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Remove RawVec::double.

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It's only used once, for `VecDeque`, and can easily be replaced by
something else. The commit changes `grow_if_necessary` to `grow` to
avoid some small regressions caused by changed inlining.

The commit also removes `Strategy::Double`, and streamlines the
remaining variants of `Strategy`.

It's a compile time win on some benchmarks because the many
instantations of `RawVec::grow` are a little smaller.
This commit is contained in:
Nicholas Nethercote 2020-05-11 12:26:59 +10:00
parent a3cc435f57
commit f420726566
2 changed files with 23 additions and 88 deletions
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20
src/liballoc/collections/vec_deque.rs
View File

@ -1354,7 +1354,9 @@ impl<T> VecDeque<T> {
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn push_front(&mut self, value: T) {
self.grow_if_necessary();
if self.is_full() {
self.grow();
}
self.tail = self.wrap_sub(self.tail, 1);
let tail = self.tail;
@ -1377,7 +1379,9 @@ impl<T> VecDeque<T> {
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn push_back(&mut self, value: T) {
self.grow_if_necessary();
if self.is_full() {
self.grow();
}
let head = self.head;
self.head = self.wrap_add(self.head, 1);
@ -1485,7 +1489,9 @@ impl<T> VecDeque<T> {
#[stable(feature = "deque_extras_15", since = "1.5.0")]
pub fn insert(&mut self, index: usize, value: T) {
assert!(index <= self.len(), "index out of bounds");
self.grow_if_necessary();
if self.is_full() {
self.grow();
}
// Move the least number of elements in the ring buffer and insert
// the given object
@ -2003,11 +2009,13 @@ impl<T> VecDeque<T> {
}
// This may panic or abort
#[inline]
fn grow_if_necessary(&mut self) {
#[inline(never)]
fn grow(&mut self) {
if self.is_full() {
let old_cap = self.cap();
self.buf.double();
// Double the buffer size.
self.buf.reserve_exact(old_cap, old_cap);
assert!(self.cap() == old_cap * 2);
unsafe {
self.handle_capacity_increase(old_cap);
}

91
src/liballoc/raw_vec.rs
View File

@ -211,64 +211,6 @@ impl<T, A: AllocRef> RawVec<T, A> {
}
}
/// Doubles the size of the type's backing allocation. 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.
///
/// This function is ideal for when pushing elements one-at-a-time because
/// you don't need to incur the costs of the more general computations
/// reserve needs to do to guard against overflow. You do however need to
/// manually check if your `len == capacity`.
///
/// # 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.
///
/// # Aborts
///
/// Aborts on OOM
///
/// # Examples
///
/// ```
/// # #![feature(raw_vec_internals)]
/// # extern crate alloc;
/// # use std::ptr;
/// # use alloc::raw_vec::RawVec;
/// struct MyVec<T> {
/// buf: RawVec<T>,
/// len: usize,
/// }
///
/// impl<T> MyVec<T> {
/// pub fn push(&mut self, elem: T) {
/// if self.len == self.buf.capacity() { self.buf.double(); }
/// // double would have aborted or panicked if the len exceeded
/// // `isize::MAX` so this is safe to do unchecked now.
/// unsafe {
/// ptr::write(self.buf.ptr().add(self.len), elem);
/// }
/// self.len += 1;
/// }
/// }
/// # fn main() {
/// # let mut vec = MyVec { buf: RawVec::new(), len: 0 };
/// # vec.push(1);
/// # }
/// ```
#[inline(never)]
#[cold]
pub fn double(&mut self) {
match self.grow(Double, MayMove, Uninitialized) {
Err(CapacityOverflow) => capacity_overflow(),
Err(AllocError { layout, .. }) => handle_alloc_error(layout),
Ok(()) => { /* yay */ }
}
}
/// Ensures that the buffer contains at least enough space to hold
/// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
/// enough capacity, will reallocate enough space plus comfortable slack
@ -336,7 +278,7 @@ impl<T, A: AllocRef> RawVec<T, A> {
needed_extra_capacity: usize,
) -> Result<(), TryReserveError> {
if self.needs_to_grow(used_capacity, needed_extra_capacity) {
self.grow(Amortized { used_capacity, needed_extra_capacity }, MayMove, Uninitialized)
self.grow(Amortized, used_capacity, needed_extra_capacity, MayMove, Uninitialized)
} else {
Ok(())
}
@ -363,7 +305,7 @@ impl<T, A: AllocRef> RawVec<T, A> {
// This is more readable than putting this in one line:
// `!self.needs_to_grow(...) || self.grow(...).is_ok()`
if self.needs_to_grow(used_capacity, needed_extra_capacity) {
self.grow(Amortized { used_capacity, needed_extra_capacity }, InPlace, Uninitialized)
self.grow(Amortized, used_capacity, needed_extra_capacity, InPlace, Uninitialized)
.is_ok()
} else {
true
@ -405,7 +347,7 @@ impl<T, A: AllocRef> RawVec<T, A> {
needed_extra_capacity: usize,
) -> Result<(), TryReserveError> {
if self.needs_to_grow(used_capacity, needed_extra_capacity) {
self.grow(Exact { used_capacity, needed_extra_capacity }, MayMove, Uninitialized)
self.grow(Exact, used_capacity, needed_extra_capacity, MayMove, Uninitialized)
} else {
Ok(())
}
@ -432,9 +374,8 @@ impl<T, A: AllocRef> RawVec<T, A> {
#[derive(Copy, Clone)]
enum Strategy {
Double,
Amortized { used_capacity: usize, needed_extra_capacity: usize },
Exact { used_capacity: usize, needed_extra_capacity: usize },
Amortized,
Exact,
}
use Strategy::*;
@ -459,6 +400,8 @@ impl<T, A: AllocRef> RawVec<T, A> {
fn grow(
&mut self,
strategy: Strategy,
used_capacity: usize,
needed_extra_capacity: usize,
placement: ReallocPlacement,
init: AllocInit,
) -> Result<(), TryReserveError> {
@ -469,23 +412,7 @@ impl<T, A: AllocRef> RawVec<T, A> {
return Err(CapacityOverflow);
}
let new_layout = match strategy {
Double => unsafe {
// 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.
// Additionally the alignment will never be too large as to "not be satisfiable",
// so `Layout::from_size_align` will always return `Some`.
//
// TL;DR, we bypass runtime checks due to dynamic assertions in this module,
// allowing us to use `from_size_align_unchecked`.
let cap = if self.cap == 0 {
// Skip to 4 because tiny `Vec`'s are dumb; but not if that would cause overflow.
if elem_size > usize::MAX / 8 { 1 } else { 4 }
} else {
self.cap * 2
};
Layout::from_size_align_unchecked(cap * elem_size, mem::align_of::<T>())
},
Amortized { used_capacity, needed_extra_capacity } => {
Amortized => {
// Nothing we can really do about these checks, sadly.
let required_cap =
used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
@ -495,7 +422,7 @@ impl<T, A: AllocRef> RawVec<T, A> {
let cap = cmp::max(double_cap, required_cap);
Layout::array::<T>(cap).map_err(|_| CapacityOverflow)?
}
Exact { used_capacity, needed_extra_capacity } => {
Exact => {
let cap =
used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
Layout::array::<T>(cap).map_err(|_| CapacityOverflow)?