diff --git a/src/extension/infer.rs b/src/extension/infer.rs index dd29d71c4..47eb88e67 100644 --- a/src/extension/infer.rs +++ b/src/extension/infer.rs @@ -22,7 +22,7 @@ use super::validate::ExtensionError; use petgraph::graph as pg; -use std::collections::{HashMap, HashSet}; +use std::collections::{HashMap, HashSet, VecDeque}; use thiserror::Error; @@ -521,57 +521,46 @@ impl UnificationContext { pub fn results(&self) -> Result { // Check that all of the metavariables associated with nodes of the // graph are solved + let depended_upon = { + let mut h: HashMap> = HashMap::new(); + for (m, m2) in self.constraints.iter().flat_map(|(m, cs)| { + cs.iter().flat_map(|c| match c { + Constraint::Plus(_, m2) => Some((*m, self.resolve(*m2))), + _ => None, + }) + }) { + h.entry(m2).or_default().push(m); + } + h + }; + // Calculate everything dependent upon a variable. + // Note it would be better to find metas ALL of whose dependencies were (transitively) + // on variables, but this is more complex, and hard to define if there are cycles + // of PLUS constraints, so leaving that as a TODO until we've handled such cycles. + let mut depends_on_var = HashSet::new(); + let mut queue = VecDeque::from_iter(self.variables.iter()); + while let Some(m) = queue.pop_front() { + if depends_on_var.insert(m) { + if let Some(d) = depended_upon.get(m) { + queue.extend(d.iter()) + } + } + } + let mut results: ExtensionSolution = HashMap::new(); for (loc, meta) in self.extensions.iter() { if let Some(rs) = self.get_solution(meta) { if loc.1 == Direction::Incoming { results.insert(loc.0, rs.clone()); } - } else if self.live_var(meta).is_some() { - // If it depends on some other live meta, that's bad news. - return Err(InferExtensionError::Unsolved { location: *loc }); - } - // If it only depends on graph variables, then we don't have - // a *solution*, but it's fine - } - debug_assert!(self.live_metas().is_empty()); - Ok(results) - } - - // Get the live var associated with a meta. - // TODO: This should really be a list - fn live_var(&self, m: &Meta) -> Option { - if self.variables.contains(m) || self.variables.contains(&self.resolve(*m)) { - return None; - } - - // TODO: We should be doing something to ensure that these are the same check... - if self.get_solution(m).is_none() { - if let Some(cs) = self.get_constraints(m) { - for c in cs { - match c { - Constraint::Plus(_, m) => return self.live_var(m), - _ => panic!("we shouldn't be here!"), - } + } else { + // Unsolved nodes must be unsolved because they depend on graph variables. + if !depends_on_var.contains(&self.resolve(*meta)) { + return Err(InferExtensionError::Unsolved { location: *loc }); } } - Some(*m) - } else { - None } - } - - /// Return the set of "live" metavariables in the context. - /// "Live" here means a metavariable: - /// - Is associated to a location in the graph in `UnifyContext.extensions` - /// - Is still unsolved - /// - Isn't a variable - fn live_metas(&self) -> HashSet { - self.extensions - .values() - .filter_map(|m| self.live_var(m)) - .filter(|m| !self.variables.contains(m)) - .collect() + Ok(results) } /// Iterates over a set of metas (the argument) and tries to solve @@ -665,12 +654,16 @@ mod test { use super::*; use crate::builder::test::closed_dfg_root_hugr; + use crate::builder::{DFGBuilder, Dataflow, DataflowHugr}; + use crate::extension::prelude::QB_T; use crate::extension::ExtensionId; use crate::extension::{prelude::PRELUDE_REGISTRY, ExtensionSet}; use crate::hugr::{validate::ValidationError, Hugr, HugrMut, HugrView, NodeType}; use crate::macros::const_extension_ids; - use crate::ops::OpType; + use crate::ops::custom::{ExternalOp, OpaqueOp}; use crate::ops::{self, dataflow::IOTrait, handle::NodeHandle, OpTrait}; + use crate::ops::{LeafOp, OpType}; + use crate::type_row; use crate::types::{FunctionType, Type, TypeRow}; @@ -1539,4 +1532,53 @@ mod test { Ok(()) } + + /// This was stack-overflowing approx 50% of the time, + /// see https://github.com/CQCL/hugr/issues/633 + #[test] + fn plus_on_self() -> Result<(), Box> { + let ext = ExtensionId::new("unknown1").unwrap(); + let delta = ExtensionSet::singleton(&ext); + let ft = FunctionType::new_linear(type_row![QB_T, QB_T]).with_extension_delta(&delta); + let mut dfg = DFGBuilder::new(ft.clone())?; + + // While https://github.com/CQCL-DEV/hugr/issues/388 is unsolved, + // most operations have empty extension_reqs (not including their own extension). + // Define some that do. + let binop: LeafOp = ExternalOp::Opaque(OpaqueOp::new( + ext.clone(), + "2qb_op", + String::new(), + vec![], + Some(ft), + )) + .into(); + let unary_sig = FunctionType::new_linear(type_row![QB_T]) + .with_extension_delta(&ExtensionSet::singleton(&ext)); + let unop: LeafOp = ExternalOp::Opaque(OpaqueOp::new( + ext, + "1qb_op", + String::new(), + vec![], + Some(unary_sig), + )) + .into(); + // Constrain q1,q2 as PLUS(ext1, inputs): + let [q1, q2] = dfg + .add_dataflow_op(binop.clone(), dfg.input_wires())? + .outputs_arr(); + // Constrain q1 as PLUS(ext2, q2): + let [q1] = dfg.add_dataflow_op(unop, [q1])?.outputs_arr(); + // Constrain q1 as EQUALS(q2) by using both together + dfg.finish_hugr_with_outputs([q1, q2], &PRELUDE_REGISTRY)?; + // The combined q1+q2 variable now has two PLUS constraints - on itself and the inputs. + Ok(()) + } + + /// [plus_on_self] had about a 50% rate of failing with stack overflow. + /// So if we run 10 times, that would succeed about 1 run in 2^10, i.e. <0.1% + #[test] + fn plus_on_self_10_times() { + [0; 10].iter().for_each(|_| plus_on_self().unwrap()) + } }