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Remove the reserve_in_place calls in {Typed,Dropless}Arena::grow.

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They are pointless. No reasonable allocator will be able to satisfy a
`reserve_in_place` request that *doubles* the size of an allocation when
dealing with allocations that are 4 KiB and larger.

Just to be sure, I confirmed on Linux that the `reserve_in_place` calls
never succeed.

(Note however that the `reserve_in_place` call for `DroplessArena::grow`
did occasionally succeed prior to the off-by-one fix in the previous
commit, because we would sometimes do a `reserve_in_place` request for
the chunk's current size, which would trivially succeed!)
This commit is contained in:
Nicholas Nethercote 2020-05-21 11:03:33 +10:00
parent 5ceff6b96a
commit 7145b87751
1 changed files with 22 additions and 30 deletions
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52
src/librustc_arena/lib.rs
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@ -216,26 +216,21 @@ impl<T> TypedArena<T> {
#[cold]
fn grow(&self, n: usize) {
unsafe {
// We need the element size in to convert chunk sizes (ranging from
// We need the element size to convert chunk sizes (ranging from
// PAGE to HUGE_PAGE bytes) to element counts.
let elem_size = cmp::max(1, mem::size_of::<T>());
let mut chunks = self.chunks.borrow_mut();
let (chunk, mut new_capacity);
let mut new_capacity;
if let Some(last_chunk) = chunks.last_mut() {
let used_bytes = self.ptr.get() as usize - last_chunk.start() as usize;
let currently_used_cap = used_bytes / mem::size_of::<T>();
last_chunk.entries = currently_used_cap;
if last_chunk.storage.reserve_in_place(currently_used_cap, n) {
self.end.set(last_chunk.end());
return;
} else {
// If the previous chunk's capacity is less than HUGE_PAGE
// bytes, then this chunk will be least double the previous
// chunk's size.
new_capacity = last_chunk.storage.capacity();
if new_capacity < HUGE_PAGE / elem_size {
new_capacity = new_capacity.checked_mul(2).unwrap();
}
last_chunk.entries = used_bytes / mem::size_of::<T>();
// If the previous chunk's capacity is less than HUGE_PAGE
// bytes, then this chunk will be least double the previous
// chunk's size.
new_capacity = last_chunk.storage.capacity();
if new_capacity < HUGE_PAGE / elem_size {
new_capacity = new_capacity.checked_mul(2).unwrap();
}
} else {
new_capacity = PAGE / elem_size;
@ -243,7 +238,7 @@ impl<T> TypedArena<T> {
// Also ensure that this chunk can fit `n`.
new_capacity = cmp::max(n, new_capacity);
chunk = TypedArenaChunk::<T>::new(new_capacity);
let chunk = TypedArenaChunk::<T>::new(new_capacity);
self.ptr.set(chunk.start());
self.end.set(chunk.end());
chunks.push(chunk);
@ -350,20 +345,17 @@ impl DroplessArena {
fn grow(&self, needed_bytes: usize) {
unsafe {
let mut chunks = self.chunks.borrow_mut();
let (chunk, mut new_capacity);
let mut new_capacity;
if let Some(last_chunk) = chunks.last_mut() {
let used_bytes = self.ptr.get() as usize - last_chunk.start() as usize;
if last_chunk.storage.reserve_in_place(used_bytes, needed_bytes) {
self.end.set(last_chunk.end());
return;
} else {
// If the previous chunk's capacity is less than HUGE_PAGE
// bytes, then this chunk will be least double the previous
// chunk's size.
new_capacity = last_chunk.storage.capacity();
if new_capacity < HUGE_PAGE {
new_capacity = new_capacity.checked_mul(2).unwrap();
}
// There is no need to update `last_chunk.entries` because that
// field isn't used by `DroplessArena`.
// If the previous chunk's capacity is less than HUGE_PAGE
// bytes, then this chunk will be least double the previous
// chunk's size.
new_capacity = last_chunk.storage.capacity();
if new_capacity < HUGE_PAGE {
new_capacity = new_capacity.checked_mul(2).unwrap();
}
} else {
new_capacity = PAGE;
@ -371,7 +363,7 @@ impl DroplessArena {
// Also ensure that this chunk can fit `needed_bytes`.
new_capacity = cmp::max(needed_bytes, new_capacity);
chunk = TypedArenaChunk::<u8>::new(new_capacity);
let chunk = TypedArenaChunk::<u8>::new(new_capacity);
self.ptr.set(chunk.start());
self.end.set(chunk.end());
chunks.push(chunk);