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Make RawVec::grow mostly non-generic. #72013

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merged 3 commits into from
May 13, 2020
Merged

Make RawVec::grow mostly non-generic. #72013

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a3cc435
Remove `RawVec::double_in_place`.
nnethercote May 11, 2020
f420726
Remove `RawVec::double`.
nnethercote May 11, 2020
68b7503
Split `RawVec::grow` up.
nnethercote May 11, 2020
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20 changes: 14 additions & 6 deletions src/liballoc/collections/vec_deque.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand All @@ -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);
Expand Down Expand Up @@ -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
Expand Down Expand Up @@ -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() {
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this check is duplicated now, isn't it?

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Agree - a better change might have been to keep the inline function but have it call a never inlined grow_always instead. Should lead to the same code generation in the end.

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);
}
Expand Down
224 changes: 81 additions & 143 deletions src/liballoc/raw_vec.rs
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
#![unstable(feature = "raw_vec_internals", reason = "implementation detail", issue = "none")]
#![doc(hidden)]

use core::alloc::MemoryBlock;
use core::alloc::{LayoutErr, MemoryBlock};
use core::cmp;
use core::mem::{self, ManuallyDrop, MaybeUninit};
use core::ops::Drop;
Expand Down Expand Up @@ -211,82 +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 */ }
}
}

/// 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.
///
/// # 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 {
self.grow(Double, InPlace, Uninitialized).is_ok()
}

/// 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
Expand Down Expand Up @@ -354,7 +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)
} else {
Ok(())
}
Expand All @@ -381,8 +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)
.is_ok()
self.grow_amortized(used_capacity, needed_extra_capacity, InPlace).is_ok()
} else {
true
}
Expand Down Expand Up @@ -423,7 +346,7 @@ 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)
} else {
Ok(())
}
Expand All @@ -448,14 +371,6 @@ 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 },
}
use Strategy::*;

impl<T, A: AllocRef> RawVec<T, A> {
/// Returns if the buffer needs to grow to fulfill the needed extra capacity.
/// Mainly used to make inlining reserve-calls possible without inlining `grow`.
Expand All @@ -473,68 +388,59 @@ impl<T, A: AllocRef> RawVec<T, A> {
self.cap = Self::capacity_from_bytes(memory.size);
}

/// Single method to handle all possibilities of growing the buffer.
fn grow(
// This method is usually instantiated many times. So we want it to be as
// small as possible, to improve compile times. But we also want as much of
// its contents to be statically computable as possible, to make the
// generated code run faster. Therefore, this method is carefully written
// so that all of the code that depends on `T` is within it, while as much
// of the code that doesn't depend on `T` as possible is in functions that
// are non-generic over `T`.
fn grow_amortized(
&mut self,
strategy: Strategy,
used_capacity: usize,
needed_extra_capacity: usize,
placement: ReallocPlacement,
init: AllocInit,
) -> Result<(), TryReserveError> {
let elem_size = mem::size_of::<T>();
if elem_size == 0 {
if mem::size_of::<T>() == 0 {
// Since we return a capacity of `usize::MAX` when `elem_size` is
// 0, getting to here necessarily means the `RawVec` is overfull.
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 } => {
// Nothing we can really do about these checks, sadly.
let required_cap =
used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
// Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
let double_cap = self.cap * 2;
// `double_cap` guarantees exponential growth.
let cap = cmp::max(double_cap, required_cap);
Layout::array::<T>(cap).map_err(|_| CapacityOverflow)?
}
Exact { used_capacity, needed_extra_capacity } => {
let cap =
used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
Layout::array::<T>(cap).map_err(|_| CapacityOverflow)?
}
};
alloc_guard(new_layout.size())?;

let memory = if let Some((ptr, old_layout)) = self.current_memory() {
debug_assert_eq!(old_layout.align(), new_layout.align());
unsafe {
self.alloc
.grow(ptr, old_layout, new_layout.size(), placement, init)
.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?
}
} else {
match placement {
MayMove => self.alloc.alloc(new_layout, init),
InPlace => Err(AllocErr),
}
.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?
};
// Nothing we can really do about these checks, sadly.
let required_cap =
used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
// Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
let double_cap = self.cap * 2;
// `double_cap` guarantees exponential growth.
let cap = cmp::max(double_cap, required_cap);
let new_layout = Layout::array::<T>(cap);

// `finish_grow` is non-generic over `T`.
let memory = finish_grow(new_layout, placement, self.current_memory(), &mut self.alloc)?;
self.set_memory(memory);
Ok(())
}

// The constraints on this method are much the same as those on
// `grow_amortized`, but this method is usually instantiated less often so
// it's less critical.
fn grow_exact(
&mut self,
used_capacity: usize,
needed_extra_capacity: usize,
) -> Result<(), TryReserveError> {
if mem::size_of::<T>() == 0 {
// Since we return a capacity of `usize::MAX` when the type size is
// 0, getting to here necessarily means the `RawVec` is overfull.
return Err(CapacityOverflow);
}

let cap = used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
let new_layout = Layout::array::<T>(cap);

// `finish_grow` is non-generic over `T`.
let memory = finish_grow(new_layout, MayMove, self.current_memory(), &mut self.alloc)?;
self.set_memory(memory);
Ok(())
}
Expand Down Expand Up @@ -562,6 +468,38 @@ impl<T, A: AllocRef> RawVec<T, A> {
}
}

// This function is outside `RawVec` to minimize compile times. See the comment
// above `RawVec::grow_amortized` for details. (The `A` parameter isn't
// significant, because the number of different `A` types seen in practice is
// much smaller than the number of `T` types.)
fn finish_grow<A>(
new_layout: Result<Layout, LayoutErr>,
placement: ReallocPlacement,
current_memory: Option<(NonNull<u8>, Layout)>,
alloc: &mut A,
) -> Result<MemoryBlock, TryReserveError>
where
A: AllocRef,
{
// Check for the error here to minimize the size of `RawVec::grow_*`.
let new_layout = new_layout.map_err(|_| CapacityOverflow)?;

alloc_guard(new_layout.size())?;

let memory = if let Some((ptr, old_layout)) = current_memory {
debug_assert_eq!(old_layout.align(), new_layout.align());
unsafe { alloc.grow(ptr, old_layout, new_layout.size(), placement, Uninitialized) }
} else {
match placement {
MayMove => alloc.alloc(new_layout, Uninitialized),
InPlace => Err(AllocErr),
}
}
.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?;

Ok(memory)
}

impl<T> RawVec<T, Global> {
/// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
///
Expand Down