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Micro optimize extend #362

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Oct 31, 2024
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Micro optimize extend #362

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84 changes: 20 additions & 64 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -964,20 +964,6 @@ impl<T, const N: usize> SmallVec<T, N> {
unsafe { self.set_len(len + 1) }
}

#[inline]
unsafe fn push_heap(&mut self, value: T) {
// SAFETY: see above
debug_assert!(self.spilled());
let len = self.len();
let cap = self.raw.heap.1;
if len == cap {
self.reserve(1);
}
let ptr = self.raw.heap.0;
ptr.as_ptr().add(len).write(value);
self.set_len(len + 1)
}

#[inline]
pub fn pop(&mut self) -> Option<T> {
if self.is_empty() {
Expand Down Expand Up @@ -1502,39 +1488,33 @@ impl<T, const N: usize> SmallVec<T, N> {

fn extend_impl<I: Iterator<Item = T>>(&mut self, iter: I) {
let mut iter = iter.fuse();
let len = self.len();
let (lower_bound, _) = iter.size_hint();
self.reserve(lower_bound);
let capacity = self.capacity();
unsafe {
let ptr = self.as_mut_ptr();
// SAFETY: ptr is valid for `capacity - len` writes
let count = extend_batch(ptr, capacity - len, len, &mut iter);
self.set_len(len + count);
}

if let Some(item) = iter.next() {
self.push(item);
} else {
return;
}

// either we ran out of items, in which case this loop doesn't get executed. or we still
// have items to push, and in that case we must be on the heap, since we filled up the
// capacity and then pushed one item
let mut len = self.len();
let mut capacity = self.capacity();
let mut ptr = self.as_mut_ptr();
unsafe {
loop {
// SAFETY: ptr is valid for `capacity - len` writes
ptr = ptr.add(len);
let mut guard = DropGuard { ptr, len: 0 };
iter.by_ref().take(capacity - len).for_each(|item| {
ptr.add(guard.len).write(item);
guard.len += 1;
});
len += guard.len;
core::mem::forget(guard);
self.set_len(len);
// At this point we either consumed all capacity or the iterator is exhausted (fused)
if let Some(item) = iter.next() {
self.push_heap(item);
self.push(item);
} else {
break;
return;
}
let len = self.len();
let (ptr, capacity) = self.raw.heap;
let ptr = ptr.as_ptr();
// SAFETY: ptr is valid for `capacity - len` writes
let count = extend_batch(ptr, capacity - len, len, &mut iter);
self.set_len(len + count);
// SAFETY: The push above would have spilled it
let (heap_ptr, heap_capacity) = self.raw.heap;
ptr = heap_ptr.as_ptr();
capacity = heap_capacity;
}
}
}
Expand Down Expand Up @@ -1703,30 +1683,6 @@ unsafe fn insert_many_batch<T, I: Iterator<Item = T>>(
count
}

// `ptr..ptr + remaining_capacity` must be valid for writes
#[inline]
unsafe fn extend_batch<T, I: Iterator<Item = T>>(
ptr: *mut T,
remaining_capacity: usize,
len: usize,
iter: &mut I,
) -> usize {
let ptr_end = ptr.add(len);
let mut guard = DropGuard {
ptr: ptr_end,
len: 0,
};
iter.take(remaining_capacity)
.enumerate()
.for_each(|(i, item)| {
ptr_end.add(i).write(item);
guard.len = i + 1;
});
let count = guard.len;
core::mem::forget(guard);
count
}

impl<T, const N: usize> Extend<T> for SmallVec<T, N> {
#[inline]
fn extend<I: IntoIterator<Item = T>>(&mut self, iterable: I) {
Expand Down