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mod allocation;
use allocation::Allocation;
use std::{
self,
cell::UnsafeCell,
mem::{size_of, MaybeUninit},
ptr, slice,
};
thread_local!(
static THREAD_LOCAL: Stack = Stack::new()
);
/// A Stack that is managed separately from the threadlocal one.
/// Typically, using the threadlocal APIs
/// is encouraged because they enable sharing across libraries, where each
/// re-use lowers the amortized cost of maintaining allocations. But, if
/// full control is necessary this API may be used.
pub struct Stack(UnsafeCell<Allocation>);
impl Drop for Stack {
fn drop(&mut self) {
let stack = self.0.get_mut();
// It's ok to use force_dealloc here instead of try_dealloc
// because we know the allocation cannot be in-use. By eliding
// the check, this allows the allocation to be freed when there
// was a panic
unsafe {
stack.force_dealloc();
}
}
}
impl Stack {
pub fn new() -> Self {
Self(UnsafeCell::new(Allocation::null()))
}
/// Place a potentially very large value on this stack.
pub fn uninit<T, R, F>(&self, f: F) -> R
where
F: FnOnce(&mut MaybeUninit<T>) -> R,
{
// Delegate implementation to uninit_slice just to get this working.
// Performance could be slightly improved with a bespoke implementation
// of this method.
self.uninit_slice(1, |slice| f(&mut slice[0]))
}
/// Allocates an uninit slice from this stack.
pub fn uninit_slice<T, F, R>(&self, len: usize, f: F) -> R
where
F: FnOnce(&mut [MaybeUninit<T>]) -> R,
{
// Special case for ZST that disregards the rest of the code,
// so that none of that code need account for ZSTs.
// The reason this is convenient is that a ZST may use
// the stack without bumping the pointer, which will
// lead other code to free that memory while still in-use.
// See also: 2ec61cda-e074-4b26-a9a5-a01b70706585
// There may be other issues also.
if std::mem::size_of::<T>() == 0 {
let mut tmp = Vec::<T>::with_capacity(len);
// We do need to take a slice here, because suprisingly
// tmp.capacity() returns 18446744073709551615
let slice = &mut tmp.spare_capacity_mut()[..len];
return f(slice);
}
// Required for correctness
// See also: 26936c11-5b7c-472e-8f63-7922e63a5425
if len == 0 {
return f(&mut []);
}
// Get the new slice, and the old allocation to
// restore once the function is finished running.
let (_restore, (ptr, len)) = unsafe {
let stack = &mut *self.0.get();
stack.get_slice(&self.0, len)
};
let slice = unsafe { slice::from_raw_parts_mut(ptr as *mut MaybeUninit<T>, len) };
f(slice)
}
/// Buffers an iterator to a slice on this stack and gives temporary access to that slice.
/// Do not use with an unbounded iterator, because this will eventually run out of memory and panic.
pub fn buffer<T, F, R, I>(&self, i: I, f: F) -> R
where
I: Iterator<Item = T>,
F: FnOnce(&mut [T]) -> R,
{
// Special case for ZST
if size_of::<T>() == 0 {
let mut v: Vec<_> = i.collect();
return f(&mut v);
}
// Data goes in a struct in case user code panics.
// User code includes Iterator::next, FnOnce, and Drop::drop
struct Writer<'a, T> {
restore: Option<DropStack<'a>>,
base: *mut T,
len: usize,
capacity: usize,
}
impl<T> Writer<'_, T> {
unsafe fn write(&mut self, item: T) {
self.base.add(self.len).write(item);
self.len += 1;
}
fn try_reuse(&mut self, stack: &mut Allocation) -> bool {
if let Some(prev) = &self.restore {
if prev.restore.ref_eq(stack) {
// If we are already are using this stack, we know the
// end ptr is already aligned. To double in size,
// we would need as many bytes as there are currently
// and do not need to align
let required_bytes = size_of::<T>() * self.capacity;
if stack.remaining_bytes() >= required_bytes {
stack.len += required_bytes;
self.capacity *= 2;
return true;
}
}
}
false
}
}
impl<T> Drop for Writer<'_, T> {
fn drop(&mut self) {
unsafe {
for i in 0..self.len {
self.base.add(i).drop_in_place()
}
}
}
}
unsafe {
let mut writer = Writer {
restore: None,
base: ptr::null_mut(),
capacity: 0,
len: 0,
};
for next in i {
if writer.capacity == writer.len {
let stack = &mut *self.0.get();
// First try to use the same stack, but if that fails
// copy over to the upsized stack
if !writer.try_reuse(stack) {
// This will always be a different allocation, otherwise
// try_reuse would have succeeded
let (restore, (base, capacity)) =
stack.get_slice(&self.0, (writer.len * 2).max(1));
// Check for 0 is to avoid copy from null ptr (miri violation)
if writer.len != 0 {
ptr::copy_nonoverlapping(writer.base, base, writer.len);
}
// This attempts to restore the old allocation when
// writer.restore is Some, but we know that there
// is a new allocation at this point, so the only
// thing it can do is free memory
writer.restore = Some(restore);
writer.capacity = capacity;
writer.base = base;
}
}
writer.write(next);
}
// TODO: (Performance?) Drop reserve of unused stack, if any. We have over-allocated.
// TODO: (Performance?) Consider using size_hint
let buffer = slice::from_raw_parts_mut(writer.base, writer.len);
f(buffer)
}
}
}
/// Allocates an uninit slice from the threadlocal stack.
pub fn uninit_slice<T, F, R>(len: usize, f: F) -> R
where
F: FnOnce(&mut [MaybeUninit<T>]) -> R,
{
THREAD_LOCAL.with(|stack| stack.uninit_slice(len, f))
}
/// Place a potentially very large value on the threadlocal second stack.
pub fn uninit<T, F, R>(f: F) -> R
where
F: FnOnce(&mut MaybeUninit<T>) -> R,
{
THREAD_LOCAL.with(|stack| stack.uninit(f))
}
/// Buffers an iterator to a slice on the threadlocal stack and gives temporary access to that slice.
/// Panics when running out of memory if the iterator is unbounded.
pub fn buffer<T, F, R, I>(i: I, f: F) -> R
where
I: Iterator<Item = T>,
F: FnOnce(&mut [T]) -> R,
{
THREAD_LOCAL.with(|stack| stack.buffer(i, f))
}
// The logic to drop our Allocation goes into a drop impl so that if there
// is a panic the drop logic is still run and we don't leak any memory.
pub(crate) struct DropStack<'a> {
pub restore: Allocation,
pub location: &'a UnsafeCell<Allocation>,
}
impl Drop for DropStack<'_> {
fn drop(&mut self) {
unsafe {
let mut current = &mut *self.location.get();
if current.ref_eq(&self.restore) {
current.len = self.restore.len;
} else {
self.restore.try_dealloc();
}
}
}
}