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Remove recursion from sccc walking
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This allows constructing the sccc for large that visit many nodes before
finding a single cycle of sccc, for example lists. When used to find
dependencies in borrow checking the list case is what occurs in very
long functions.
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@HeroicKatora
HeroicKatora committed Nov 8, 2020
1 parent 355904d commit eb597f5
Showing 1 changed file with 182 additions and 73 deletions.
255 changes: 182 additions & 73 deletions compiler/rustc_data_structures/src/graph/scc/mod.rs
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Expand Up @@ -231,20 +231,30 @@ where

let scc_indices = (0..num_nodes)
.map(G::Node::new)
.map(|node| match this.walk_node(0, node) {
.map(|node| match this.start_walk_from(node) {
WalkReturn::Complete { scc_index } => scc_index,
WalkReturn::Cycle { min_depth } => {
panic!("`walk_node(0, {:?})` returned cycle with depth {:?}", node, min_depth)
}
WalkReturn::Cycle { min_depth } => panic!(
"`start_walk_node({:?})` returned cycle with depth {:?}",
node, min_depth
),
})
.collect();

Sccs { scc_indices, scc_data: this.scc_data }
}

/// Visits a node during the DFS. We first examine its current
/// state -- if it is not yet visited (`NotVisited`), we can push
/// it onto the stack and start walking its successors.
fn start_walk_from(&mut self, node: G::Node) -> WalkReturn<S> {
if let Some(result) = self.inspect_node(node) {
result
} else {
self.walk_unvisited_node(node)
}
}

/// Inspect a node during the DFS. We first examine its current
/// state -- if it is not yet visited (`NotVisited`), return `None` so
/// that the caller might push it onto the stack and start walking its
/// successors.
///
/// If it is already on the DFS stack it will be in the state
/// `BeingVisited`. In that case, we have found a cycle and we
Expand All @@ -253,20 +263,19 @@ where
/// Otherwise, we are looking at a node that has already been
/// completely visited. We therefore return `WalkReturn::Complete`
/// with its associated SCC index.
fn walk_node(&mut self, depth: usize, node: G::Node) -> WalkReturn<S> {
debug!("walk_node(depth = {:?}, node = {:?})", depth, node);
match self.find_state(node) {
fn inspect_node(&mut self, node: G::Node) -> Option<WalkReturn<S>> {
Some(match self.find_state(node) {
NodeState::InCycle { scc_index } => WalkReturn::Complete { scc_index },

NodeState::BeingVisited { depth: min_depth } => WalkReturn::Cycle { min_depth },

NodeState::NotVisited => self.walk_unvisited_node(depth, node),
NodeState::NotVisited => return None,

NodeState::InCycleWith { parent } => panic!(
"`find_state` returned `InCycleWith({:?})`, which ought to be impossible",
parent
),
}
})
}

/// Fetches the state of the node `r`. If `r` is recorded as being
Expand Down Expand Up @@ -392,74 +401,174 @@ where
}

/// Walks a node that has never been visited before.
fn walk_unvisited_node(&mut self, depth: usize, node: G::Node) -> WalkReturn<S> {
debug!("walk_unvisited_node(depth = {:?}, node = {:?})", depth, node);

debug_assert!(matches!(self.node_states[node], NodeState::NotVisited));

// Push `node` onto the stack.
self.node_states[node] = NodeState::BeingVisited { depth };
self.node_stack.push(node);

// Walk each successor of the node, looking to see if any of
// them can reach a node that is presently on the stack. If
// so, that means they can also reach us.
let mut min_depth = depth;
let mut min_cycle_root = node;
let successors_len = self.successors_stack.len();
for successor_node in self.graph.successors(node) {
debug!("walk_unvisited_node: node = {:?} successor_ode = {:?}", node, successor_node);
match self.walk_node(depth + 1, successor_node) {
WalkReturn::Cycle { min_depth: successor_min_depth } => {
// Track the minimum depth we can reach.
assert!(successor_min_depth <= depth);
if successor_min_depth < min_depth {
///
/// Call this method when `inspect_node` has returned `None`. Having the
/// caller decide avoids mutual recursion between the two methods and allows
/// us to maintain an allocated stack for nodes on the path between calls.
fn walk_unvisited_node(&mut self, initial: G::Node) -> WalkReturn<S> {
struct VisitingNodeFrame<G: DirectedGraph, Successors> {
node: G::Node,
iter: Option<Successors>,
depth: usize,
min_depth: usize,
successors_len: usize,
min_cycle_root: G::Node,
successor_node: G::Node,
}

// Move the stack to a local variable. We want to utilize the existing allocation and
// mutably borrow it without borrowing self at the same time.
let mut successors_stack = core::mem::take(&mut self.successors_stack);
debug_assert_eq!(successors_stack.len(), 0);

let mut stack: Vec<VisitingNodeFrame<G, _>> = vec![VisitingNodeFrame {
node: initial,
depth: 0,
min_depth: 0,
iter: None,
successors_len: 0,
min_cycle_root: initial,
successor_node: initial,
}];

let mut return_value = None;

'recurse: while let Some(frame) = stack.last_mut() {
let VisitingNodeFrame {
node,
depth,
iter,
successors_len,
min_depth,
min_cycle_root,
successor_node,
} = frame;

let node = *node;
let depth = *depth;

let successors = match iter {
Some(iter) => iter,
None => {
// This None marks that we still have the initialize this node's frame.
debug!("walk_unvisited_node(depth = {:?}, node = {:?})", depth, node);

debug_assert!(matches!(self.node_states[node], NodeState::NotVisited));

// Push `node` onto the stack.
self.node_states[node] = NodeState::BeingVisited { depth };
self.node_stack.push(node);

// Walk each successor of the node, looking to see if any of
// them can reach a node that is presently on the stack. If
// so, that means they can also reach us.
*successors_len = successors_stack.len();
// Set and return a reference, this is currently empty.
iter.get_or_insert(self.graph.successors(node))
}
};

// Now that iter is initialized, this is a constant for this frame.
let successors_len = *successors_len;

// Construct iterators for the nodes and walk results. There are two cases:
// * The walk of a successor node returned.
// * The remaining successor nodes.
let returned_walk =
return_value.take().into_iter().map(|walk| (*successor_node, Some(walk)));

let successor_walk = successors.by_ref().map(|successor_node| {
debug!(
"walk_unvisited_node: node = {:?} successor_ode = {:?}",
node, successor_node
);
(successor_node, self.inspect_node(successor_node))
});

for (successor_node, walk) in returned_walk.chain(successor_walk) {
match walk {
Some(WalkReturn::Cycle { min_depth: successor_min_depth }) => {
// Track the minimum depth we can reach.
assert!(successor_min_depth <= depth);
if successor_min_depth < *min_depth {
debug!(
"walk_unvisited_node: node = {:?} successor_min_depth = {:?}",
node, successor_min_depth
);
*min_depth = successor_min_depth;
*min_cycle_root = successor_node;
}
}

Some(WalkReturn::Complete { scc_index: successor_scc_index }) => {
// Push the completed SCC indices onto
// the `successors_stack` for later.
debug!(
"walk_unvisited_node: node = {:?} successor_min_depth = {:?}",
node, successor_min_depth
"walk_unvisited_node: node = {:?} successor_scc_index = {:?}",
node, successor_scc_index
);
min_depth = successor_min_depth;
min_cycle_root = successor_node;
successors_stack.push(successor_scc_index);
}
}

WalkReturn::Complete { scc_index: successor_scc_index } => {
// Push the completed SCC indices onto
// the `successors_stack` for later.
debug!(
"walk_unvisited_node: node = {:?} successor_scc_index = {:?}",
node, successor_scc_index
);
self.successors_stack.push(successor_scc_index);
None => {
let depth = depth + 1;
debug!("walk_node(depth = {:?}, node = {:?})", depth, successor_node);
// Remember which node the return value will come from.
frame.successor_node = successor_node;
// Start a new stack frame the step into it.
stack.push(VisitingNodeFrame {
node: successor_node,
depth,
iter: None,
successors_len: 0,
min_depth: depth,
min_cycle_root: successor_node,
successor_node: successor_node,
});
continue 'recurse;
}
}
}
}

// Completed walk, remove `node` from the stack.
let r = self.node_stack.pop();
debug_assert_eq!(r, Some(node));

// If `min_depth == depth`, then we are the root of the
// cycle: we can't reach anyone further down the stack.
if min_depth == depth {
// Note that successor stack may have duplicates, so we
// want to remove those:
let deduplicated_successors = {
let duplicate_set = &mut self.duplicate_set;
duplicate_set.clear();
self.successors_stack
.drain(successors_len..)
.filter(move |&i| duplicate_set.insert(i))
};
let scc_index = self.scc_data.create_scc(deduplicated_successors);
self.node_states[node] = NodeState::InCycle { scc_index };
WalkReturn::Complete { scc_index }
} else {
// We are not the head of the cycle. Return back to our
// caller. They will take ownership of the
// `self.successors` data that we pushed.
self.node_states[node] = NodeState::InCycleWith { parent: min_cycle_root };
WalkReturn::Cycle { min_depth }
// Completed walk, remove `node` from the stack.
let r = self.node_stack.pop();
debug_assert_eq!(r, Some(node));

// Remove the frame, it's done.
let frame = stack.pop().unwrap();

// If `min_depth == depth`, then we are the root of the
// cycle: we can't reach anyone further down the stack.

// Pass the 'return value' down the stack.
// We return one frame at a time so there can't be another return value.
debug_assert!(return_value.is_none());
return_value = Some(if frame.min_depth == depth {
// Note that successor stack may have duplicates, so we
// want to remove those:
let deduplicated_successors = {
let duplicate_set = &mut self.duplicate_set;
duplicate_set.clear();
successors_stack
.drain(successors_len..)
.filter(move |&i| duplicate_set.insert(i))
};
let scc_index = self.scc_data.create_scc(deduplicated_successors);
self.node_states[node] = NodeState::InCycle { scc_index };
WalkReturn::Complete { scc_index }
} else {
// We are not the head of the cycle. Return back to our
// caller. They will take ownership of the
// `self.successors` data that we pushed.
self.node_states[node] = NodeState::InCycleWith { parent: frame.min_cycle_root };
WalkReturn::Cycle { min_depth: frame.min_depth }
});
}

// Keep the allocation we used for successors_stack.
self.successors_stack = successors_stack;
debug_assert_eq!(self.successors_stack.len(), 0);

return_value.unwrap()
}
}

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