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Implement in-place growth for RawVec

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This commit is contained in:
Piotr Czarnecki 2015-08-12 05:53:58 +02:00
parent d4b67cd7cc
commit 2674b2ca98
1 changed files with 107 additions and 11 deletions
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118
src/liballoc/raw_vec.rs
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@ -240,6 +240,47 @@ impl<T> RawVec<T> {
}
}
/// Attempts to double the size of the type's backing allocation in place. This is common
/// enough to want to do that it's easiest to just have a dedicated method. Slightly
/// more efficient logic can be provided for this than the general case.
///
/// Returns true if the reallocation attempt has succeeded, or false otherwise.
///
/// # Panics
///
/// * Panics if T is zero-sized on the assumption that you managed to exhaust
/// all `usize::MAX` slots in your imaginary buffer.
/// * Panics on 32-bit platforms if the requested capacity exceeds
/// `isize::MAX` bytes.
#[inline(never)]
#[cold]
pub fn double_in_place(&mut self) -> bool {
unsafe {
let elem_size = mem::size_of::<T>();
let align = mem::align_of::<T>();
// since we set the capacity to usize::MAX when elem_size is
// 0, getting to here necessarily means the RawVec is overfull.
assert!(elem_size != 0, "capacity overflow");
// Since we guarantee that we never allocate more than isize::MAX bytes,
// `elem_size * self.cap <= isize::MAX` as a precondition, so this can't overflow
let new_cap = 2 * self.cap;
let new_alloc_size = new_cap * elem_size;
alloc_guard(new_alloc_size);
let size = heap::reallocate_inplace(self.ptr() as *mut _,
self.cap * elem_size,
new_alloc_size,
align);
if size >= new_alloc_size {
// We can't directly divide `size`.
self.cap = new_cap;
}
size >= new_alloc_size
}
}
/// Ensures that the buffer contains at least enough space to hold
/// `used_cap + needed_extra_cap` elements. If it doesn't already,
/// will reallocate the minimum possible amount of memory necessary.
@ -300,6 +341,22 @@ impl<T> RawVec<T> {
}
}
/// Calculates the buffer's new size given that it'll hold `used_cap +
/// needed_extra_cap` elements. This logic is used in amortized reserve methods.
/// Returns `(new_capacity, new_alloc_size)`.
fn amortized_new_size(&self, used_cap: usize, needed_extra_cap: usize) -> (usize, usize) {
let elem_size = mem::size_of::<T>();
// Nothing we can really do about these checks :(
let required_cap = used_cap.checked_add(needed_extra_cap)
.expect("capacity overflow");
// Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
let double_cap = self.cap * 2;
// `double_cap` guarantees exponential growth.
let new_cap = cmp::max(double_cap, required_cap);
let new_alloc_size = new_cap.checked_mul(elem_size).expect("capacity overflow");
(new_cap, new_alloc_size)
}
/// Ensures that the buffer contains at least enough space to hold
/// `used_cap + needed_extra_cap` elements. If it doesn't already have
/// enough capacity, will reallocate enough space plus comfortable slack
@ -360,17 +417,7 @@ impl<T> RawVec<T> {
return;
}
// Nothing we can really do about these checks :(
let required_cap = used_cap.checked_add(needed_extra_cap)
.expect("capacity overflow");
// Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
let double_cap = self.cap * 2;
// `double_cap` guarantees exponential growth.
let new_cap = cmp::max(double_cap, required_cap);
let new_alloc_size = new_cap.checked_mul(elem_size).expect("capacity overflow");
let (new_cap, new_alloc_size) = self.amortized_new_size(used_cap, needed_extra_cap);
// FIXME: may crash and burn on over-reserve
alloc_guard(new_alloc_size);
@ -393,6 +440,55 @@ impl<T> RawVec<T> {
}
}
/// Attempts to ensure that the buffer contains at least enough space to hold
/// `used_cap + needed_extra_cap` elements. If it doesn't already have
/// enough capacity, will reallocate in place enough space plus comfortable slack
/// space to get amortized `O(1)` behaviour. Will limit this behaviour
/// if it would needlessly cause itself to panic.
///
/// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
/// the requested space. This is not really unsafe, but the unsafe
/// code *you* write that relies on the behaviour of this function may break.
///
/// Returns true if the reallocation attempt has succeeded, or false otherwise.
///
/// # Panics
///
/// * Panics if the requested capacity exceeds `usize::MAX` bytes.
/// * Panics on 32-bit platforms if the requested capacity exceeds
/// `isize::MAX` bytes.
pub fn reserve_in_place(&mut self, used_cap: usize, needed_extra_cap: usize) -> bool {
unsafe {
let elem_size = mem::size_of::<T>();
let align = mem::align_of::<T>();
// NOTE: we don't early branch on ZSTs here because we want this
// to actually catch "asking for more than usize::MAX" in that case.
// If we make it past the first branch then we are guaranteed to
// panic.
// Don't actually need any more capacity. If the current `cap` is 0, we can't
// reallocate in place.
// Wrapping in case they give a bad `used_cap`
if self.cap().wrapping_sub(used_cap) >= needed_extra_cap || self.cap == 0 {
return false;
}
let (_, new_alloc_size) = self.amortized_new_size(used_cap, needed_extra_cap);
// FIXME: may crash and burn on over-reserve
alloc_guard(new_alloc_size);
let size = heap::reallocate_inplace(self.ptr() as *mut _,
self.cap * elem_size,
new_alloc_size,
align);
if size >= new_alloc_size {
self.cap = new_alloc_size / elem_size;
}
size >= new_alloc_size
}
}
/// Shrinks the allocation down to the specified amount. If the given amount
/// is 0, actually completely deallocates.
///