Implement in-place growth for RawVec

Author: pczarn

src/liballoc/raw_vec.rs

@@ -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.
  ///