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Use MaybeUninit in liballoc #54924

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merged 5 commits into from
Oct 12, 2018
Merged

Use MaybeUninit in liballoc #54924

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aeea992
alloc: fix deprecated warnings
japaric Aug 19, 2018
6644e18
address RalfJung's comment
japaric Aug 23, 2018
4d8310c
gdb_rust_pretty_printing: adapt to the changes in the layout of btree…
japaric Sep 1, 2018
faa733d
fix tidy
japaric Sep 1, 2018
e4434be
remove a now outdated comment
RalfJung Oct 10, 2018
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16 changes: 12 additions & 4 deletions src/etc/gdb_rust_pretty_printing.py
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Original file line number Diff line number Diff line change
Expand Up @@ -322,8 +322,11 @@ def to_string(self):
def children(self):
(length, data_ptr) = \
rustpp.extract_length_and_ptr_from_std_btreeset(self.__val)
val = GdbValue(data_ptr.get_wrapped_value().dereference()).get_child_at_index(3)
gdb_ptr = val.get_wrapped_value()
leaf_node = GdbValue(data_ptr.get_wrapped_value().dereference())
maybe_uninit_keys = leaf_node.get_child_at_index(3)
manually_drop_keys = maybe_uninit_keys.get_child_at_index(1)
keys = manually_drop_keys.get_child_at_index(0)
gdb_ptr = keys.get_wrapped_value()
for index in xrange(length):
yield (str(index), gdb_ptr[index])

Expand All @@ -345,9 +348,14 @@ def to_string(self):
def children(self):
(length, data_ptr) = \
rustpp.extract_length_and_ptr_from_std_btreemap(self.__val)
keys = GdbValue(data_ptr.get_wrapped_value().dereference()).get_child_at_index(3)
leaf_node = GdbValue(data_ptr.get_wrapped_value().dereference())
maybe_uninit_keys = leaf_node.get_child_at_index(3)
manually_drop_keys = maybe_uninit_keys.get_child_at_index(1)
keys = manually_drop_keys.get_child_at_index(0)
keys_ptr = keys.get_wrapped_value()
vals = GdbValue(data_ptr.get_wrapped_value().dereference()).get_child_at_index(4)
maybe_uninit_vals = leaf_node.get_child_at_index(4)
manually_drop_vals = maybe_uninit_vals.get_child_at_index(1)
vals = manually_drop_vals.get_child_at_index(0)
vals_ptr = vals.get_wrapped_value()
for index in xrange(length):
yield (str(index), keys_ptr[index])
Expand Down
40 changes: 20 additions & 20 deletions src/liballoc/collections/btree/node.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -42,7 +42,7 @@
// This implies that even an empty internal node has at least one edge.

use core::marker::PhantomData;
use core::mem;
use core::mem::{self, MaybeUninit};
use core::ptr::{self, Unique, NonNull};
use core::slice;

Expand Down Expand Up @@ -73,7 +73,7 @@ struct LeafNode<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`.
/// This is only guaranteed to be initialized when `parent` is nonnull.
parent_idx: u16,
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parent_idx: MaybeUninit<u16>,

/// The number of keys and values this node stores.
///
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Can you confirm that despite introducing MaybeUninit we are still the same size? (i.e., same 32-bit word layout as indicated by the comment on len)?

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MaybeUninit won't introduce any additional alignment requirements that aren't present in the type it wraps, and will only increase the size of the resulting type as a result of inhibiting niche-filling optimizations (and u16 has not niches, so this shouldn't be an issue).

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What would be a good way to test this? We have a way to "hack" a static_assert!, but the size alone won't do it here because there's a ptr right before this, so on 64bit system it'll be 2 ptr-words in size at least. And we cannot yet do an offset! macro in CTFE. I could add a debug_assert! to test the offset at run-time?

However, I agree with what @cramertj said and this is repr(C), so I am not expecting any surprises. Still, would be nice to be sure.

Expand All @@ -83,8 +83,8 @@ 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],
keys: MaybeUninit<[K; CAPACITY]>,
vals: MaybeUninit<[V; CAPACITY]>,
}

impl<K, V> LeafNode<K, V> {
Expand All @@ -94,10 +94,10 @@ impl<K, V> LeafNode<K, V> {
LeafNode {
// As a general policy, we leave fields uninitialized if they can be, as this should
// be both slightly faster and easier to track in Valgrind.
keys: mem::uninitialized(),
vals: mem::uninitialized(),
keys: MaybeUninit::uninitialized(),
vals: MaybeUninit::uninitialized(),
parent: ptr::null(),
parent_idx: mem::uninitialized(),
parent_idx: MaybeUninit::uninitialized(),
len: 0
}
}
Expand All @@ -115,10 +115,10 @@ unsafe impl Sync for LeafNode<(), ()> {}
// ever take a pointer past the first key.
static EMPTY_ROOT_NODE: LeafNode<(), ()> = LeafNode {
parent: ptr::null(),
parent_idx: 0,
parent_idx: MaybeUninit::uninitialized(),
len: 0,
keys: [(); CAPACITY],
vals: [(); CAPACITY],
keys: MaybeUninit::uninitialized(),
vals: MaybeUninit::uninitialized(),
};

/// The underlying representation of internal nodes. As with `LeafNode`s, these should be hidden
Expand Down Expand Up @@ -430,7 +430,7 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
root: self.root,
_marker: PhantomData
},
idx: self.as_leaf().parent_idx as usize,
idx: unsafe { usize::from(*self.as_leaf().parent_idx.get_ref()) },
_marker: PhantomData
})
} else {
Expand Down Expand Up @@ -567,7 +567,7 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Immut<'a>, K, V, Type> {
// the node, which is allowed by LLVM.
unsafe {
slice::from_raw_parts(
self.as_leaf().keys.as_ptr(),
self.as_leaf().keys.as_ptr() as *const K,
self.len()
)
}
Expand All @@ -578,7 +578,7 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Immut<'a>, K, V, Type> {
debug_assert!(!self.is_shared_root());
unsafe {
slice::from_raw_parts(
self.as_leaf().vals.as_ptr(),
self.as_leaf().vals.as_ptr() as *const V,
self.len()
)
}
Expand All @@ -605,7 +605,7 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
} else {
unsafe {
slice::from_raw_parts_mut(
&mut self.as_leaf_mut().keys as *mut [K] as *mut K,
self.as_leaf_mut().keys.get_mut() as *mut [K] as *mut K,
self.len()
)
}
Expand All @@ -616,7 +616,7 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
debug_assert!(!self.is_shared_root());
unsafe {
slice::from_raw_parts_mut(
&mut self.as_leaf_mut().vals as *mut [V] as *mut V,
self.as_leaf_mut().vals.get_mut() as *mut [V] as *mut V,
self.len()
)
}
Expand Down Expand Up @@ -1013,7 +1013,7 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::
let ptr = self.node.as_internal_mut() as *mut _;
let mut child = self.descend();
child.as_leaf_mut().parent = ptr;
child.as_leaf_mut().parent_idx = idx;
child.as_leaf_mut().parent_idx.set(idx);
}

/// Unsafely asserts to the compiler some static information about whether the underlying
Expand Down Expand Up @@ -1152,12 +1152,12 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::KV>

ptr::copy_nonoverlapping(
self.node.keys().as_ptr().add(self.idx + 1),
new_node.keys.as_mut_ptr(),
new_node.keys.as_mut_ptr() as *mut K,
new_len
);
ptr::copy_nonoverlapping(
self.node.vals().as_ptr().add(self.idx + 1),
new_node.vals.as_mut_ptr(),
new_node.vals.as_mut_ptr() as *mut V,
new_len
);

Expand Down Expand Up @@ -1210,12 +1210,12 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::

ptr::copy_nonoverlapping(
self.node.keys().as_ptr().add(self.idx + 1),
new_node.data.keys.as_mut_ptr(),
new_node.data.keys.as_mut_ptr() as *mut K,
new_len
);
ptr::copy_nonoverlapping(
self.node.vals().as_ptr().add(self.idx + 1),
new_node.data.vals.as_mut_ptr(),
new_node.data.vals.as_mut_ptr() as *mut V,
new_len
);
ptr::copy_nonoverlapping(
Expand Down
1 change: 1 addition & 0 deletions src/liballoc/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -119,6 +119,7 @@
#![feature(rustc_const_unstable)]
#![feature(const_vec_new)]
#![feature(slice_partition_dedup)]
#![feature(maybe_uninit)]

// Allow testing this library

Expand Down