::PatEntry)> = vec![]; + let mut entries: Vec<TrackedFormal> = vec![]; let mut arguments = HashMap::default(); for entry in pattern.pat_entries() { let ident = entry.ident().unwrap(); let idx = self.declare_local(&ident, ident.to_string()); - let has_default = entry.default().is_some(); - entries.push((idx, entry)); - arguments.insert(ident.into(), has_default); + + arguments.insert(ident.into(), entry.default().is_some()); + + if let Some(default_expr) = entry.default() { + entries.push(TrackedFormal::WithDefault { + local_idx: idx, + // This phantom is used to track at runtime (!) whether we need to + // finalise the local's stack slot or not. The relevant instructions are + // emitted in the second pass where the mechanism is explained as well. + finalise_request_idx: { + let span = self.span_for(&default_expr); + self.scope_mut().declare_phantom(span, false) + }, + default_expr, + pattern_entry: entry, + }); + } else { + entries.push(TrackedFormal::NoDefault { + local_idx: idx, + pattern_entry: entry, + }); + } } // For each of the bindings, push the set on the stack and // attempt to select from it. let stack_idx = self.scope().stack_index(set_idx); - for (idx, entry) in (&entries).into_iter() { + for tracked_formal in (&entries).into_iter() { self.push_op(OpCode::OpGetLocal(stack_idx), pattern); - self.emit_literal_ident(&entry.ident().unwrap()); + self.emit_literal_ident(&tracked_formal.pattern_entry().ident().unwrap()); + + let idx = tracked_formal.local_idx(); // Use the same mechanism as `compile_select_or` if a // default value was provided, or simply select otherwise. - if let Some(default_expr) = entry.default() { - self.push_op(OpCode::OpAttrsTrySelect, &entry.ident().unwrap()); - - let jump_to_default = - self.push_op(OpCode::OpJumpIfNotFound(JumpOffset(0)), &default_expr); - - let jump_over_default = self.push_op(OpCode::OpJump(JumpOffset(0)), &default_expr); - - self.patch_jump(jump_to_default); - - // Does not need to thunked since compile() already does so when necessary - self.compile(*idx, default_expr); - - self.patch_jump(jump_over_default); - } else { - self.push_op(OpCode::OpAttrsSelect, &entry.ident().unwrap()); + match tracked_formal { + TrackedFormal::WithDefault { + default_expr, + pattern_entry, + .. + } => { + // The tricky bit about compiling a formal argument with a default value + // is that the default may be a thunk that may depend on the value of + // other formal arguments, i.e. may need to be finalised. This + // finalisation can only happen if we are actually using the default + // value—otherwise OpFinalise will crash on an already finalised (or + // non-thunk) value. + // + // Thus we use an additional local to track whether we wound up + // defaulting or not. `FinaliseRequest(false)` indicates that we should + // not finalise, as we did not default. + // + // We are being wasteful with VM stack space in case of default + // expressions that don't end up needing to be finalised. Unfortunately + // 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.emit_constant(Value::FinaliseRequest(false), default_expr); + + let jump_over_default = + self.push_op(OpCode::OpJump(JumpOffset(0)), default_expr); + + self.patch_jump(jump_to_default); + + // Does not need to thunked since compile() already does so when necessary + self.compile(idx, default_expr.clone()); + + self.emit_constant(Value::FinaliseRequest(true), default_expr); + + self.patch_jump(jump_over_default); + } + TrackedFormal::NoDefault { pattern_entry, .. } => { + self.push_op(OpCode::OpAttrsSelect, &pattern_entry.ident().unwrap()); + } } - self.scope_mut().mark_initialised(*idx); + self.scope_mut().mark_initialised(idx); + if let TrackedFormal::WithDefault { + finalise_request_idx, + .. + } = tracked_formal + { + self.scope_mut().mark_initialised(*finalise_request_idx); + } } - for (idx, _) in (&entries).into_iter() { - if self.scope()[*idx].needs_finaliser { - let stack_idx = self.scope().stack_index(*idx); - self.push_op(OpCode::OpFinalise(stack_idx), pattern); + for tracked_formal in (&entries).into_iter() { + if self.scope()[tracked_formal.local_idx()].needs_finaliser { + let stack_idx = self.scope().stack_index(tracked_formal.local_idx()); + match tracked_formal { + TrackedFormal::NoDefault { .. } => + panic!("Tvix bug: local for pattern formal needs finaliser, but has no default expr"), + TrackedFormal::WithDefault { finalise_request_idx, .. } => { + 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 + ); + let jump_over_finalise = + self.push_op( + OpCode::OpJumpIfNoFinaliseRequest( + JumpOffset(0)), + pattern + ); + self.push_op( + OpCode::OpFinalise(stack_idx), + pattern, + ); + self.patch_jump(jump_over_finalise); + // Get rid of finaliser request value on the stack + self.push_op(OpCode::OpPop, pattern); + } + } } } @@ -1196,7 +1326,8 @@ impl Compiler<'_> { OpCode::OpJump(n) | OpCode::OpJumpIfFalse(n) | OpCode::OpJumpIfTrue(n) - | OpCode::OpJumpIfNotFound(n) => { + | OpCode::OpJumpIfNotFound(n) + | OpCode::OpJumpIfNoFinaliseRequest(n) => { *n = offset; } |