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Make LeafNode #[repr(C)] and put the metadata before generic items

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This way we can safely statically allocate a LeafNode to use as the
placeholder before allocating, and any type accessing it will be able to
access the metadata at the same offset.
This commit is contained in:
C Jones 2018-04-30 15:24:59 -04:00
parent 64e6dda0bc
commit 669bd8223b
1 changed files with 12 additions and 8 deletions
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20
src/liballoc/btree/node.rs
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@ -60,12 +60,12 @@ pub const CAPACITY: usize = 2 * B - 1;
///
/// See also rust-lang/rfcs#197, which would make this structure significantly more safe by
/// avoiding accidentally dropping unused and uninitialized keys and values.
///
/// We put the metadata first so that its position is the same for every `K` and `V`, in order
/// to statically allocate a single dummy node to avoid allocations. This struct is `repr(C)` to
/// prevent them from being reordered.
#[repr(C)]
struct LeafNode<K, V> {
/// The arrays storing the actual data of the node. Only the first `len` elements of each
/// array are initialized and valid.
keys: [K; CAPACITY],
vals: [V; CAPACITY],
/// 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>,
@ -77,10 +77,14 @@ struct LeafNode<K, V> {
/// The number of keys and values this node stores.
///
/// This is at the end of the node's representation and next to `parent_idx` to encourage
/// the compiler to join `len` and `parent_idx` into the same 32-bit word, reducing space
/// overhead.
/// This next to `parent_idx` to encourage the compiler to join `len` and
/// `parent_idx` into the same 32-bit word, reducing space overhead.
len: u16,
/// The arrays storing the actual data of the node. Only the first `len` elements of each
/// array are initialized and valid.
keys: [K; CAPACITY],
vals: [V; CAPACITY],
}
impl<K, V> LeafNode<K, V> {