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BTreeMap: wrap node's raw parent pointer in NonNull #76929

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Sep 19, 2020
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BTreeMap: wrap node's raw parent pointer in NonNull #76929

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42 changes: 22 additions & 20 deletions library/alloc/src/collections/btree/node.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@
// edges: if height > 0 {
// [Box<Node<K, V, height - 1>>; 2 * B]
// } else { () },
// parent: *const Node<K, V, height + 1>,
// parent: Option<NonNull<Node<K, V, height + 1>>>,
// parent_idx: u16,
// len: u16,
// }
Expand Down Expand Up @@ -50,9 +50,8 @@ const EDGE_IDX_RIGHT_OF_CENTER: usize = B;
/// The underlying representation of leaf nodes.
#[repr(C)]
struct LeafNode<K, V> {
/// We use `*const` as opposed to `*mut` so as to be covariant in `K` and `V`.
/// This either points to an actual node or is null.
parent: *const InternalNode<K, V>,
/// We want to be covariant in `K` and `V`.
parent: Option<NonNull<InternalNode<K, V>>>,

/// This node's index into the parent node's `edges` array.
/// `*node.parent.edges[node.parent_idx]` should be the same thing as `node`.
Expand Down Expand Up @@ -80,7 +79,7 @@ impl<K, V> LeafNode<K, V> {
// be both slightly faster and easier to track in Valgrind.
keys: MaybeUninit::uninit_array(),
vals: MaybeUninit::uninit_array(),
parent: ptr::null(),
parent: None,
parent_idx: MaybeUninit::uninit(),
len: 0,
}
Expand Down Expand Up @@ -224,7 +223,7 @@ impl<K, V> Root<K, V> {
)
};
self.height -= 1;
self.node_as_mut().as_leaf_mut().parent = ptr::null();
self.node_as_mut().as_leaf_mut().parent = None;

unsafe {
Global.dealloc(NonNull::from(top).cast(), Layout::new::<InternalNode<K, V>>());
Expand Down Expand Up @@ -309,7 +308,7 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
pub fn len(&self) -> usize {
// Crucially, we only access the `len` field here. If BorrowType is marker::ValMut,
// there might be outstanding mutable references to values that we must not invalidate.
unsafe { (*self.as_leaf_ptr()).len as usize }
unsafe { usize::from((*self.as_leaf_ptr()).len) }
}

/// Returns the height of this node in the whole tree. Zero height denotes the
Expand Down Expand Up @@ -365,16 +364,19 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
) -> Result<Handle<NodeRef<BorrowType, K, V, marker::Internal>, marker::Edge>, Self> {
// We need to use raw pointers to nodes because, if BorrowType is marker::ValMut,
// there might be outstanding mutable references to values that we must not invalidate.
let parent_as_leaf = unsafe { (*self.as_leaf_ptr()).parent as *const LeafNode<K, V> };
if let Some(non_zero) = NonNull::new(parent_as_leaf as *mut _) {
Ok(Handle {
node: NodeRef { height: self.height + 1, node: non_zero, _marker: PhantomData },
idx: unsafe { usize::from(*(*self.as_leaf_ptr()).parent_idx.as_ptr()) },
let leaf_ptr = self.as_leaf_ptr();
unsafe { (*leaf_ptr).parent }
.as_ref()
.map(|parent| Handle {
node: NodeRef {
height: self.height + 1,
node: parent.cast(),
_marker: PhantomData,
},
idx: unsafe { usize::from((*leaf_ptr).parent_idx.assume_init()) },
_marker: PhantomData,
})
} else {
Err(self)
}
.ok_or(self)
}

pub fn first_edge(self) -> Handle<Self, marker::Edge> {
Expand Down Expand Up @@ -572,7 +574,7 @@ impl<'a, K: 'a, V: 'a> NodeRef<marker::Mut<'a>, K, V, marker::Leaf> {
/// Adds a key/value pair to the end of the node.
pub fn push(&mut self, key: K, val: V) {
let len = &mut self.as_leaf_mut().len;
let idx = *len as usize;
let idx = usize::from(*len);
assert!(idx < CAPACITY);
*len += 1;
unsafe {
Expand Down Expand Up @@ -617,7 +619,7 @@ impl<'a, K: 'a, V: 'a> NodeRef<marker::Mut<'a>, K, V, marker::Internal> {
assert!(edge.height == self.height - 1);

let len = &mut self.as_leaf_mut().len;
let idx = *len as usize;
let idx = usize::from(*len);
assert!(idx < CAPACITY);
*len += 1;
unsafe {
Expand Down Expand Up @@ -672,7 +674,7 @@ impl<'a, K: 'a, V: 'a> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
let edge =
ptr::read(internal.as_internal().edges.get_unchecked(idx + 1).as_ptr());
let mut new_root = Root { node: edge, height: internal.height - 1 };
new_root.node_as_mut().as_leaf_mut().parent = ptr::null();
new_root.node_as_mut().as_leaf_mut().parent = None;
Some(new_root)
}
};
Expand Down Expand Up @@ -704,7 +706,7 @@ impl<'a, K: 'a, V: 'a> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
);

let mut new_root = Root { node: edge, height: internal.height - 1 };
new_root.node_as_mut().as_leaf_mut().parent = ptr::null();
new_root.node_as_mut().as_leaf_mut().parent = None;

for i in 0..old_len {
Handle::new_edge(internal.reborrow_mut(), i).correct_parent_link();
Expand Down Expand Up @@ -956,7 +958,7 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::
/// when the ordering of edges has been changed, such as in the various `insert` methods.
fn correct_parent_link(mut self) {
let idx = self.idx as u16;
let ptr = self.node.as_internal_mut() as *mut _;
let ptr = NonNull::new(self.node.as_internal_mut());
let mut child = self.descend();
child.as_leaf_mut().parent = ptr;
child.as_leaf_mut().parent_idx.write(idx);
Expand Down