servo/components/style/rule_tree/mod.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#![allow(unsafe_code)]

//! The rule tree.

use applicable_declarations::ApplicableDeclarationList;
#[cfg(feature = "servo")]
use heapsize::HeapSizeOf;
#[cfg(feature = "gecko")]
use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
use properties::{Importance, LonghandIdSet, PropertyDeclarationBlock};
use servo_arc::{Arc, ArcBorrow, NonZeroPtrMut};
use shared_lock::{Locked, StylesheetGuards, SharedRwLockReadGuard};
use smallvec::SmallVec;
use std::io::{self, Write};
use std::mem;
use std::ptr;
use std::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
use stylesheets::StyleRule;
use thread_state;

/// The rule tree, the structure servo uses to preserve the results of selector
/// matching.
///
/// This is organized as a tree of rules. When a node matches a set of rules,
/// they're inserted in order in the tree, starting with the less specific one.
///
/// When a rule is inserted in the tree, other elements may share the path up to
/// a given rule. If that's the case, we don't duplicate child nodes, but share
/// them.
///
/// When the rule node refcount drops to zero, it doesn't get freed. It gets
/// instead put into a free list, and it is potentially GC'd after a while in a
/// single-threaded fashion.
///
/// That way, a rule node that represents a likely-to-match-again rule (like a
/// :hover rule) can be reused if we haven't GC'd it yet.
///
/// See the discussion at https://github.com/servo/servo/pull/15562 and the IRC
/// logs at http://logs.glob.uno/?c=mozilla%23servo&s=3+Apr+2017&e=3+Apr+2017
/// logs from http://logs.glob.uno/?c=mozilla%23servo&s=3+Apr+2017&e=3+Apr+2017#c644094
/// to se a discussion about the different memory orderings used here.
#[derive(Debug)]
#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
pub struct RuleTree {
    root: StrongRuleNode,
}

impl Drop for RuleTree {
    fn drop(&mut self) {
        // GC the rule tree.
        unsafe { self.gc(); }

        // After the GC, the free list should be empty.
        debug_assert!(self.root.get().next_free.load(Ordering::Relaxed) == FREE_LIST_SENTINEL);

        // Remove the sentinel. This indicates that GCs will no longer occur.
        // Any further drops of StrongRuleNodes must occur on the main thread,
        // and will trigger synchronous dropping of the Rule nodes.
        self.root.get().next_free.store(ptr::null_mut(), Ordering::Relaxed);
    }
}

#[cfg(feature = "gecko")]
impl MallocSizeOf for RuleTree {
    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
        let mut n = ops.malloc_size_of(self.root.ptr());
        n += self.root.get().size_of(ops);
        n
    }
}

/// A style source for the rule node. It can either be a CSS style rule or a
/// declaration block.
///
/// Note that, even though the declaration block from inside the style rule
/// could be enough to implement the rule tree, keeping the whole rule provides
/// more debuggability, and also the ability of show those selectors to
/// devtools.
#[derive(Clone, Debug)]
pub enum StyleSource {
    /// A style rule stable pointer.
    Style(Arc<Locked<StyleRule>>),
    /// A declaration block stable pointer.
    Declarations(Arc<Locked<PropertyDeclarationBlock>>),
    /// Indicates no style source. Used to save an Option wrapper around the stylesource in
    /// RuleNode
    None,
}

impl PartialEq for StyleSource {
    fn eq(&self, other: &Self) -> bool {
        self.ptr_equals(other)
    }
}

impl StyleSource {
    #[inline]
    fn ptr_equals(&self, other: &Self) -> bool {
        use self::StyleSource::*;
        match (self, other) {
            (&Style(ref one), &Style(ref other)) => Arc::ptr_eq(one, other),
            (&Declarations(ref one), &Declarations(ref other)) => Arc::ptr_eq(one, other),
            (&None, _) | (_, &None) => panic!("Should not check for equality between null StyleSource objects"),
            _ => false,
        }
    }

    fn dump<W: Write>(&self, guard: &SharedRwLockReadGuard, writer: &mut W) {
        use self::StyleSource::*;

        if let Style(ref rule) = *self {
            let rule = rule.read_with(guard);
            let _ = write!(writer, "{:?}", rule.selectors);
        }

        let _ = write!(writer, "  -> {:?}", self.read(guard).declarations());
    }

    /// Read the style source guard, and obtain thus read access to the
    /// underlying property declaration block.
    #[inline]
    pub fn read<'a>(&'a self, guard: &'a SharedRwLockReadGuard) -> &'a PropertyDeclarationBlock {
        let block = match *self {
            StyleSource::Style(ref rule) => &rule.read_with(guard).block,
            StyleSource::Declarations(ref block) => block,
            StyleSource::None => panic!("Cannot call read on StyleSource::None"),
        };
        block.read_with(guard)
    }

    /// Indicates if this StyleSource has a value
    pub fn is_some(&self) -> bool {
        match *self {
            StyleSource::None => false,
            _ => true,
        }
    }
}

/// This value exists here so a node that pushes itself to the list can know
/// that is in the free list by looking at is next pointer, and comparing it
/// with null.
///
/// The root node doesn't have a null pointer in the free list, but this value.
const FREE_LIST_SENTINEL: *mut RuleNode = 0x01 as *mut RuleNode;

/// A second sentinel value for the free list, indicating that it's locked (i.e.
/// another thread is currently adding an entry). We spin if we find this value.
const FREE_LIST_LOCKED: *mut RuleNode = 0x02 as *mut RuleNode;

impl RuleTree {
    /// Construct a new rule tree.
    pub fn new() -> Self {
        RuleTree {
            root: StrongRuleNode::new(Box::new(RuleNode::root())),
        }
    }

    /// Get the root rule node.
    pub fn root(&self) -> &StrongRuleNode {
        &self.root
    }

    fn dump<W: Write>(&self, guards: &StylesheetGuards, writer: &mut W) {
        let _ = writeln!(writer, " + RuleTree");
        self.root.get().dump(guards, writer, 0);
    }

    /// Dump the rule tree to stdout.
    pub fn dump_stdout(&self, guards: &StylesheetGuards) {
        let mut stdout = io::stdout();
        self.dump(guards, &mut stdout);
    }

    /// Inserts the given rules, that must be in proper order by specifity, and
    /// returns the corresponding rule node representing the last inserted one.
    ///
    /// !important rules are detected and inserted into the appropriate position
    /// in the rule tree. This allows selector matching to ignore importance,
    /// while still maintaining the appropriate cascade order in the rule tree.
    pub fn insert_ordered_rules_with_important<'a, I>(
        &self,
        iter: I,
        guards: &StylesheetGuards
    ) -> StrongRuleNode
    where
        I: Iterator<Item=(StyleSource, CascadeLevel)>,
    {
        use self::CascadeLevel::*;
        let mut current = self.root.clone();
        let mut last_level = current.get().level;

        let mut found_important = false;
        let mut important_style_attr = None;
        let mut important_author = SmallVec::<[StyleSource; 4]>::new();
        let mut important_user = SmallVec::<[StyleSource; 4]>::new();
        let mut important_ua = SmallVec::<[StyleSource; 4]>::new();
        let mut transition = None;

        for (source, level) in iter {
            debug_assert!(last_level <= level, "Not really ordered");
            debug_assert!(!level.is_important(), "Important levels handled internally");
            let any_important = {
                let pdb = source.read(level.guard(guards));
                pdb.any_important()
            };

            if any_important {
                found_important = true;
                match level {
                    AuthorNormal => important_author.push(source.clone()),
                    UANormal => important_ua.push(source.clone()),
                    UserNormal => important_user.push(source.clone()),
                    StyleAttributeNormal => {
                        debug_assert!(important_style_attr.is_none());
                        important_style_attr = Some(source.clone());
                    },
                    _ => {},
                };
            }

            // We don't optimize out empty rules, even though we could.
            //
            // Inspector relies on every rule being inserted in the normal level
            // at least once, in order to return the rules with the correct
            // specificity order.
            //
            // TODO(emilio): If we want to apply these optimizations without
            // breaking inspector's expectations, we'd need to run
            // selector-matching again at the inspector's request. That may or
            // may not be a better trade-off.
            if matches!(level, Transitions) && found_important {
                // There can be at most one transition, and it will come at
                // the end of the iterator. Stash it and apply it after
                // !important rules.
                debug_assert!(transition.is_none());
                transition = Some(source);
            } else {
                current = current.ensure_child(self.root.downgrade(), source, level);
            }
            last_level = level;
        }

        // Early-return in the common case of no !important declarations.
        if !found_important {
            return current;
        }

        //
        // Insert important declarations, in order of increasing importance,
        // followed by any transition rule.
        //

        for source in important_author.drain() {
            current = current.ensure_child(self.root.downgrade(), source, AuthorImportant);
        }

        if let Some(source) = important_style_attr {
            current = current.ensure_child(self.root.downgrade(), source, StyleAttributeImportant);
        }

        for source in important_user.drain() {
            current = current.ensure_child(self.root.downgrade(), source, UserImportant);
        }

        for source in important_ua.drain() {
            current = current.ensure_child(self.root.downgrade(), source, UAImportant);
        }

        if let Some(source) = transition {
            current = current.ensure_child(self.root.downgrade(), source, Transitions);
        }

        current
    }

    /// Given a list of applicable declarations, insert the rules and return the
    /// corresponding rule node.
    pub fn compute_rule_node(
        &self,
        applicable_declarations: &mut ApplicableDeclarationList,
        guards: &StylesheetGuards
    ) -> StrongRuleNode {
        let rules = applicable_declarations.drain().map(|d| d.order_and_level());
        let rule_node = self.insert_ordered_rules_with_important(rules, guards);
        rule_node
    }

    /// Insert the given rules, that must be in proper order by specifity, and
    /// return the corresponding rule node representing the last inserted one.
    pub fn insert_ordered_rules<'a, I>(&self, iter: I) -> StrongRuleNode
        where I: Iterator<Item=(StyleSource, CascadeLevel)>,
    {
        self.insert_ordered_rules_from(self.root.clone(), iter)
    }

    fn insert_ordered_rules_from<'a, I>(&self,
                                        from: StrongRuleNode,
                                        iter: I) -> StrongRuleNode
        where I: Iterator<Item=(StyleSource, CascadeLevel)>,
    {
        let mut current = from;
        let mut last_level = current.get().level;
        for (source, level) in iter {
            debug_assert!(last_level <= level, "Not really ordered");
            current = current.ensure_child(self.root.downgrade(), source, level);
            last_level = level;
        }
        current
    }

    /// This can only be called when no other threads is accessing this tree.
    pub unsafe fn gc(&self) {
        self.root.gc();
    }

    /// This can only be called when no other threads is accessing this tree.
    pub unsafe fn maybe_gc(&self) {
        self.root.maybe_gc();
    }

    /// Replaces a rule in a given level (if present) for another rule.
    ///
    /// Returns the resulting node that represents the new path, or None if
    /// the old path is still valid.
    pub fn update_rule_at_level(&self,
                                level: CascadeLevel,
                                pdb: Option<ArcBorrow<Locked<PropertyDeclarationBlock>>>,
                                path: &StrongRuleNode,
                                guards: &StylesheetGuards,
                                important_rules_changed: &mut bool)
                                -> Option<StrongRuleNode> {
        debug_assert!(level.is_unique_per_element());
        // TODO(emilio): Being smarter with lifetimes we could avoid a bit of
        // the refcount churn.
        let mut current = path.clone();
        *important_rules_changed = false;

        // First walk up until the first less-or-equally specific rule.
        let mut children = SmallVec::<[_; 10]>::new();
        while current.get().level > level {
            children.push((current.get().source.clone(), current.get().level));
            current = current.parent().unwrap().clone();
        }

        // Then remove the one at the level we want to replace, if any.
        //
        // NOTE: Here we assume that only one rule can be at the level we're
        // replacing.
        //
        // This is certainly true for HTML style attribute rules, animations and
        // transitions, but could not be so for SMIL animations, which we'd need
        // to special-case (isn't hard, it's just about removing the `if` and
        // special cases, and replacing them for a `while` loop, avoiding the
        // optimizations).
        if current.get().level == level {
            *important_rules_changed |= level.is_important();

            if let Some(pdb) = pdb {
                // If the only rule at the level we're replacing is exactly the
                // same as `pdb`, we're done, and `path` is still valid.
                //
                // TODO(emilio): Another potential optimization is the one where
                // we can just replace the rule at that level for `pdb`, and
                // then we don't need to re-create the children, and `path` is
                // also equally valid. This is less likely, and would require an
                // in-place mutation of the source, which is, at best, fiddly,
                // so let's skip it for now.
                let is_here_already = match &current.get().source {
                    &StyleSource::Declarations(ref already_here) => {
                        pdb.with_arc(|arc| Arc::ptr_eq(arc, already_here))
                    },
                    _ => unreachable!("Replacing non-declarations style?"),
                };

                if is_here_already {
                    debug!("Picking the fast path in rule replacement");
                    return None;
                }
            }
            current = current.parent().unwrap().clone();
        }
        debug_assert!(current.get().level != level,
                      "Multiple rules should've been replaced?");

        // Insert the rule if it's relevant at this level in the cascade.
        //
        // These optimizations are likely to be important, because the levels
        // where replacements apply (style and animations) tend to trigger
        // pretty bad styling cases already.
        if let Some(pdb) = pdb {
            if level.is_important() {
                if pdb.read_with(level.guard(guards)).any_important() {
                    current = current.ensure_child(self.root.downgrade(),
                                                   StyleSource::Declarations(pdb.clone_arc()),
                                                   level);
                }
            } else {
                if pdb.read_with(level.guard(guards)).any_normal() {
                    current = current.ensure_child(self.root.downgrade(),
                                                   StyleSource::Declarations(pdb.clone_arc()),
                                                   level);
                }
            }
        }

        // Now the rule is in the relevant place, push the children as
        // necessary.
        let rule =
            self.insert_ordered_rules_from(current, children.drain().rev());
        Some(rule)
    }

    /// Returns new rule nodes without Transitions level rule.
    pub fn remove_transition_rule_if_applicable(&self, path: &StrongRuleNode) -> StrongRuleNode {
        // Return a clone if there is no transition level.
        if path.cascade_level() != CascadeLevel::Transitions {
            return path.clone();
        }

        path.parent().unwrap().clone()
    }

    /// Returns new rule node without rules from declarative animations.
    pub fn remove_animation_rules(&self, path: &StrongRuleNode) -> StrongRuleNode {
        // Return a clone if there are no animation rules.
        if !path.has_animation_or_transition_rules() {
            return path.clone();
        }

        let iter = path.self_and_ancestors().take_while(
            |node| node.cascade_level() >= CascadeLevel::SMILOverride);
        let mut last = path;
        let mut children = SmallVec::<[_; 10]>::new();
        for node in iter {
            if !node.cascade_level().is_animation() {
                children.push((node.get().source.clone(), node.cascade_level()));
            }
            last = node;
        }

        let rule = self.insert_ordered_rules_from(last.parent().unwrap().clone(), children.drain().rev());
        rule
    }
}

/// The number of RuleNodes added to the free list before we will consider
/// doing a GC when calling maybe_gc().  (The value is copied from Gecko,
/// where it likely did not result from a rigorous performance analysis.)
const RULE_TREE_GC_INTERVAL: usize = 300;

/// The cascade level these rules are relevant at, as per[1].
///
/// The order of variants declared here is significant, and must be in
/// _ascending_ order of precedence.
///
/// [1]: https://drafts.csswg.org/css-cascade/#cascade-origin
#[repr(u8)]
#[derive(Clone, Copy, Debug, Eq, PartialEq, PartialOrd)]
#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
pub enum CascadeLevel {
    /// Normal User-Agent rules.
    UANormal = 0,
    /// Presentational hints.
    PresHints,
    /// User normal rules.
    UserNormal,
    /// XBL <stylesheet> rules.
    XBL,
    /// Author normal rules.
    AuthorNormal,
    /// Style attribute normal rules.
    StyleAttributeNormal,
    /// SVG SMIL animations.
    SMILOverride,
    /// CSS animations and script-generated animations.
    Animations,
    /// Author-supplied important rules.
    AuthorImportant,
    /// Style attribute important rules.
    StyleAttributeImportant,
    /// User important rules.
    UserImportant,
    /// User-agent important rules.
    UAImportant,
    /// Transitions
    ///
    /// NB: If this changes from being last, change from_byte below.
    Transitions,
}

impl CascadeLevel {
    /// Converts a raw byte to a CascadeLevel.
    pub unsafe fn from_byte(byte: u8) -> Self {
        debug_assert!(byte <= CascadeLevel::Transitions as u8);
        mem::transmute(byte)
    }

    /// Select a lock guard for this level
    pub fn guard<'a>(&self, guards: &'a StylesheetGuards<'a>) -> &'a SharedRwLockReadGuard<'a> {
        match *self {
            CascadeLevel::UANormal |
            CascadeLevel::UserNormal |
            CascadeLevel::UserImportant |
            CascadeLevel::UAImportant => guards.ua_or_user,
            _ => guards.author,
        }
    }

    /// Returns whether this cascade level is unique per element, in which case
    /// we can replace the path in the cascade without fear.
    pub fn is_unique_per_element(&self) -> bool {
        match *self {
            CascadeLevel::Transitions |
            CascadeLevel::Animations |
            CascadeLevel::SMILOverride |
            CascadeLevel::StyleAttributeNormal |
            CascadeLevel::StyleAttributeImportant => true,
            _ => false,
        }
    }

    /// Returns whether this cascade level represents important rules of some
    /// sort.
    #[inline]
    pub fn is_important(&self) -> bool {
        match *self {
            CascadeLevel::AuthorImportant |
            CascadeLevel::StyleAttributeImportant |
            CascadeLevel::UserImportant |
            CascadeLevel::UAImportant => true,
            _ => false,
        }
    }

    /// Returns the importance relevant for this rule. Pretty similar to
    /// `is_important`.
    #[inline]
    pub fn importance(&self) -> Importance {
        if self.is_important() {
            Importance::Important
        } else {
            Importance::Normal
        }
    }

    /// Returns whether this cascade level represents an animation rules.
    #[inline]
    pub fn is_animation(&self) -> bool {
        match *self {
            CascadeLevel::SMILOverride |
            CascadeLevel::Animations |
            CascadeLevel::Transitions => true,
            _ => false,
        }
    }
}

// The root node never has siblings, but needs a free count. We use the same
// storage for both to save memory.
struct PrevSiblingOrFreeCount(AtomicPtr<RuleNode>);
impl PrevSiblingOrFreeCount {
    fn new() -> Self {
        PrevSiblingOrFreeCount(AtomicPtr::new(ptr::null_mut()))
    }

    unsafe fn as_prev_sibling(&self) -> &AtomicPtr<RuleNode> {
        &self.0
    }

    unsafe fn as_free_count(&self) -> &AtomicUsize {
        unsafe {
            mem::transmute(&self.0)
        }
    }
}

/// A node in the rule tree.
pub struct RuleNode {
    /// The root node. Only the root has no root pointer, for obvious reasons.
    root: Option<WeakRuleNode>,

    /// The parent rule node. Only the root has no parent.
    parent: Option<StrongRuleNode>,

    /// The actual style source, either coming from a selector in a StyleRule,
    /// or a raw property declaration block (like the style attribute).
    source: StyleSource,

    /// The cascade level this rule is positioned at.
    level: CascadeLevel,

    refcount: AtomicUsize,
    first_child: AtomicPtr<RuleNode>,
    next_sibling: AtomicPtr<RuleNode>,

    /// Previous sibling pointer for all non-root nodes.
    ///
    /// For the root, stores the of RuleNodes we have added to the free list
    /// since the last GC. (We don't update this if we rescue a RuleNode from
    /// the free list.  It's just used as a heuristic to decide when to run GC.)
    prev_sibling_or_free_count: PrevSiblingOrFreeCount,

    /// The next item in the rule tree free list, that starts on the root node.
    ///
    /// When this is set to null, that means that the rule tree has been torn
    /// down, and GCs will no longer occur. When this happens, StrongRuleNodes
    /// may only be dropped on the main thread, and teardown happens
    /// synchronously.
    next_free: AtomicPtr<RuleNode>,
}

unsafe impl Sync for RuleTree {}
unsafe impl Send for RuleTree {}

// On Gecko builds, hook into the leak checking machinery.
#[cfg(feature = "gecko")]
#[cfg(debug_assertions)]
mod gecko_leak_checking {
use std::mem::size_of;
use std::os::raw::{c_char, c_void};
use super::RuleNode;

extern "C" {
    pub fn NS_LogCtor(aPtr: *const c_void, aTypeName: *const c_char, aSize: u32);
    pub fn NS_LogDtor(aPtr: *const c_void, aTypeName: *const c_char, aSize: u32);
}

static NAME: &'static [u8] = b"RuleNode\0";

/// Logs the creation of a heap-allocated object to Gecko's leak-checking machinery.
pub fn log_ctor(ptr: *const RuleNode) {
    let s = NAME as *const [u8] as *const u8 as *const c_char;
    unsafe {
        NS_LogCtor(ptr as *const c_void, s, size_of::<RuleNode>() as u32);
    }
}

/// Logs the destruction of a heap-allocated object to Gecko's leak-checking machinery.
pub fn log_dtor(ptr: *const RuleNode) {
    let s = NAME as *const [u8] as *const u8 as *const c_char;
    unsafe {
        NS_LogDtor(ptr as *const c_void, s, size_of::<RuleNode>() as u32);
    }
}

}

#[inline(always)]
fn log_new(_ptr: *const RuleNode) {
    #[cfg(feature = "gecko")]
    #[cfg(debug_assertions)]
    gecko_leak_checking::log_ctor(_ptr);
}

#[inline(always)]
fn log_drop(_ptr: *const RuleNode) {
    #[cfg(feature = "gecko")]
    #[cfg(debug_assertions)]
    gecko_leak_checking::log_dtor(_ptr);
}

impl RuleNode {
    fn new(root: WeakRuleNode,
           parent: StrongRuleNode,
           source: StyleSource,
           level: CascadeLevel) -> Self {
        debug_assert!(root.upgrade().parent().is_none());
        RuleNode {
            root: Some(root),
            parent: Some(parent),
            source: source,
            level: level,
            refcount: AtomicUsize::new(1),
            first_child: AtomicPtr::new(ptr::null_mut()),
            next_sibling: AtomicPtr::new(ptr::null_mut()),
            prev_sibling_or_free_count: PrevSiblingOrFreeCount::new(),
            next_free: AtomicPtr::new(ptr::null_mut()),
        }
    }

    fn root() -> Self {
        RuleNode {
            root: None,
            parent: None,
            source: StyleSource::None,
            level: CascadeLevel::UANormal,
            refcount: AtomicUsize::new(1),
            first_child: AtomicPtr::new(ptr::null_mut()),
            next_sibling: AtomicPtr::new(ptr::null_mut()),
            prev_sibling_or_free_count: PrevSiblingOrFreeCount::new(),
            next_free: AtomicPtr::new(FREE_LIST_SENTINEL),
        }
    }

    fn is_root(&self) -> bool {
        self.parent.is_none()
    }

    fn next_sibling(&self) -> Option<WeakRuleNode> {
        // We use acquire semantics here to ensure proper synchronization while
        // inserting in the child list.
        let ptr = self.next_sibling.load(Ordering::Acquire);
        if ptr.is_null() {
            None
        } else {
            Some(WeakRuleNode::from_ptr(ptr))
        }
    }

    fn prev_sibling(&self) -> &AtomicPtr<RuleNode> {
        debug_assert!(!self.is_root());
        unsafe { self.prev_sibling_or_free_count.as_prev_sibling() }
    }

    fn free_count(&self) -> &AtomicUsize {
        debug_assert!(self.is_root());
        unsafe { self.prev_sibling_or_free_count.as_free_count() }
    }

    /// Remove this rule node from the child list.
    ///
    /// This method doesn't use proper synchronization, and it's expected to be
    /// called in a single-threaded fashion, thus the unsafety.
    ///
    /// This is expected to be called before freeing the node from the free
    /// list.
    unsafe fn remove_from_child_list(&self) {
        debug!("Remove from child list: {:?}, parent: {:?}",
               self as *const RuleNode, self.parent.as_ref().map(|p| p.ptr()));
        // NB: The other siblings we use in this function can also be dead, so
        // we can't use `get` here, since it asserts.
        let prev_sibling =
            self.prev_sibling().swap(ptr::null_mut(), Ordering::Relaxed);

        let next_sibling =
            self.next_sibling.swap(ptr::null_mut(), Ordering::Relaxed);

        // Store the `next` pointer as appropriate, either in the previous
        // sibling, or in the parent otherwise.
        if prev_sibling.is_null() {
            let parent = self.parent.as_ref().unwrap();
            parent.get().first_child.store(next_sibling, Ordering::Relaxed);
        } else {
            let previous = &*prev_sibling;
            previous.next_sibling.store(next_sibling, Ordering::Relaxed);
        }

        // Store the previous sibling pointer in the next sibling if present,
        // otherwise we're done.
        if !next_sibling.is_null() {
            let next = &*next_sibling;
            next.prev_sibling().store(prev_sibling, Ordering::Relaxed);
        }
    }

    fn dump<W: Write>(&self, guards: &StylesheetGuards, writer: &mut W, indent: usize) {
        const INDENT_INCREMENT: usize = 4;

        for _ in 0..indent {
            let _ = write!(writer, " ");
        }

        let _ = writeln!(writer, " - {:?} (ref: {:?}, parent: {:?})",
                         self as *const _, self.refcount.load(Ordering::Relaxed),
                         self.parent.as_ref().map(|p| p.ptr()));

        for _ in 0..indent {
            let _ = write!(writer, " ");
        }

        if self.source.is_some() {
            self.source.dump(self.level.guard(guards), writer);
        } else {
            if indent != 0 {
                warn!("How has this happened?");
            }
            let _ = write!(writer, "(root)");
        }

        let _ = write!(writer, "\n");
        for child in self.iter_children() {
            child.upgrade().get().dump(guards, writer, indent + INDENT_INCREMENT);
        }
    }

    fn iter_children(&self) -> RuleChildrenListIter {
        // See next_sibling to see why we need Acquire semantics here.
        let first_child = self.first_child.load(Ordering::Acquire);
        RuleChildrenListIter {
            current: if first_child.is_null() {
                None
            } else {
                Some(WeakRuleNode::from_ptr(first_child))
            }
        }
    }
}

#[cfg(feature = "gecko")]
impl MallocSizeOf for RuleNode {
    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
        let mut n = 0;
        for child in self.iter_children() {
            n += ops.malloc_size_of(child.ptr());
            n += unsafe { (*child.ptr()).size_of(ops) };
        }
        n
    }
}

#[derive(Clone)]
struct WeakRuleNode {
    p: NonZeroPtrMut<RuleNode>,
}

/// A strong reference to a rule node.
#[derive(Debug, PartialEq)]
pub struct StrongRuleNode {
    p: NonZeroPtrMut<RuleNode>,
}

#[cfg(feature = "servo")]
impl HeapSizeOf for StrongRuleNode {
    fn heap_size_of_children(&self) -> usize { 0 }
}


impl StrongRuleNode {
    fn new(n: Box<RuleNode>) -> Self {
        debug_assert!(n.parent.is_none() == !n.source.is_some());

        let ptr = Box::into_raw(n);
        log_new(ptr);

        debug!("Creating rule node: {:p}", ptr);

        StrongRuleNode::from_ptr(ptr)
    }

    fn from_ptr(ptr: *mut RuleNode) -> Self {
        StrongRuleNode {
            p: NonZeroPtrMut::new(ptr)
        }
    }

    fn downgrade(&self) -> WeakRuleNode {
        WeakRuleNode::from_ptr(self.ptr())
    }

    fn parent(&self) -> Option<&StrongRuleNode> {
        self.get().parent.as_ref()
    }

    fn ensure_child(
        &self,
        root: WeakRuleNode,
        source: StyleSource,
        level: CascadeLevel
    ) -> StrongRuleNode {
        let mut last = None;

        // NB: This is an iterator over _weak_ nodes.
        //
        // It's fine though, because nothing can make us GC while this happens,
        // and this happens to be hot.
        //
        // TODO(emilio): We could actually make this even less hot returning a
        // WeakRuleNode, and implementing this on WeakRuleNode itself...
        for child in self.get().iter_children() {
            let child_node = unsafe { &*child.ptr() };
            if child_node.level == level &&
                child_node.source.ptr_equals(&source) {
                return child.upgrade();
            }
            last = Some(child);
        }

        let mut node = Box::new(RuleNode::new(root,
                                              self.clone(),
                                              source.clone(),
                                              level));
        let new_ptr: *mut RuleNode = &mut *node;

        loop {
            let next;

            {
                let last_node = last.as_ref().map(|l| unsafe { &*l.ptr() });
                let next_sibling_ptr = match last_node {
                    Some(ref l) => &l.next_sibling,
                    None => &self.get().first_child,
                };

                // We use `AqcRel` semantics to ensure the initializing writes
                // in `node` are visible after the swap succeeds.
                let existing =
                    next_sibling_ptr.compare_and_swap(ptr::null_mut(),
                                                      new_ptr,
                                                      Ordering::AcqRel);

                if existing.is_null() {
                    // Now we know we're in the correct position in the child
                    // list, we can set the back pointer, knowing that this will
                    // only be accessed again in a single-threaded manner when
                    // we're sweeping possibly dead nodes.
                    if let Some(ref l) = last {
                        node.prev_sibling().store(l.ptr(), Ordering::Relaxed);
                    }

                    return StrongRuleNode::new(node);
                }

                // Existing is not null: some thread inserted a child node since
                // we accessed `last`.
                next = WeakRuleNode::from_ptr(existing);

                if unsafe { &*next.ptr() }.source.ptr_equals(&source) {
                    // That node happens to be for the same style source, use
                    // that, and let node fall out of scope.
                    return next.upgrade();
                }
            }

            // Try again inserting after the new last child.
            last = Some(next);
        }
    }

    /// Raw pointer to the RuleNode
    pub fn ptr(&self) -> *mut RuleNode {
        self.p.ptr()
    }

    fn get(&self) -> &RuleNode {
        if cfg!(debug_assertions) {
            let node = unsafe { &*self.ptr() };
            assert!(node.refcount.load(Ordering::Relaxed) > 0);
        }
        unsafe { &*self.ptr() }
    }

    /// Get the style source corresponding to this rule node. May return `None`
    /// if it's the root node, which means that the node hasn't matched any
    /// rules.
    pub fn style_source(&self) -> &StyleSource {
        &self.get().source
    }

    /// The cascade level for this node
    pub fn cascade_level(&self) -> CascadeLevel {
        self.get().level
    }

    /// Get the importance that this rule node represents.
    pub fn importance(&self) -> Importance {
        self.get().level.importance()
    }

    /// Get an iterator for this rule node and its ancestors.
    pub fn self_and_ancestors(&self) -> SelfAndAncestors {
        SelfAndAncestors {
            current: Some(self)
        }
    }

    /// Returns whether this node has any child, only intended for testing
    /// purposes, and called on a single-threaded fashion only.
    pub unsafe fn has_children_for_testing(&self) -> bool {
        !self.get().first_child.load(Ordering::Relaxed).is_null()
    }

    unsafe fn pop_from_free_list(&self) -> Option<WeakRuleNode> {
        // NB: This can run from the root node destructor, so we can't use
        // `get()`, since it asserts the refcount is bigger than zero.
        let me = &*self.ptr();

        debug_assert!(me.is_root());

        // FIXME(#14213): Apparently the layout data can be gone from script.
        //
        // That's... suspicious, but it's fine if it happens for the rule tree
        // case, so just don't crash in the case we're doing the final GC in
        // script.

        debug_assert!(!thread_state::get().is_worker() &&
                      (thread_state::get().is_layout() ||
                       thread_state::get().is_script()));

        let current = me.next_free.load(Ordering::Relaxed);
        if current == FREE_LIST_SENTINEL {
            return None;
        }

        debug_assert!(!current.is_null(),
                      "Multiple threads are operating on the free list at the \
                       same time?");
        debug_assert!(current != self.ptr(),
                      "How did the root end up in the free list?");

        let next = (*current).next_free.swap(ptr::null_mut(), Ordering::Relaxed);

        debug_assert!(!next.is_null(),
                      "How did a null pointer end up in the free list?");

        me.next_free.store(next, Ordering::Relaxed);

        debug!("Popping from free list: cur: {:?}, next: {:?}", current, next);

        Some(WeakRuleNode::from_ptr(current))
    }

    unsafe fn assert_free_list_has_no_duplicates_or_null(&self) {
        assert!(cfg!(debug_assertions), "This is an expensive check!");
        use hash::HashSet;

        let me = &*self.ptr();
        assert!(me.is_root());

        let mut current = self.ptr();
        let mut seen = HashSet::new();
        while current != FREE_LIST_SENTINEL {
            let next = (*current).next_free.load(Ordering::Relaxed);
            assert!(!next.is_null());
            assert!(!seen.contains(&next));
            seen.insert(next);

            current = next;
        }
    }

    unsafe fn gc(&self) {
        if cfg!(debug_assertions) {
            self.assert_free_list_has_no_duplicates_or_null();
        }

        // NB: This can run from the root node destructor, so we can't use
        // `get()`, since it asserts the refcount is bigger than zero.
        let me = &*self.ptr();

        debug_assert!(me.is_root(), "Can't call GC on a non-root node!");

        while let Some(weak) = self.pop_from_free_list() {
            let node = &*weak.ptr();
            if node.refcount.load(Ordering::Relaxed) != 0 {
                // Nothing to do, the node is still alive.
                continue;
            }

            debug!("GC'ing {:?}", weak.ptr());
            node.remove_from_child_list();
            log_drop(weak.ptr());
            let _ = Box::from_raw(weak.ptr());
        }

        me.free_count().store(0, Ordering::Relaxed);

        debug_assert!(me.next_free.load(Ordering::Relaxed) == FREE_LIST_SENTINEL);
    }

    unsafe fn maybe_gc(&self) {
        debug_assert!(self.get().is_root(), "Can't call GC on a non-root node!");
        if self.get().free_count().load(Ordering::Relaxed) > RULE_TREE_GC_INTERVAL {
            self.gc();
        }
    }

    /// Implementation of `nsRuleNode::HasAuthorSpecifiedRules` for Servo rule
    /// nodes.
    ///
    /// Returns true if any properties specified by `rule_type_mask` was set by
    /// an author rule.
    #[cfg(feature = "gecko")]
    pub fn has_author_specified_rules<E>(&self,
                                         mut element: E,
                                         guards: &StylesheetGuards,
                                         rule_type_mask: u32,
                                         author_colors_allowed: bool)
        -> bool
        where E: ::dom::TElement
    {
        use gecko_bindings::structs::{NS_AUTHOR_SPECIFIED_BACKGROUND, NS_AUTHOR_SPECIFIED_BORDER};
        use gecko_bindings::structs::{NS_AUTHOR_SPECIFIED_PADDING, NS_AUTHOR_SPECIFIED_TEXT_SHADOW};
        use properties::{CSSWideKeyword, LonghandId, LonghandIdSet};
        use properties::{PropertyDeclaration, PropertyDeclarationId};
        use std::borrow::Cow;
        use values::specified::Color;

        // Reset properties:
        const BACKGROUND_PROPS: &'static [LonghandId] = &[
            LonghandId::BackgroundColor,
            LonghandId::BackgroundImage,
        ];

        const BORDER_PROPS: &'static [LonghandId] = &[
            LonghandId::BorderTopColor,
            LonghandId::BorderTopStyle,
            LonghandId::BorderTopWidth,
            LonghandId::BorderRightColor,
            LonghandId::BorderRightStyle,
            LonghandId::BorderRightWidth,
            LonghandId::BorderBottomColor,
            LonghandId::BorderBottomStyle,
            LonghandId::BorderBottomWidth,
            LonghandId::BorderLeftColor,
            LonghandId::BorderLeftStyle,
            LonghandId::BorderLeftWidth,
            LonghandId::BorderTopLeftRadius,
            LonghandId::BorderTopRightRadius,
            LonghandId::BorderBottomRightRadius,
            LonghandId::BorderBottomLeftRadius,

            LonghandId::BorderInlineStartColor,
            LonghandId::BorderInlineStartStyle,
            LonghandId::BorderInlineStartWidth,
            LonghandId::BorderInlineEndColor,
            LonghandId::BorderInlineEndStyle,
            LonghandId::BorderInlineEndWidth,
            LonghandId::BorderBlockStartColor,
            LonghandId::BorderBlockStartStyle,
            LonghandId::BorderBlockStartWidth,
            LonghandId::BorderBlockEndColor,
            LonghandId::BorderBlockEndStyle,
            LonghandId::BorderBlockEndWidth,
        ];

        const PADDING_PROPS: &'static [LonghandId] = &[
            LonghandId::PaddingTop,
            LonghandId::PaddingRight,
            LonghandId::PaddingBottom,
            LonghandId::PaddingLeft,

            LonghandId::PaddingInlineStart,
            LonghandId::PaddingInlineEnd,
            LonghandId::PaddingBlockStart,
            LonghandId::PaddingBlockEnd,
        ];

        // Inherited properties:
        const TEXT_SHADOW_PROPS: &'static [LonghandId] = &[
            LonghandId::TextShadow,
        ];

        fn inherited(id: LonghandId) -> bool {
            id == LonghandId::TextShadow
        }

        // Set of properties that we are currently interested in.
        let mut properties = LonghandIdSet::new();

        if rule_type_mask & NS_AUTHOR_SPECIFIED_BACKGROUND != 0 {
            for id in BACKGROUND_PROPS {
                properties.insert(*id);
            }
        }
        if rule_type_mask & NS_AUTHOR_SPECIFIED_BORDER != 0 {
            for id in BORDER_PROPS {
                properties.insert(*id);
            }
        }
        if rule_type_mask & NS_AUTHOR_SPECIFIED_PADDING != 0 {
            for id in PADDING_PROPS {
                properties.insert(*id);
            }
        }
        if rule_type_mask & NS_AUTHOR_SPECIFIED_TEXT_SHADOW != 0 {
            for id in TEXT_SHADOW_PROPS {
                properties.insert(*id);
            }
        }

        // If author colors are not allowed, only claim to have author-specified
        // rules if we're looking at a non-color property or if we're looking at
        // the background color and it's set to transparent.
        const IGNORED_WHEN_COLORS_DISABLED: &'static [LonghandId]  = &[
            LonghandId::BackgroundImage,
            LonghandId::BorderTopColor,
            LonghandId::BorderRightColor,
            LonghandId::BorderBottomColor,
            LonghandId::BorderLeftColor,
            LonghandId::BorderInlineStartColor,
            LonghandId::BorderInlineEndColor,
            LonghandId::BorderBlockStartColor,
            LonghandId::BorderBlockEndColor,
            LonghandId::TextShadow,
        ];

        if !author_colors_allowed {
            for id in IGNORED_WHEN_COLORS_DISABLED {
                properties.remove(*id);
            }
        }

        let mut element_rule_node = Cow::Borrowed(self);

        loop {
            // We need to be careful not to count styles covered up by
            // user-important or UA-important declarations.  But we do want to
            // catch explicit inherit styling in those and check our parent
            // element to see whether we have user styling for those properties.
            // Note that we don't care here about inheritance due to lack of a
            // specified value, since all the properties we care about are reset
            // properties.
            //
            // FIXME: The above comment is copied from Gecko, but the last
            // sentence is no longer correct since 'text-shadow' support was
            // added.
            //
            // This is a bug in Gecko, replicated in Stylo for now:
            //
            // https://bugzilla.mozilla.org/show_bug.cgi?id=1363088

            let mut inherited_properties = LonghandIdSet::new();
            let mut have_explicit_ua_inherit = false;

            for node in element_rule_node.self_and_ancestors() {
                let source = node.style_source();
                let declarations = if source.is_some() {
                    source.read(node.cascade_level().guard(guards)).declaration_importance_iter()
                } else {
                    continue
                };

                // Iterate over declarations of the longhands we care about.
                let node_importance = node.importance();
                let longhands = declarations.rev()
                    .filter_map(|(declaration, importance)| {
                        if importance != node_importance { return None }
                        match declaration.id() {
                            PropertyDeclarationId::Longhand(id) => {
                                Some((id, declaration))
                            }
                            _ => None
                        }
                    });

                match node.cascade_level() {
                    // Non-author rules:
                    CascadeLevel::UANormal |
                    CascadeLevel::UAImportant |
                    CascadeLevel::UserNormal |
                    CascadeLevel::UserImportant  => {
                        for (id, declaration) in longhands {
                            if properties.contains(id) {
                                // This property was set by a non-author rule.
                                // Stop looking for it in this element's rule
                                // nodes.
                                properties.remove(id);

                                // However, if it is inherited, then it might be
                                // inherited from an author rule from an
                                // ancestor element's rule nodes.
                                if declaration.get_css_wide_keyword() == Some(CSSWideKeyword::Inherit) ||
                                    (declaration.get_css_wide_keyword() == Some(CSSWideKeyword::Unset) &&
                                     inherited(id))
                                {
                                    have_explicit_ua_inherit = true;
                                    inherited_properties.insert(id);
                                }
                            }
                        }
                    }
                    // Author rules:
                    CascadeLevel::PresHints |
                    CascadeLevel::XBL |
                    CascadeLevel::AuthorNormal |
                    CascadeLevel::StyleAttributeNormal |
                    CascadeLevel::SMILOverride |
                    CascadeLevel::Animations |
                    CascadeLevel::AuthorImportant |
                    CascadeLevel::StyleAttributeImportant |
                    CascadeLevel::Transitions => {
                        for (id, declaration) in longhands {
                            if properties.contains(id) {
                                if !author_colors_allowed {
                                    if let PropertyDeclaration::BackgroundColor(ref color) = *declaration {
                                        return *color == Color::transparent()
                                    }
                                }
                                return true
                            }
                        }
                    }
                }
            }

            if !have_explicit_ua_inherit { break }

            // Continue to the parent element and search for the inherited properties.
            element = match element.inheritance_parent() {
                Some(parent) => parent,
                None => break
            };

            let parent_data = element.mutate_data().unwrap();
            let parent_rule_node = parent_data.styles.primary().rules().clone();
            element_rule_node = Cow::Owned(parent_rule_node);

            properties = inherited_properties;
        }

        false
    }

    /// Returns true if there is either animation or transition level rule.
    pub fn has_animation_or_transition_rules(&self) -> bool {
        self.self_and_ancestors()
            .take_while(|node| node.cascade_level() >= CascadeLevel::SMILOverride)
            .any(|node| node.cascade_level().is_animation())
    }

    /// Get a set of properties whose CascadeLevel are higher than Animations
    /// but not equal to Transitions.
    ///
    /// If there are any custom properties, we set the boolean value of the
    /// returned tuple to true.
    pub fn get_properties_overriding_animations(&self,
                                                guards: &StylesheetGuards)
                                                -> (LonghandIdSet, bool) {
        use properties::PropertyDeclarationId;

        // We want to iterate over cascade levels that override the animations
        // level, i.e.  !important levels and the transitions level.
        //
        // However, we actually want to skip the transitions level because
        // although it is higher in the cascade than animations, when both
        // transitions and animations are present for a given element and
        // property, transitions are suppressed so that they don't actually
        // override animations.
        let iter =
            self.self_and_ancestors()
                .skip_while(|node| node.cascade_level() == CascadeLevel::Transitions)
                .take_while(|node| node.cascade_level() > CascadeLevel::Animations);
        let mut result = (LonghandIdSet::new(), false);
        for node in iter {
            let style = node.style_source();
            for (decl, important) in style.read(node.cascade_level().guard(guards))
                                               .declaration_importance_iter() {
                // Although we are only iterating over cascade levels that
                // override animations, in a given property declaration block we
                // can have a mixture of !important and non-!important
                // declarations but only the !important declarations actually
                // override animations.
                if important.important() {
                    match decl.id() {
                        PropertyDeclarationId::Longhand(id) => result.0.insert(id),
                        PropertyDeclarationId::Custom(_) => result.1 = true
                    }
                }
            }
        }
        result
    }

    /// Returns PropertyDeclarationBlock for this node.
    /// This function must be called only for animation level node.
    fn get_animation_style(&self) -> &Arc<Locked<PropertyDeclarationBlock>> {
        debug_assert!(self.cascade_level().is_animation(),
                      "The cascade level should be an animation level");
        match *self.style_source() {
            StyleSource::Declarations(ref block) => block,
            StyleSource::Style(_) => unreachable!("animating style should not be a style rule"),
            StyleSource::None => unreachable!("animating style should not be none"),
        }
    }

    /// Returns SMIL override declaration block if exists.
    pub fn get_smil_animation_rule(&self) -> Option<&Arc<Locked<PropertyDeclarationBlock>>> {
        if cfg!(feature = "servo") {
            // Servo has no knowledge of a SMIL rule, so just avoid looking for it.
            return None;
        }

        self.self_and_ancestors()
            .take_while(|node| node.cascade_level() >= CascadeLevel::SMILOverride)
            .find(|node| node.cascade_level() == CascadeLevel::SMILOverride)
            .map(|node| node.get_animation_style())
    }
}

/// An iterator over a rule node and its ancestors.
#[derive(Clone)]
pub struct SelfAndAncestors<'a> {
    current: Option<&'a StrongRuleNode>,
}

impl<'a> Iterator for SelfAndAncestors<'a> {
    type Item = &'a StrongRuleNode;

    fn next(&mut self) -> Option<Self::Item> {
        self.current.map(|node| {
            self.current = node.parent();
            node
        })
    }
}

impl Clone for StrongRuleNode {
    fn clone(&self) -> Self {
        debug!("{:?}: {:?}+", self.ptr(), self.get().refcount.load(Ordering::Relaxed));
        debug_assert!(self.get().refcount.load(Ordering::Relaxed) > 0);
        self.get().refcount.fetch_add(1, Ordering::Relaxed);
        StrongRuleNode::from_ptr(self.ptr())
    }
}

impl Drop for StrongRuleNode {
    fn drop(&mut self) {
        let node = unsafe { &*self.ptr() };

        debug!("{:?}: {:?}-", self.ptr(), node.refcount.load(Ordering::Relaxed));
        debug!("Dropping node: {:?}, root: {:?}, parent: {:?}",
               self.ptr(),
               node.root.as_ref().map(|r| r.ptr()),
               node.parent.as_ref().map(|p| p.ptr()));
        let should_drop = {
            debug_assert!(node.refcount.load(Ordering::Relaxed) > 0);
            node.refcount.fetch_sub(1, Ordering::Relaxed) == 1
        };

        if !should_drop {
            return
        }

        debug_assert_eq!(node.first_child.load(Ordering::Acquire),
                         ptr::null_mut());
        if node.parent.is_none() {
            debug!("Dropping root node!");
            // The free list should be null by this point
            debug_assert!(node.next_free.load(Ordering::Relaxed).is_null());
            log_drop(self.ptr());
            let _ = unsafe { Box::from_raw(self.ptr()) };
            return;
        }

        let root = unsafe { &*node.root.as_ref().unwrap().ptr() };
        let free_list = &root.next_free;
        let mut old_head = free_list.load(Ordering::Relaxed);

        // If the free list is null, that means that the rule tree has been
        // formally torn down, and the last standard GC has already occurred.
        // We require that any callers using the rule tree at this point are
        // on the main thread only, which lets us trigger a synchronous GC
        // here to avoid leaking anything. We use the GC machinery, rather
        // than just dropping directly, so that we benefit from the iterative
        // destruction and don't trigger unbounded recursion during drop. See
        // [1] and the associated crashtest.
        //
        // [1] https://bugzilla.mozilla.org/show_bug.cgi?id=439184
        if old_head.is_null() {
            debug_assert!(!thread_state::get().is_worker() &&
                          (thread_state::get().is_layout() ||
                           thread_state::get().is_script()));
            // Add the node as the sole entry in the free list.
            debug_assert!(node.next_free.load(Ordering::Relaxed).is_null());
            node.next_free.store(FREE_LIST_SENTINEL, Ordering::Relaxed);
            free_list.store(node as *const _ as *mut _, Ordering::Relaxed);

            // Invoke the GC.
            //
            // Note that we need hold a strong reference to the root so that it
            // doesn't go away during the GC (which would happen if we're freeing
            // the last external reference into the rule tree). This is nicely
            // enforced by having the gc() method live on StrongRuleNode rather than
            // RuleNode.
            let strong_root: StrongRuleNode = node.root.as_ref().unwrap().upgrade();
            unsafe { strong_root.gc(); }

            // Leave the free list null, like we found it, such that additional
            // drops for straggling rule nodes will take this same codepath.
            debug_assert_eq!(root.next_free.load(Ordering::Relaxed),
                             FREE_LIST_SENTINEL);
            root.next_free.store(ptr::null_mut(), Ordering::Relaxed);

            // Return. If strong_root is the last strong reference to the root,
            // this re-enter StrongRuleNode::drop, and take the root-dropping
            // path earlier in this function.
            return;
        }

        // We're sure we're already in the free list, don't spinloop if we're.
        // Note that this is just a fast path, so it doesn't need to have an
        // strong memory ordering.
        if node.next_free.load(Ordering::Relaxed) != ptr::null_mut() {
            return;
        }

        // Ensure we "lock" the free list head swapping it with FREE_LIST_LOCKED.
        //
        // Note that we use Acquire/Release semantics for the free list
        // synchronization, in order to guarantee that the next_free
        // reads/writes we do below are properly visible from multiple threads
        // racing.
        loop {
            match free_list.compare_exchange_weak(old_head,
                                                  FREE_LIST_LOCKED,
                                                  Ordering::Acquire,
                                                  Ordering::Relaxed) {
                Ok(..) => {
                    if old_head != FREE_LIST_LOCKED {
                        break;
                    }
                },
                Err(new) => old_head = new,
            }
        }

        // If other thread has raced with use while using the same rule node,
        // just store the old head again, we're done.
        //
        // Note that we can use relaxed operations for loading since we're
        // effectively locking the free list with Acquire/Release semantics, and
        // the memory ordering is already guaranteed by that locking/unlocking.
        if node.next_free.load(Ordering::Relaxed) != ptr::null_mut() {
            free_list.store(old_head, Ordering::Release);
            return;
        }

        // Else store the old head as the next pointer, and store ourselves as
        // the new head of the free list.
        //
        // This can be relaxed since this pointer won't be read until GC.
        node.next_free.store(old_head, Ordering::Relaxed);

        // Increment the free count. This doesn't need to be an RMU atomic
        // operation, because the free list is "locked".
        let old_free_count = root.free_count().load(Ordering::Relaxed);
        root.free_count().store(old_free_count + 1, Ordering::Relaxed);

        // This can be release because of the locking of the free list, that
        // ensures that all the other nodes racing with this one are using
        // `Acquire`.
        free_list.store(self.ptr(), Ordering::Release);
    }
}

impl<'a> From<&'a StrongRuleNode> for WeakRuleNode {
    fn from(node: &'a StrongRuleNode) -> Self {
        WeakRuleNode::from_ptr(node.ptr())
    }
}

impl WeakRuleNode {
    fn upgrade(&self) -> StrongRuleNode {
        debug!("Upgrading weak node: {:p}", self.ptr());

        let node = unsafe { &*self.ptr() };
        node.refcount.fetch_add(1, Ordering::Relaxed);
        StrongRuleNode::from_ptr(self.ptr())
    }

    fn from_ptr(ptr: *mut RuleNode) -> Self {
        WeakRuleNode {
            p: NonZeroPtrMut::new(ptr)
        }
    }

    fn ptr(&self) -> *mut RuleNode {
        self.p.ptr()
    }
}

struct RuleChildrenListIter {
    current: Option<WeakRuleNode>,
}

impl Iterator for RuleChildrenListIter {
    type Item = WeakRuleNode;

    fn next(&mut self) -> Option<Self::Item> {
        self.current.take().map(|current| {
            self.current = unsafe { &*current.ptr() }.next_sibling();
            current
        })
    }
}