From ccfb971dc54cb77522d70f1ecbf1e9080e7ba0ca Mon Sep 17 00:00:00 2001 From: Vincent Ambo Date: Sun, 28 Aug 2022 03:45:45 +0300 Subject: fix(tvix/eval): correctly thread through dynamic upvalues This puts together the puzzle pieces for threading dynamic upvalues (that is, upvalues resolved from the `with`-stack) all the way through. Reading the test case enclosed in this commit and walking through it is recommended to understand what problem is being tackled here. In short, because the compiler can not statically know *which* with-scope a dynamic argument is resolved from it needs to lay the groundwork for resolving from *all* possible scopes. There are multiple different approaches to doing this. The approach chosen in this commit is that if a dynamic upvalue is detected, the compiler will emit instructions to close over this dynamic value in *all* enclosing lambda contexts. It uses a new instruction for this that will leave around a sentinel value in case an identifier could not be resolved, and wire the location of this found value (or sentinel) up through the upvalues to the next level of nesting. In this tradeoff, tvix potentially closes over more upvalues than are needed (but in practice, how often do people create *really* deep `with`-stacks? and in *this* kind of code situation? maybe we should even warn for this!) but avoids keeping the entire attribute sets themselves around. Looking at the test case, each surrounding closure will close over *all* dynamic identifiers that are referenced later on visible to it, but only the last one for each identifier will actually end up being used. This also covers our bases for an additional edge-case this creates, in which an identifier potentially resolves to a dynamic upvalue *and* to a dynamic value within the function's own scope (again, would anyone really do this?) by introducing a resolution instruction for that particular case. There is likely some potential for cleaning up this code which is quite ugly in some parts, but as this implementation is now carefully calibrated to work I decided it is time to commit it and clean it up in subsequent commits. Change-Id: Ib701e3e6da39bd2c95938d1384036ff4f9fb3749 Reviewed-on: https://cl.tvl.fyi/c/depot/+/6322 Tested-by: BuildkiteCI Reviewed-by: sterni --- tvix/eval/src/compiler/mod.rs | 24 ++++++-- tvix/eval/src/opcode.rs | 2 + .../eval-okay-deeply-nested-with-closure.exp | 1 + .../eval-okay-deeply-nested-with-closure.nix | 18 ++++++ tvix/eval/src/value/mod.rs | 8 ++- tvix/eval/src/vm.rs | 71 +++++++++++++++++++--- 6 files changed, 112 insertions(+), 12 deletions(-) create mode 100644 tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.exp create mode 100644 tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.nix (limited to 'tvix/eval') diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs index 11d6d8231f09..78fe76ca01a9 100644 --- a/tvix/eval/src/compiler/mod.rs +++ b/tvix/eval/src/compiler/mod.rs @@ -702,6 +702,15 @@ impl Compiler { if let Some(idx) = self.resolve_dynamic_upvalue(self.contexts.len() - 1, ident.text()) { + // 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_constant(Value::String(ident.text().into())); + self.chunk().push_op(OpCode::OpResolveWithOrUpvalue(idx)); + return; + } + self.chunk().push_op(OpCode::OpGetUpvalue(idx)); return; } @@ -810,11 +819,18 @@ impl Compiler { self.chunk().push_op(OpCode::OpClosure(closure_idx)); for upvalue in compiled.scope.upvalues { match upvalue { - Upvalue::Stack(idx) => self.chunk().push_op(OpCode::DataLocalIdx(idx)), - Upvalue::Upvalue(idx) => self.chunk().push_op(OpCode::DataUpvalueIdx(idx)), - Upvalue::Dynamic { name, .. } => { + Upvalue::Stack(idx) => { + self.chunk().push_op(OpCode::DataLocalIdx(idx)); + } + Upvalue::Upvalue(idx) => { + self.chunk().push_op(OpCode::DataUpvalueIdx(idx)); + } + Upvalue::Dynamic { name, up } => { let idx = self.chunk().push_constant(Value::String(name.into())); - self.chunk().push_op(OpCode::DataDynamicIdx(idx)) + self.chunk().push_op(OpCode::DataDynamicIdx(idx)); + if let Some(up) = up { + self.chunk().push_op(OpCode::DataDynamicAncestor(up)); + } } }; } diff --git a/tvix/eval/src/opcode.rs b/tvix/eval/src/opcode.rs index edf7a8a5dc4a..5f1f33f2b3e5 100644 --- a/tvix/eval/src/opcode.rs +++ b/tvix/eval/src/opcode.rs @@ -82,6 +82,7 @@ pub enum OpCode { OpPushWith(StackIdx), OpPopWith, OpResolveWith, + OpResolveWithOrUpvalue(UpvalueIdx), // Lists OpList(Count), @@ -116,4 +117,5 @@ pub enum OpCode { DataLocalIdx(StackIdx), DataUpvalueIdx(UpvalueIdx), DataDynamicIdx(ConstantIdx), + DataDynamicAncestor(UpvalueIdx), } diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.exp new file mode 100644 index 000000000000..3bed31f76e3f --- /dev/null +++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.exp @@ -0,0 +1 @@ +[ 1 2 3 4 ] diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.nix new file mode 100644 index 000000000000..7f13f1f27030 --- /dev/null +++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.nix @@ -0,0 +1,18 @@ +# This convoluted test constructs a situation in which dynamically +# resolved upvalues refer `with` blocks introduced at different lambda +# context boundaries, i.e. the access to a, b in the innermost closure +# must be threaded through upvalues in several levels. + +(_: +with { a = 1; b = 1; }; + +_: +with { b = 2; c = 2; }; + +_: +with { c = 3; d = 3; }; + +_: +with { d = 4; }; + +[ a b c d ]) null null null null diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs index eb23605bd0b2..54211e8ba313 100644 --- a/tvix/eval/src/value/mod.rs +++ b/tvix/eval/src/value/mod.rs @@ -34,6 +34,7 @@ pub enum Value { // are never returned to or created directly by users. AttrPath(Vec), AttrNotFound, + DynamicUpvalueMissing(NixString), } impl Value { @@ -54,7 +55,9 @@ impl Value { Value::Closure(_) | Value::Builtin(_) => "lambda", // Internal types - Value::AttrPath(_) | Value::AttrNotFound => "internal", + Value::AttrPath(_) | Value::AttrNotFound | Value::DynamicUpvalueMissing(_) => { + "internal" + } } } @@ -163,6 +166,9 @@ impl Display for Value { // internal types Value::AttrPath(path) => write!(f, "internal[attrpath({})]", path.len()), Value::AttrNotFound => f.write_str("internal[not found]"), + Value::DynamicUpvalueMissing(name) => { + write!(f, "internal[no_dyn_upvalue({name})]") + } } } } diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs index 23d9c3555153..5d5cb57dbabf 100644 --- a/tvix/eval/src/vm.rs +++ b/tvix/eval/src/vm.rs @@ -5,8 +5,8 @@ use std::{cell::Ref, rc::Rc}; use crate::{ chunk::Chunk, - errors::{ErrorKind, EvalResult}, - opcode::{Count, JumpOffset, OpCode, StackIdx}, + errors::{Error, ErrorKind, EvalResult}, + opcode::{ConstantIdx, Count, JumpOffset, OpCode, StackIdx}, value::{Closure, Lambda, NixAttrs, NixList, Value}, }; @@ -100,6 +100,10 @@ impl VM { op } + fn peek_op(&self) -> OpCode { + self.chunk().code[self.frame().ip] + } + fn pop(&mut self) -> Value { self.stack.pop().expect("runtime stack empty") } @@ -337,6 +341,25 @@ impl VM { self.push(value) } + OpCode::OpResolveWithOrUpvalue(idx) => { + let ident = self.pop().to_string()?; + 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().closure.upvalue(idx).clone(); + self.push(value); + } + + Err(err) => return Err(err), + } + } + OpCode::OpAssert => { if !self.pop().as_bool()? { return Err(ErrorKind::AssertionFailed.into()); @@ -389,10 +412,8 @@ impl VM { } OpCode::DataDynamicIdx(ident_idx) => { - let chunk = self.chunk(); - let ident = chunk.constant(ident_idx).as_str()?.to_string(); - drop(chunk); // some lifetime trickery due to cell::Ref - closure.push_upvalue(self.resolve_with(&ident)?); + let value = self.resolve_dynamic_upvalue(ident_idx)?; + closure.push_upvalue(value); } _ => panic!("compiler error: missing closure operand"), @@ -402,7 +423,10 @@ impl VM { // Data-carrying operands should never be executed, // that is a critical error in the VM. - OpCode::DataLocalIdx(_) | OpCode::DataUpvalueIdx(_) | OpCode::DataDynamicIdx(_) => { + OpCode::DataLocalIdx(_) + | OpCode::DataUpvalueIdx(_) + | OpCode::DataDynamicIdx(_) + | OpCode::DataDynamicAncestor(_) => { panic!("VM bug: attempted to execute data-carrying operand") } } @@ -452,6 +476,39 @@ impl VM { Ok(()) } + fn resolve_dynamic_upvalue(&mut self, ident_idx: ConstantIdx) -> EvalResult { + let chunk = self.chunk(); + let ident = chunk.constant(ident_idx).as_str()?.to_string(); + drop(chunk); // some lifetime trickery due to cell::Ref + + // Peek at the current instruction (note: IP has already + // advanced!) to see if it is actually data indicating a + // "fallback upvalue" in case the dynamic could not be + // resolved at this level. + let up = match self.peek_op() { + OpCode::DataDynamicAncestor(idx) => { + // advance ip past this data + self.inc_ip(); + Some(idx) + } + _ => None, + }; + + match self.resolve_with(&ident) { + Ok(v) => Ok(v), + + Err(Error { + kind: ErrorKind::UnknownDynamicVariable(_), + .. + }) => match up { + Some(idx) => Ok(self.frame().closure.upvalue(idx).clone()), + None => Ok(Value::DynamicUpvalueMissing(ident.into())), + }, + + Err(err) => Err(err), + } + } + /// Resolve a dynamic identifier through the with-stack at runtime. fn resolve_with(&self, ident: &str) -> EvalResult { for idx in self.with_stack.iter().rev() { -- cgit 1.4.1