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rustc_arena: handle recursive allocation, don't trust size_hint #79154

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rustc_arena: handle recursive allocation, don't trust size_hint #79154

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72 changes: 18 additions & 54 deletions compiler/rustc_arena/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -191,6 +191,8 @@ impl<T> TypedArena<T> {
#[inline]
pub fn alloc_from_iter<I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
assert!(mem::size_of::<T>() != 0);
// To handle the possibility that the act of iterating itself causes allocations on
// the arena, we must collect into a local buffer first, then copy it into the arena.
let mut vec: SmallVec<[_; 8]> = iter.into_iter().collect();
if vec.is_empty() {
return &mut [];
Expand Down Expand Up @@ -431,66 +433,26 @@ impl DroplessArena {
}
}

#[inline]
unsafe fn write_from_iter<T, I: Iterator<Item = T>>(
&self,
mut iter: I,
len: usize,
mem: *mut T,
) -> &mut [T] {
let mut i = 0;
// Use a manual loop since LLVM manages to optimize it better for
// slice iterators
loop {
let value = iter.next();
if i >= len || value.is_none() {
// We only return as many items as the iterator gave us, even
// though it was supposed to give us `len`
return slice::from_raw_parts_mut(mem, i);
}
ptr::write(mem.add(i), value.unwrap());
i += 1;
}
}

#[inline]
pub fn alloc_from_iter<T, I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
let iter = iter.into_iter();
assert!(mem::size_of::<T>() != 0);
assert!(!mem::needs_drop::<T>());

let size_hint = iter.size_hint();

match size_hint {
(min, Some(max)) if min == max => {
// We know the exact number of elements the iterator will produce here
let len = min;

if len == 0 {
return &mut [];
}

let mem = self.alloc_raw(Layout::array::<T>(len).unwrap()) as *mut T;
unsafe { self.write_from_iter(iter, len, mem) }
}
(_, _) => {
cold_path(move || -> &mut [T] {
let mut vec: SmallVec<[_; 8]> = iter.collect();
if vec.is_empty() {
return &mut [];
}
// Move the content to the arena by copying it and then forgetting
// the content of the SmallVec
unsafe {
let len = vec.len();
let start_ptr =
self.alloc_raw(Layout::for_value::<[T]>(vec.as_slice())) as *mut T;
vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
vec.set_len(0);
slice::from_raw_parts_mut(start_ptr, len)
}
})
}
// To handle the possibility that the act of iterating itself causes allocations on
// the arena, we must collect into a local buffer first, then copy it into the arena.
let mut vec: SmallVec<[_; 8]> = iter.collect();
if vec.is_empty() {
return &mut [];
}
// Move the content to the arena by copying it and then forgetting
// the content of the SmallVec
unsafe {
let len = vec.len();
let start_ptr = self.alloc_raw(Layout::for_value::<[T]>(vec.as_slice())) as *mut T;
vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
vec.set_len(0);
slice::from_raw_parts_mut(start_ptr, len)
}
}
}
Expand Down Expand Up @@ -542,6 +504,8 @@ impl DropArena {

#[inline]
pub unsafe fn alloc_from_iter<T, I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
// To handle the possibility that the act of iterating itself causes allocations on
// the arena, we must collect into a local buffer first, then copy it into the arena.
let mut vec: SmallVec<[_; 8]> = iter.into_iter().collect();
if vec.is_empty() {
return &mut [];
Expand Down