thin_slice/lib.rs
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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
//! An owned slice that tries to use only one word of storage.
//!
//! `ThinBoxedSlice<T>` can be used in place of `Box<[T]>` on the `x86_64`
//! architecture to hold ownership of a slice when it's important to reduce
//! memory usage of the box itself. When the slice length is less than
//! `0xffff`, a single word is used to encode the slice pointer and length.
//! When it is greater than `0xffff`, a heap allocation is used to store the
//! fat pointer representing the slice.
//!
//! A `ThinBoxedSlice<T>` is always created by converting from a `Box<[T]>`.
//!
//! On any architecture other than `x86_64`, a `ThinBoxedSlice<T>` will simply
//! use a `Box<[T]>` internally.
//!
//! # Examples
//!
//! Creating a `ThinBoxedSlice`:
//!
//! ```
//! # use thin_slice::ThinBoxedSlice;
//! let fat_pointer = vec![10, 20, 30].into_boxed_slice();
//! let thin_pointer: ThinBoxedSlice<_> = fat_pointer.into();
//! ```
use std::cmp::Ordering;
use std::fmt;
use std::hash::{Hash, Hasher};
#[cfg(target_arch = "x86_64")]
use std::marker::PhantomData;
#[cfg(target_arch = "x86_64")]
use std::mem;
use std::ops::{Deref, DerefMut};
#[cfg(target_arch = "x86_64")]
use std::ptr::NonNull;
#[cfg(target_arch = "x86_64")]
use std::slice;
/// An owned slice that tries to use only one word of storage.
///
/// See the [module-level documentation](index.html) for more.
pub struct ThinBoxedSlice<T> {
/// Storage for the slice.
///
/// Once `std::num::NonZeroUsize` has stabilized, we can switch to that.
///
/// The value stored here depends on the length of the slice.
///
/// If len = 0, `data` will be `1usize`.
///
/// If 0 < len < 0xffff, then len will be stored in the top 16 bits of
/// data, and the lower 48 bits will be the pointer to the elements.
///
/// If len >= 0xffff, then the top 16 bits of data will be 0xffff, and
/// the lower 48 bits will be a pointer to a heap allocated `Box<[T]>`.
#[cfg(target_arch = "x86_64")]
data: NonNull<()>,
#[cfg(not(target_arch = "x86_64"))]
data: Box<[T]>,
#[cfg(target_arch = "x86_64")]
_phantom: PhantomData<Box<[T]>>,
}
#[cfg(target_arch = "x86_64")]
const TAG_MASK: usize = 0xffff000000000000;
#[cfg(target_arch = "x86_64")]
const PTR_MASK: usize = 0x0000ffffffffffff;
#[cfg(target_arch = "x86_64")]
const PTR_HIGH: usize = 0x0000800000000000;
#[cfg(target_arch = "x86_64")]
const TAG_SHIFT: usize = 48;
#[cfg(target_arch = "x86_64")]
const TAG_LIMIT: usize = TAG_MASK >> TAG_SHIFT;
#[cfg(target_arch = "x86_64")]
enum Storage<T> {
Inline(*mut T, usize),
Spilled(*mut Box<[T]>),
}
#[cfg(target_arch = "x86_64")]
impl<T> ThinBoxedSlice<T> {
/// Constructs a `ThinBoxedSlice` from a raw pointer.
///
/// Like `Box::from_raw`, after calling this function, the raw pointer is
/// owned by the resulting `ThinBoxedSlice`.
///
/// # Examples
///
/// ```
/// # use thin_slice::ThinBoxedSlice;
/// let x = vec![10, 20, 30].into_boxed_slice(); // a Box<[i32]>
/// let ptr = Box::into_raw(x); // a *mut [i32]
/// let x = unsafe { ThinBoxedSlice::from_raw(ptr) }; // a ThinBoxedSlice<i32>
/// ```
#[inline]
pub unsafe fn from_raw(raw: *mut [T]) -> ThinBoxedSlice<T> {
let len = (*raw).len();
let ptr = (*raw).as_mut_ptr();
let storage = if len == 0 {
Storage::Inline(1usize as *mut _, 0)
} else if len < TAG_LIMIT {
Storage::Inline(ptr, len)
} else {
let boxed_slice = Box::from_raw(raw);
Storage::Spilled(Box::into_raw(Box::new(boxed_slice)))
};
ThinBoxedSlice {
data: storage.into_data(),
_phantom: PhantomData,
}
}
/// Consumes the `ThinBoxedSlice`, returning a raw pointer to the slice
/// it owned.
///
/// Like `Box::into_raw`, after calling this function, the caller is
/// responsible for the memory previously managed by the `ThinBoxedSlice`.
/// In particular, the caller should properly destroy the `[T]` and release
/// the memory. The proper way to do so is to convert the raw pointer back
/// into a `Box` or a `ThinBoxedSlice`, with either the `Box::from_raw` or
/// `ThinBoxedSlice::from_raw` functions.
///
/// # Examples
///
/// ```
/// # use thin_slice::ThinBoxedSlice;
/// let x = vec![10, 20, 30].into_boxed_slice();
/// let x = ThinBoxedSlice::from(x);
/// let ptr = ThinBoxedSlice::into_raw(x);
/// ```
#[inline]
pub fn into_raw(b: ThinBoxedSlice<T>) -> *mut [T] {
unsafe {
match b.into_storage() {
Storage::Inline(ptr, len) => {
slice::from_raw_parts_mut(ptr, len)
}
Storage::Spilled(ptr) => {
Box::into_raw(*Box::from_raw(ptr))
}
}
}
}
/// Consumes and leaks the `ThinBoxedSlice`, returning a mutable reference,
/// `&'a mut [T]`. Here, the lifetime `'a` may be chosen to be `'static`.
///
/// Like `Box::leak`, this function is mainly useful for data that lives
/// for the remainder of the program's life. Dropping the returned
/// reference will cause a memory leak. If this is not acceptable, the
/// reference should first be wrapped with the `Box::from_raw` function
/// producing a `Box`, or with the `ThinBoxedSlice::from_raw` function
/// producing a `ThinBoxedSlice`. This value can then be dropped which will
/// properly destroy `[T]` and release the allocated memory.
///
/// # Examples
///
/// ```
/// # use thin_slice::ThinBoxedSlice;
/// fn main() {
/// let x = ThinBoxedSlice::from(vec![1, 2, 3].into_boxed_slice());
/// let static_ref = ThinBoxedSlice::leak(x);
/// static_ref[0] = 4;
/// assert_eq!(*static_ref, [4, 2, 3]);
/// }
/// ```
#[inline]
pub fn leak(b: ThinBoxedSlice<T>) -> &'static mut [T] {
unsafe { &mut *ThinBoxedSlice::into_raw(b) }
}
/// Returns a pointer to the heap allocation that stores the fat pointer
/// to the slice, if any. This is useful for systems that need to measure
/// memory allocation, but is otherwise an opaque pointer.
#[inline]
pub fn spilled_storage(&self) -> Option<*const ()> {
match self.storage() {
Storage::Inline(..) => None,
Storage::Spilled(ptr) => Some(ptr as *const ()),
}
}
#[inline]
fn storage(&self) -> Storage<T> {
Storage::from_data(self.data.clone())
}
#[inline]
fn into_storage(self) -> Storage<T> {
let storage = self.storage();
mem::forget(self);
storage
}
}
#[cfg(not(target_arch = "x86_64"))]
impl<T> ThinBoxedSlice<T> {
/// Constructs a `ThinBoxedSlice` from a raw pointer.
///
/// Like `Box::from_raw`, after calling this function, the raw pointer is
/// owned by the resulting `ThinBoxedSlice`.
///
/// # Examples
///
/// ```
/// # use thin_slice::ThinBoxedSlice;
/// let x = vec![10, 20, 30].into_boxed_slice(); // a Box<[i32]>
/// let ptr = Box::into_raw(x); // a *mut [i32]
/// let x = unsafe { ThinBoxedSlice::from_raw(ptr) }; // a ThinBoxedSlice<i32>
/// ```
#[inline]
pub unsafe fn from_raw(raw: *mut [T]) -> ThinBoxedSlice<T> {
ThinBoxedSlice {
data: Box::from_raw(raw),
}
}
/// Consumes the `ThinBoxedSlice`, returning a raw pointer to the slice
/// it owned.
///
/// Like `Box::into_raw`, after calling this function, the caller is
/// responsible for the memory previously managed by the `ThinBoxedSlice`.
/// In particular, the caller should properly destroy the `[T]` and release
/// the memory. The proper way to do so is to convert the raw pointer back
/// into a `Box` or a `ThinBoxedSlice`, with either the `Box::from_raw` or
/// `ThinBoxedSlice::from_raw` functions.
///
/// # Examples
///
/// ```
/// # use thin_slice::ThinBoxedSlice;
/// let x = vec![10, 20, 30].into_boxed_slice();
/// let x = ThinBoxedSlice::from(x);
/// let ptr = ThinBoxedSlice::into_raw(x);
/// ```
#[inline]
pub fn into_raw(b: ThinBoxedSlice<T>) -> *mut [T] {
Box::into_raw(b.data)
}
/// Consumes and leaks the `ThinBoxedSlice`, returning a mutable reference,
/// `&'a mut [T]`. Here, the lifetime `'a` may be chosen to be `'static`.
///
/// Like `Box::leak`, this function is mainly useful for data that lives
/// for the remainder of the program's life. Dropping the returned
/// reference will cause a memory leak. If this is not acceptable, the
/// reference should first be wrapped with the `Box::from_raw` function
/// producing a `Box`, or with the `ThinBoxedSlice::from_raw` function
/// producing a `ThinBoxedSlice`. This value can then be dropped which will
/// properly destroy `[T]` and release the allocated memory.
///
/// # Examples
///
/// ```
/// # use thin_slice::ThinBoxedSlice;
/// fn main() {
/// let x = ThinBoxedSlice::from(vec![1, 2, 3].into_boxed_slice());
/// let static_ref = ThinBoxedSlice::leak(x);
/// static_ref[0] = 4;
/// assert_eq!(*static_ref, [4, 2, 3]);
/// }
/// ```
#[inline]
pub fn leak<'a>(b: ThinBoxedSlice<T>) -> &'a mut [T] where T: 'a {
Box::leak(b.data)
}
/// Returns a pointer to the heap allocation that stores the fat pointer
/// to the slice, if any. This is useful for systems that need to measure
/// memory allocation, but is otherwise an opaque pointer.
#[inline]
pub fn spilled_storage(&self) -> Option<*const ()> {
None
}
}
#[cfg(target_arch = "x86_64")]
impl<T> Storage<T> {
#[inline]
fn from_data(data: NonNull<()>) -> Storage<T> {
let data = data.as_ptr() as usize;
let len = (data & TAG_MASK) >> TAG_SHIFT;
let mut ptr = data & PTR_MASK;
if (ptr & PTR_HIGH) == PTR_HIGH {
// Canonical linear addresses on x86_64 are sign extended from
// bit 48.
ptr |= TAG_MASK;
}
if len < TAG_LIMIT {
Storage::Inline(ptr as *mut T, len)
} else {
Storage::Spilled(ptr as *mut Box<[T]>)
}
}
#[inline]
fn into_data(self) -> NonNull<()> {
let data = match self {
Storage::Inline(ptr, len) => {
(len << TAG_SHIFT) | ((ptr as usize) & PTR_MASK)
}
Storage::Spilled(ptr) => {
TAG_MASK | ((ptr as usize) & PTR_MASK)
}
};
unsafe {
NonNull::new_unchecked(data as *mut _)
}
}
}
impl<T> From<Box<[T]>> for ThinBoxedSlice<T> {
fn from(value: Box<[T]>) -> ThinBoxedSlice<T> {
let ptr = Box::into_raw(value);
unsafe {
ThinBoxedSlice::from_raw(ptr)
}
}
}
impl<T> Into<Box<[T]>> for ThinBoxedSlice<T> {
fn into(self) -> Box<[T]> {
let ptr = ThinBoxedSlice::into_raw(self);
unsafe {
Box::from_raw(ptr)
}
}
}
unsafe impl<T: Send> Send for ThinBoxedSlice<T> {}
unsafe impl<T: Sync> Sync for ThinBoxedSlice<T> {}
#[cfg(target_arch = "x86_64")]
impl<T> Drop for ThinBoxedSlice<T> {
fn drop(&mut self) {
let _ = Into::<Box<[T]>>::into(
ThinBoxedSlice {
data: self.data.clone(),
_phantom: PhantomData,
}
);
}
}
impl<T: Clone> Clone for ThinBoxedSlice<T> {
#[cfg(target_arch = "x86_64")]
fn clone(&self) -> Self {
unsafe {
match self.storage() {
Storage::Inline(ptr, len) => {
slice::from_raw_parts_mut(ptr, len)
.to_vec()
.into_boxed_slice()
.into()
}
Storage::Spilled(ptr) => {
(*ptr).clone().into()
}
}
}
}
#[cfg(not(target_arch = "x86_64"))]
fn clone(&self) -> Self {
ThinBoxedSlice {
data: self.data.clone(),
}
}
}
impl<T> AsRef<[T]> for ThinBoxedSlice<T> {
fn as_ref(&self) -> &[T] {
&**self
}
}
impl<T> AsMut<[T]> for ThinBoxedSlice<T> {
fn as_mut(&mut self) -> &mut [T] {
&mut **self
}
}
impl<T> Deref for ThinBoxedSlice<T> {
type Target = [T];
#[cfg(target_arch = "x86_64")]
fn deref(&self) -> &[T] {
unsafe {
match self.storage() {
Storage::Inline(ptr, len) => {
slice::from_raw_parts(ptr, len)
}
Storage::Spilled(ptr) => {
&**ptr
}
}
}
}
#[cfg(not(target_arch = "x86_64"))]
fn deref(&self) -> &[T] {
&*self.data
}
}
impl<T> DerefMut for ThinBoxedSlice<T> {
#[cfg(target_arch = "x86_64")]
fn deref_mut(&mut self) -> &mut [T] {
unsafe {
match self.storage() {
Storage::Inline(ptr, len) => {
slice::from_raw_parts_mut(ptr, len)
}
Storage::Spilled(ptr) => {
&mut **ptr
}
}
}
}
#[cfg(not(target_arch = "x86_64"))]
fn deref_mut(&mut self) -> &mut [T] {
&mut *self.data
}
}
impl<T> Default for ThinBoxedSlice<T> {
fn default() -> Self {
Box::<[T]>::default().into()
}
}
impl<T: fmt::Debug> fmt::Debug for ThinBoxedSlice<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
impl<T: Eq> Eq for ThinBoxedSlice<T> {}
impl<T: Hash> Hash for ThinBoxedSlice<T> {
fn hash<H: Hasher>(&self, state: &mut H) {
(**self).hash(state);
}
}
impl<T: PartialEq> PartialEq for ThinBoxedSlice<T> {
#[inline]
fn eq(&self, other: &ThinBoxedSlice<T>) -> bool {
PartialEq::eq(&**self, &**other)
}
#[inline]
fn ne(&self, other: &ThinBoxedSlice<T>) -> bool {
PartialEq::ne(&**self, &**other)
}
}
impl<T: PartialOrd> PartialOrd for ThinBoxedSlice<T> {
#[inline]
fn partial_cmp(&self, other: &ThinBoxedSlice<T>) -> Option<Ordering> {
PartialOrd::partial_cmp(&**self, &**other)
}
#[inline]
fn lt(&self, other: &ThinBoxedSlice<T>) -> bool {
PartialOrd::lt(&**self, &**other)
}
#[inline]
fn le(&self, other: &ThinBoxedSlice<T>) -> bool {
PartialOrd::le(&**self, &**other)
}
#[inline]
fn ge(&self, other: &ThinBoxedSlice<T>) -> bool {
PartialOrd::ge(&**self, &**other)
}
#[inline]
fn gt(&self, other: &ThinBoxedSlice<T>) -> bool {
PartialOrd::gt(&**self, &**other)
}
}
impl<T> fmt::Pointer for ThinBoxedSlice<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let ptr = &**self;
fmt::Pointer::fmt(&ptr, f)
}
}
#[cfg(target_arch = "x86_64")]
#[test]
fn test_spilled_storage() {
let x = ThinBoxedSlice::from(vec![0; TAG_LIMIT - 1].into_boxed_slice());
assert!(x.spilled_storage().is_none());
let x = ThinBoxedSlice::from(vec![0; TAG_LIMIT].into_boxed_slice());
assert!(x.spilled_storage().is_some());
}
#[cfg(target_arch = "x86_64")]
#[test]
fn test_from_raw_large() {
let mut vec = vec![0; TAG_LIMIT];
vec[123] = 456;
let ptr = Box::into_raw(vec.into_boxed_slice());
let x = unsafe { ThinBoxedSlice::from_raw(ptr) };
assert_eq!(x[123], 456);
}
#[cfg(target_arch = "x86_64")]
#[test]
fn test_into_raw_large() {
let mut vec = vec![0; TAG_LIMIT];
vec[123] = 456;
let x = ThinBoxedSlice::from(vec.into_boxed_slice());
let ptr = ThinBoxedSlice::into_raw(x);
let y = unsafe { Box::from_raw(ptr) };
assert_eq!(y[123], 456);
}
#[cfg(target_arch = "x86_64")]
#[test]
fn test_leak_large() {
let mut vec = vec![0; TAG_LIMIT];
vec[123] = 456;
let x = ThinBoxedSlice::from(vec.into_boxed_slice());
let static_ref = ThinBoxedSlice::leak(x);
static_ref[123] *= 1000;
assert_eq!(static_ref[123], 456000);
}