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Diffstat (limited to 'tvix/eval/src/compiler/attrs.rs')
| -rw-r--r-- | tvix/eval/src/compiler/attrs.rs | 208 |
1 files changed, 208 insertions, 0 deletions
diff --git a/tvix/eval/src/compiler/attrs.rs b/tvix/eval/src/compiler/attrs.rs new file mode 100644 index 0000000000..6ad31ab850 --- /dev/null +++ b/tvix/eval/src/compiler/attrs.rs @@ -0,0 +1,208 @@ +//! This module implements compiler logic related to attribute sets +//! (construction, access operators, ...). + +use super::*; + +impl Compiler<'_, '_> { + pub(super) fn compile_attr(&mut self, slot: LocalIdx, node: ast::Attr) { + match node { + ast::Attr::Dynamic(dynamic) => { + self.compile(slot, dynamic.expr().unwrap()); + self.emit_force(&dynamic.expr().unwrap()); + } + + ast::Attr::Str(s) => { + self.compile_str(slot, s.clone()); + self.emit_force(&s); + } + + ast::Attr::Ident(ident) => self.emit_literal_ident(&ident), + } + } + + /// Compile attribute set literals into equivalent bytecode. + /// + /// This is complicated by a number of features specific to Nix + /// attribute sets, most importantly: + /// + /// 1. Keys can be dynamically constructed through interpolation. + /// 2. Keys can refer to nested attribute sets. + /// 3. Attribute sets can (optionally) be recursive. + pub(super) fn compile_attr_set(&mut self, slot: LocalIdx, node: ast::AttrSet) { + if node.rec_token().is_some() { + todo!("recursive attribute sets are not yet implemented") + } + + // Open a scope to track the positions of the temporaries used + // by the `OpAttrs` instruction. + self.begin_scope(); + + let mut count = self.compile_inherit_attrs(slot, node.inherits()); + + for kv in node.attrpath_values() { + count += 1; + + // Because attribute set literals can contain nested keys, + // there is potentially more than one key fragment. If + // this is the case, a special operation to construct a + // runtime value representing the attribute path is + // emitted. + let mut key_count = 0; + let key_span = self.span_for(&kv.attrpath().unwrap()); + let key_idx = self.scope_mut().declare_phantom(key_span, false); + + for fragment in kv.attrpath().unwrap().attrs() { + // Key fragments can contain dynamic expressions, + // which makes accounting for their stack slots very + // tricky. + // + // In order to ensure the locals are correctly cleaned + // up, we essentially treat any key fragment after the + // first one (which has a locals index that will + // become that of the final key itself) as being part + // of a separate scope, which results in the somewhat + // annoying setup logic below. + let fragment_slot = match key_count { + 0 => key_idx, + 1 => { + self.begin_scope(); + self.scope_mut().declare_phantom(key_span, false) + } + _ => self.scope_mut().declare_phantom(key_span, false), + }; + + key_count += 1; + self.compile_attr(fragment_slot, fragment); + self.scope_mut().mark_initialised(fragment_slot); + } + + // We're done with the key if there was only one fragment, + // otherwise we need to emit an instruction to construct + // the attribute path. + if key_count > 1 { + self.push_op( + OpCode::OpAttrPath(Count(key_count)), + &kv.attrpath().unwrap(), + ); + + // Close the temporary scope that was set up for the + // key fragments. + self.scope_mut().end_scope(); + } + + // The value is just compiled as normal so that its + // resulting value is on the stack when the attribute set + // is constructed at runtime. + let value_span = self.span_for(&kv.value().unwrap()); + let value_slot = self.scope_mut().declare_phantom(value_span, false); + self.compile(slot, kv.value().unwrap()); + self.scope_mut().mark_initialised(value_slot); + } + + self.push_op(OpCode::OpAttrs(Count(count)), &node); + + // Remove the temporary scope, but do not emit any additional + // cleanup (OpAttrs consumes all of these locals). + self.scope_mut().end_scope(); + } + + pub(super) fn compile_has_attr(&mut self, slot: LocalIdx, node: ast::HasAttr) { + // Put the attribute set on the stack. + self.compile(slot, node.expr().unwrap()); + + // Push all path fragments with an operation for fetching the + // next nested element, for all fragments except the last one. + for (count, fragment) in node.attrpath().unwrap().attrs().enumerate() { + if count > 0 { + self.push_op(OpCode::OpAttrsTrySelect, &fragment); + } + + self.compile_attr(slot, fragment); + } + + // After the last fragment, emit the actual instruction that + // leaves a boolean on the stack. + self.push_op(OpCode::OpAttrsIsSet, &node); + } + + pub(super) fn compile_select(&mut self, slot: LocalIdx, node: ast::Select) { + let set = node.expr().unwrap(); + let path = node.attrpath().unwrap(); + + if node.or_token().is_some() { + self.compile_select_or(slot, set, path, node.default_expr().unwrap()); + return; + } + + // Push the set onto the stack + self.compile(slot, set.clone()); + self.emit_force(&set); + + // Compile each key fragment and emit access instructions. + // + // TODO: multi-select instruction to avoid re-pushing attrs on + // nested selects. + for fragment in path.attrs() { + self.compile_attr(slot, fragment); + self.push_op(OpCode::OpAttrsSelect, &node); + } + } + + /// Compile an `or` expression into a chunk of conditional jumps. + /// + /// If at any point during attribute set traversal a key is + /// missing, the `OpAttrOrNotFound` instruction will leave a + /// special sentinel value on the stack. + /// + /// After each access, a conditional jump evaluates the top of the + /// stack and short-circuits to the default value if it sees the + /// sentinel. + /// + /// Code like `{ a.b = 1; }.a.c or 42` yields this bytecode and + /// runtime stack: + /// + /// ```notrust + /// Bytecode Runtime stack + /// ┌────────────────────────────┐ ┌─────────────────────────┐ + /// │ ... │ │ ... │ + /// │ 5 OP_ATTRS(1) │ → │ 5 [ { a.b = 1; } ] │ + /// │ 6 OP_CONSTANT("a") │ → │ 6 [ { a.b = 1; } "a" ] │ + /// │ 7 OP_ATTR_OR_NOT_FOUND │ → │ 7 [ { b = 1; } ] │ + /// │ 8 JUMP_IF_NOT_FOUND(13) │ → │ 8 [ { b = 1; } ] │ + /// │ 9 OP_CONSTANT("C") │ → │ 9 [ { b = 1; } "c" ] │ + /// │ 10 OP_ATTR_OR_NOT_FOUND │ → │ 10 [ NOT_FOUND ] │ + /// │ 11 JUMP_IF_NOT_FOUND(13) │ → │ 11 [ ] │ + /// │ 12 JUMP(14) │ │ .. jumped over │ + /// │ 13 CONSTANT(42) │ → │ 12 [ 42 ] │ + /// │ 14 ... │ │ .. .... │ + /// └────────────────────────────┘ └─────────────────────────┘ + /// ``` + fn compile_select_or( + &mut self, + slot: LocalIdx, + set: ast::Expr, + path: ast::Attrpath, + default: ast::Expr, + ) { + self.compile(slot, set.clone()); + self.emit_force(&set); + let mut jumps = vec![]; + + for fragment in path.attrs() { + self.compile_attr(slot, fragment.clone()); + self.push_op(OpCode::OpAttrsTrySelect, &fragment); + jumps.push(self.push_op(OpCode::OpJumpIfNotFound(JumpOffset(0)), &fragment)); + } + + let final_jump = self.push_op(OpCode::OpJump(JumpOffset(0)), &path); + + for jump in jumps { + self.patch_jump(jump); + } + + // Compile the default value expression and patch the final + // jump to point *beyond* it. + self.compile(slot, default); + self.patch_jump(final_jump); + } +} |