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use ManuallyDrop instead of forget inside collections

Browse Source 
This commit changes some usage of mem::forget into mem::ManuallyDrop
in some Vec, VecDeque, BTreeMap and Box methods.

Before the commit, the generated IR for some of the methods was
longer, and even after optimization, some unwinding artifacts were
still present.
This commit is contained in:
Trevor Spiteri 2020-04-04 14:24:26 +02:00
parent 1b521f5773
commit 2b718e8d9c
5 changed files with 34 additions and 38 deletions
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2
src/liballoc/boxed.rs
View File

@ -469,8 +469,8 @@ impl<T: ?Sized> Box<T> {
#[inline]
#[doc(hidden)]
pub fn into_unique(b: Box<T>) -> Unique<T> {
let b = mem::ManuallyDrop::new(b);
let mut unique = b.0;
mem::forget(b);
// Box is kind-of a library type, but recognized as a "unique pointer" by
// Stacked Borrows. This function here corresponds to "reborrowing to
// a raw pointer", but there is no actual reborrow here -- so

16
src/liballoc/collections/btree/map.rs
View File

@ -4,9 +4,10 @@ use core::fmt::Debug;
use core::hash::{Hash, Hasher};
use core::iter::{FromIterator, FusedIterator, Peekable};
use core::marker::PhantomData;
use core::mem::{self, ManuallyDrop};
use core::ops::Bound::{Excluded, Included, Unbounded};
use core::ops::{Index, RangeBounds};
use core::{fmt, mem, ptr};
use core::{fmt, ptr};
use super::node::{self, marker, ForceResult::*, Handle, InsertResult::*, NodeRef};
use super::search::{self, SearchResult::*};
@ -190,9 +191,9 @@ impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
// We can't destructure subtree directly
// because BTreeMap implements Drop
let (subroot, sublength) = unsafe {
let subtree = ManuallyDrop::new(subtree);
let root = ptr::read(&subtree.root);
let length = subtree.length;
mem::forget(subtree);
(root, length)
};
@ -1515,15 +1516,14 @@ impl<K, V> IntoIterator for BTreeMap<K, V> {
type IntoIter = IntoIter<K, V>;
fn into_iter(self) -> IntoIter<K, V> {
if self.root.is_none() {
mem::forget(self);
let me = ManuallyDrop::new(self);
if me.root.is_none() {
return IntoIter { front: None, back: None, length: 0 };
}
let root1 = unsafe { unwrap_unchecked(ptr::read(&self.root)).into_ref() };
let root2 = unsafe { unwrap_unchecked(ptr::read(&self.root)).into_ref() };
let len = self.length;
mem::forget(self);
let root1 = unsafe { unwrap_unchecked(ptr::read(&me.root)).into_ref() };
let root2 = unsafe { unwrap_unchecked(ptr::read(&me.root)).into_ref() };
let len = me.length;
IntoIter {
front: Some(root1.first_leaf_edge()),

11
src/liballoc/collections/vec_deque.rs
View File

@ -12,7 +12,7 @@ use core::cmp::{self, Ordering};
use core::fmt;
use core::hash::{Hash, Hasher};
use core::iter::{once, repeat_with, FromIterator, FusedIterator};
use core::mem::{self, replace};
use core::mem::{self, replace, ManuallyDrop};
use core::ops::Bound::{Excluded, Included, Unbounded};
use core::ops::{Index, IndexMut, RangeBounds, Try};
use core::ptr::{self, NonNull};
@ -2898,12 +2898,12 @@ impl<T> From<Vec<T>> for VecDeque<T> {
/// This avoids reallocating where possible, but the conditions for that are
/// strict, and subject to change, and so shouldn't be relied upon unless the
/// `Vec<T>` came from `From<VecDeque<T>>` and hasn't been reallocated.
fn from(mut other: Vec<T>) -> Self {
fn from(other: Vec<T>) -> Self {
unsafe {
let mut other = ManuallyDrop::new(other);
let other_buf = other.as_mut_ptr();
let mut buf = RawVec::from_raw_parts(other_buf, other.capacity());
let len = other.len();
mem::forget(other);
// We need to extend the buf if it's not a power of two, too small
// or doesn't have at least one free space
@ -2955,6 +2955,7 @@ impl<T> From<VecDeque<T>> for Vec<T> {
other.make_contiguous();
unsafe {
let other = ManuallyDrop::new(other);
let buf = other.buf.ptr();
let len = other.len();
let cap = other.cap();
@ -2962,9 +2963,7 @@ impl<T> From<VecDeque<T>> for Vec<T> {
if other.head != 0 {
ptr::copy(buf.add(other.tail), buf, len);
}
let out = Vec::from_raw_parts(buf, len, cap);
mem::forget(other);
out
Vec::from_raw_parts(buf, len, cap)
}
}
}

16
src/liballoc/raw_vec.rs
View File

@ -3,7 +3,7 @@
use core::alloc::MemoryBlock;
use core::cmp;
use core::mem::{self, MaybeUninit};
use core::mem::{self, ManuallyDrop, MaybeUninit};
use core::ops::Drop;
use core::ptr::{NonNull, Unique};
use core::slice;
@ -112,11 +112,10 @@ impl<T> RawVec<T, Global> {
}
/// Converts a `Box<[T]>` into a `RawVec<T>`.
pub fn from_box(mut slice: Box<[T]>) -> Self {
pub fn from_box(slice: Box<[T]>) -> Self {
unsafe {
let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
mem::forget(slice);
result
let mut slice = ManuallyDrop::new(slice);
RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len())
}
}
}
@ -579,11 +578,10 @@ impl<T> RawVec<T, Global> {
"`len` must be smaller than or equal to `self.capacity()`"
);
let me = ManuallyDrop::new(self);
// NOTE: not calling `capacity()` here; actually using the real `cap` field!
let slice = slice::from_raw_parts_mut(self.ptr() as *mut MaybeUninit<T>, len);
let output = Box::from_raw(slice);
mem::forget(self);
output
let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
Box::from_raw(slice)
}
}

27
src/liballoc/vec.rs
View File

@ -66,7 +66,7 @@ use core::hash::{self, Hash};
use core::intrinsics::{arith_offset, assume};
use core::iter::{FromIterator, FusedIterator, TrustedLen};
use core::marker::PhantomData;
use core::mem;
use core::mem::{self, ManuallyDrop};
use core::ops::Bound::{Excluded, Included, Unbounded};
use core::ops::{self, Index, IndexMut, RangeBounds};
use core::ptr::{self, NonNull};
@ -392,7 +392,7 @@ impl<T> Vec<T> {
/// ```
#[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
pub fn into_raw_parts(self) -> (*mut T, usize, usize) {
let mut me = mem::ManuallyDrop::new(self);
let mut me = ManuallyDrop::new(self);
(me.as_mut_ptr(), me.len(), me.capacity())
}
@ -678,9 +678,9 @@ impl<T> Vec<T> {
pub fn into_boxed_slice(mut self) -> Box<[T]> {
unsafe {
self.shrink_to_fit();
let buf = ptr::read(&self.buf);
let len = self.len();
mem::forget(self);
let me = ManuallyDrop::new(self);
let buf = ptr::read(&me.buf);
let len = me.len();
buf.into_box(len).assume_init()
}
}
@ -1949,16 +1949,16 @@ impl<T> IntoIterator for Vec<T> {
/// }
/// ```
#[inline]
fn into_iter(mut self) -> IntoIter<T> {
fn into_iter(self) -> IntoIter<T> {
unsafe {
let begin = self.as_mut_ptr();
let mut me = ManuallyDrop::new(self);
let begin = me.as_mut_ptr();
let end = if mem::size_of::<T>() == 0 {
arith_offset(begin as *const i8, self.len() as isize) as *const T
arith_offset(begin as *const i8, me.len() as isize) as *const T
} else {
begin.add(self.len()) as *const T
begin.add(me.len()) as *const T
};
let cap = self.buf.capacity();
mem::forget(self);
let cap = me.buf.capacity();
IntoIter {
buf: NonNull::new_unchecked(begin),
phantom: PhantomData,
@ -2081,9 +2081,8 @@ impl<T> SpecExtend<T, IntoIter<T>> for Vec<T> {
// has not been advanced at all.
if iterator.buf.as_ptr() as *const _ == iterator.ptr {
unsafe {
let vec = Vec::from_raw_parts(iterator.buf.as_ptr(), iterator.len(), iterator.cap);
mem::forget(iterator);
vec
let it = ManuallyDrop::new(iterator);
Vec::from_raw_parts(it.buf.as_ptr(), it.len(), it.cap)
}
} else {
let mut vector = Vec::new();