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// Copyright 2018-2021 Theodore Cipicchio // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! `Scratchpad` type implementation. use core::fmt; use core::ptr; use super::{ Buffer, Error, ErrorKind, MarkerBack, MarkerFront, StaticBuffer, Tracking, }; use core::cell::{Cell, UnsafeCell}; use core::mem::size_of; #[cfg(not(any(stable_maybe_uninit, feature = "unstable")))] use core::mem::uninitialized; #[cfg(any(stable_maybe_uninit, feature = "unstable"))] use core::mem::MaybeUninit; use core::ops::{Deref, DerefMut}; /// Front and back stacks for `Marker` tracking (used internally). pub(crate) struct MarkerStacks<TrackingT> where TrackingT: Tracking, { /// Stack data. pub(crate) data: TrackingT, /// Front stack offset. pub(crate) front: usize, /// Back stack offset. pub(crate) back: usize, } impl<TrackingT> fmt::Debug for MarkerStacks<TrackingT> where TrackingT: Tracking, { #[inline] fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "MarkerStacks {{ ... }}") } } /// `BorrowFlag` replacement for internal `RefCell` type. type BorrowFlag = usize; const UNUSED: BorrowFlag = 0usize; const WRITING: BorrowFlag = !0usize; /// `Ref` replacement for internal `RefCell` type. /// /// This only implements the parts of `core::cell::Ref` used by this crate and /// is not intended as a full replacement. pub(crate) struct Ref<'a, T> where T: 'a, { cell: &'a RefCell<T>, } impl<'a, T> Drop for Ref<'a, T> { #[inline] fn drop(&mut self) { let borrow = self.cell.borrow.get(); debug_assert_ne!(borrow, UNUSED); debug_assert_ne!(borrow, WRITING); self.cell.borrow.set(borrow - 1); } } impl<'a, T> Deref for Ref<'a, T> { type Target = T; #[inline] fn deref(&self) -> &T { unsafe { &*self.cell.value.get() } } } impl<'a, T> fmt::Debug for Ref<'a, T> where T: fmt::Debug, { #[inline] fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Debug::fmt(&**self, f) } } /// `RefMut` replacement for internal `RefCell` type. /// /// This only implements the parts of `core::cell::RefMut` used by this crate /// and is not intended as a full replacement. pub(crate) struct RefMut<'a, T> where T: 'a, { cell: &'a RefCell<T>, } impl<'a, T> Drop for RefMut<'a, T> { #[inline] fn drop(&mut self) { debug_assert_eq!(self.cell.borrow.get(), WRITING); self.cell.borrow.set(UNUSED); } } impl<'a, T> Deref for RefMut<'a, T> { type Target = T; #[inline] fn deref(&self) -> &T { unsafe { &*self.cell.value.get() } } } impl<'a, T> DerefMut for RefMut<'a, T> { #[inline] fn deref_mut(&mut self) -> &mut T { unsafe { &mut *self.cell.value.get() } } } impl<'a, T> fmt::Debug for RefMut<'a, T> where T: fmt::Debug, { #[inline] fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Debug::fmt(&**self, f) } } /// `RefCell` replacement for internal use. /// /// In order to support initialization of a `RefCell<MarkerStacks<_>>` using /// `Scratchpad::static_new_in_place()`, we would need to be able to rely on /// the internals of `RefCell` to have a specific layout. It is likely /// dangerous to assume that its internals won't change over time, so we'll /// instead use a custom type whose layout we can depend on over future /// versions. pub(crate) struct RefCell<T> { borrow: Cell<BorrowFlag>, value: UnsafeCell<T>, } impl<T> RefCell<T> { #[inline] #[cfg(feature = "unstable")] pub(crate) const fn new(value: T) -> Self { RefCell { borrow: Cell::new(UNUSED), value: UnsafeCell::new(value), } } #[inline] #[cfg(not(feature = "unstable"))] pub(crate) fn new(value: T) -> Self { RefCell { borrow: Cell::new(UNUSED), value: UnsafeCell::new(value), } } /// Creates a `RefCell` in uninitialized memory, leaving its value /// uninitialized. #[inline] pub(crate) unsafe fn new_uninitialized_value_in_place(dst: *mut Self) { // `UnsafeCell<T>` simply wraps a `T` value, so we don't need to do // any special initialization for the `value` field. ptr::write(&mut (*dst).borrow, Cell::new(UNUSED)); } #[inline] pub(crate) fn borrow(&self) -> Ref<T> { let borrow = self.borrow.get(); assert_ne!(borrow, WRITING); self.borrow.set(borrow + 1); Ref { cell: self } } #[inline] pub(crate) fn borrow_mut(&self) -> RefMut<T> { assert_eq!(self.borrow.get(), UNUSED); self.borrow.set(WRITING); RefMut { cell: self } } #[inline] pub(crate) fn get_mut(&mut self) -> &mut T { unsafe { &mut *self.value.get() } } } impl<T> fmt::Debug for RefCell<T> where T: fmt::Debug, { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { // The `Debug` implementation for `core::cell::RefCell` won't show the // cell's value if it is mutably borrowed, but that seems unnecessary // since we won't be holding on to the immutably borrowed value // outside the scope of this function (and since `RefCell` isn't // `Sync`, we can expect it to not be modified while in this // function). f.debug_struct("RefCell") .field("value", unsafe { &*self.value.get() }) .finish() } } /// Stack-like dynamic memory pool with double-ended allocation support. /// /// `Scratchpad` manages dynamic allocations from a fixed-size region of /// memory in a stack-like manner. Allocations can be made simultaneously from /// either the "front" or "back" of the scratchpad by setting a [`Marker`] /// using either [`mark_front()`][`mark_front()`] (returning a /// [`MarkerFront`]) or [`mark_back()`][`mark_back()`] (returning a /// [`MarkerBack`]). Multiple markers can be set, but only the most-recently /// set marker of a given type that is still active can be used to allocate /// objects. /// /// Individual allocations can be made from the marker, but no memory is /// actually freed back into the pool until the marker is dropped, where all /// the memory allocated through the marker is released at once. If the marker /// is not the most-recently set active marker of its type, its memory will /// simply be flagged as unused until all markers of the same type that were /// created after it are also dropped, at which point the memory will be once /// again made available for allocations. /// /// `Scratchpad`, [`Marker`] implementations, and [`Allocation`] all make use /// of static lifetimes to ensure that an object cannot be used after the /// object from which it was created is dropped (an allocation cannot outlive /// its marker, and a marker cannot outlive its scratchpad). /// /// *See also the [crate-level documentation](index.html) for more detailed /// information about how `Scratchpad` works and can be used.* /// /// [`Allocation`]: struct.Allocation.html /// [`mark_back()`]: #method.mark_back /// [`mark_front()`]: #method.mark_front /// [`Marker`]: trait.Marker.html /// [`MarkerBack`]: struct.MarkerBack.html /// [`MarkerFront`]: struct.MarkerFront.html pub struct Scratchpad<BufferT, TrackingT> where BufferT: Buffer, TrackingT: Tracking, { /// Buffer from which allocations are made. pub(crate) buffer: UnsafeCell<BufferT>, /// Dual stack containing the offsets of each active marker. If a marker /// not at the end of one of the stacks is freed, its offset is set to /// `core::usize::MAX` to indicate it is no longer active until the /// allocations that came after it (in the same stack) have also been /// freed. pub(crate) markers: RefCell<MarkerStacks<TrackingT>>, } impl<BufferT, TrackingT> Scratchpad<BufferT, TrackingT> where BufferT: Buffer, TrackingT: Tracking, { /// Creates a new scratchpad instance. /// /// Note that using large static arrays for allocation storage or marker /// tracking can cause the program to run out of stack space while calling /// this function. It is recommended to either use borrowed slices or /// boxed slices if this occurs, or alternatively use the unsafe /// [`static_new_in_place()`] function to create the `Scratchpad`. /// /// # Examples /// /// ``` /// # #[macro_use] /// # extern crate scratchpad; /// use scratchpad::Scratchpad; /// # #[cfg(not(any(stable_maybe_uninit, feature = "unstable")))] /// # use std::mem::uninitialized; /// # #[cfg(any(stable_maybe_uninit, feature = "unstable"))] /// use std::mem::MaybeUninit; /// /// # fn main() { /// // Creates a scratchpad that can hold up to 256 bytes of data and up /// // to 4 allocation markers. The initial contents of each buffer are /// // ignored, so we can provide uninitialized data in order to reduce /// // the runtime overhead of creating a scratchpad. /// # #[cfg(not(any(stable_maybe_uninit, feature = "unstable")))] /// # let scratchpad = unsafe { Scratchpad::new( /// # uninitialized::<array_type_for_bytes!(u64, 256)>(), /// # uninitialized::<array_type_for_markers!(usize, 4)>(), /// # ) }; /// # #[cfg(any(stable_maybe_uninit, feature = "unstable"))] /// let scratchpad = unsafe { Scratchpad::new( /// MaybeUninit::<array_type_for_bytes!(MaybeUninit<u64>, 256)>::uninit().assume_init(), /// MaybeUninit::<array_type_for_markers!(MaybeUninit<usize>, 4)>::uninit().assume_init(), /// ) }; /// # } /// ``` /// /// [`static_new_in_place()`]: #method.static_new_in_place #[inline(always)] #[cfg(feature = "unstable")] pub const fn new(buffer: BufferT, tracking: TrackingT) -> Self { Scratchpad { buffer: UnsafeCell::new(buffer), markers: RefCell::new(MarkerStacks { data: tracking, front: 0, back: ::core::usize::MAX, // Lazy initialization. }), } } /// Creates a new scratchpad instance. /// /// Note that using large static arrays for allocation storage or marker /// tracking can cause the program to run out of stack space while calling /// this function. It is recommended to either use borrowed slices or /// boxed slices if this occurs, or alternatively use the unsafe /// [`static_new_in_place()`] function to create the `Scratchpad`. /// /// # Examples /// /// ``` /// # #[macro_use] /// # extern crate scratchpad; /// use scratchpad::Scratchpad; /// # #[cfg(not(any(stable_maybe_uninit, feature = "unstable")))] /// # use std::mem::uninitialized; /// # #[cfg(any(stable_maybe_uninit, feature = "unstable"))] /// use std::mem::MaybeUninit; /// /// # fn main() { /// // Creates a scratchpad that can hold up to 256 bytes of data and up /// // to 4 allocation markers. The initial contents of each buffer are /// // ignored, so we can provide uninitialized data in order to reduce /// // the runtime overhead of creating a scratchpad. /// # #[cfg(not(any(stable_maybe_uninit, feature = "unstable")))] /// # let scratchpad = unsafe { Scratchpad::new( /// # uninitialized::<array_type_for_bytes!(u64, 256)>(), /// # uninitialized::<array_type_for_markers!(usize, 4)>(), /// # ) }; /// # #[cfg(any(stable_maybe_uninit, feature = "unstable"))] /// let scratchpad = unsafe { Scratchpad::new( /// MaybeUninit::<array_type_for_bytes!(MaybeUninit<u64>, 256)>::uninit().assume_init(), /// MaybeUninit::<array_type_for_markers!(MaybeUninit<usize>, 4)>::uninit().assume_init(), /// ) }; /// # } /// ``` /// /// [`static_new_in_place()`]: #method.static_new_in_place #[inline(always)] #[cfg(not(feature = "unstable"))] pub fn new(buffer: BufferT, tracking: TrackingT) -> Self { Scratchpad { buffer: UnsafeCell::new(buffer), markers: RefCell::new(MarkerStacks { back: tracking.capacity(), data: tracking, front: 0, }), } } } impl<BufferT, TrackingT> Scratchpad<BufferT, TrackingT> where BufferT: StaticBuffer, TrackingT: Tracking + StaticBuffer, { /// Creates a new instance for scratchpad types backed entirely by static /// arrays without initializing array memory. /// /// Since static array [`Buffer`] and [`Tracking`] types are owned by the /// scratchpad, and their sizes are known ahead of time to the scratchpad /// type, scratchpads using only static arrays for storage can be created /// without having to provide any parameters. /// /// Note that using large static arrays for allocation storage or marker /// tracking can cause the program to run out of stack space while calling /// this function. It is recommended to either use borrowed slices or /// boxed slices if this occurs, or alternatively use the unsafe /// [`static_new_in_place()`] function to create the `Scratchpad`. /// /// It is strongly recommended to use arrays of [`MaybeUninit`] elements /// for both allocation and tracking storage if possible when using this /// function. Even though [`ByteData`] types can safely store any pattern /// of bits without causing undefined behavior, the rules in Rust for /// using integers whose memory is specifically uninitialized have not /// been finalized. /// /// # Examples /// /// ``` /// # #[macro_use] /// # extern crate scratchpad; /// use scratchpad::Scratchpad; /// # #[cfg(any(stable_maybe_uninit, feature = "unstable"))] /// use std::mem::MaybeUninit; /// /// # fn main() { /// // Creates a scratchpad that can hold up to 256 bytes of data and up /// // to 4 allocation markers. /// # #[cfg(not(any(stable_maybe_uninit, feature = "unstable")))] /// # let scratchpad = Scratchpad::< /// # array_type_for_bytes!(u64, 256), /// # array_type_for_markers!(usize, 4), /// # >::static_new(); /// # #[cfg(any(stable_maybe_uninit, feature = "unstable"))] /// let scratchpad = Scratchpad::< /// array_type_for_bytes!(MaybeUninit<u64>, 256), /// array_type_for_markers!(MaybeUninit<usize>, 4), /// >::static_new(); /// # } /// ``` /// /// [`Buffer`]: trait.Buffer.html /// [`Tracking`]: trait.Tracking.html /// [`static_new_in_place()`]: #method.static_new_in_place /// [`MaybeUninit`]: https://doc.rust-lang.org/std/mem/union.MaybeUninit.html /// [`ByteData`]: trait.ByteData.html #[inline(always)] pub fn static_new() -> Self { #[cfg(any(stable_maybe_uninit, feature = "unstable"))] // LINT: In order to maintain backwards compatibility with // `scratchpad` versions that predate `MaybeUninit`, we still // have to allow uninitialized buffer setup for `ByteData` types // not wrapped by `MaybeUninit`. The method documentation // mentions the caveats of this and recommends using buffers of // `MaybeUninit` types whenever possible. #[cfg_attr( feature = "cargo-clippy", allow(clippy::uninit_assumed_init) )] return Scratchpad { buffer: unsafe { MaybeUninit::uninit().assume_init() }, markers: RefCell::new(MarkerStacks { data: unsafe { MaybeUninit::uninit().assume_init() }, front: 0, back: size_of::<TrackingT>() / size_of::<usize>(), }), }; #[cfg(not(any(stable_maybe_uninit, feature = "unstable")))] return Scratchpad { buffer: unsafe { uninitialized() }, markers: RefCell::new(MarkerStacks { data: unsafe { uninitialized() }, front: 0, back: size_of::<TrackingT>() / size_of::<usize>(), }), }; } /// Initializes a new instance in uninitialized memory for scratchpad /// types backed entirely by static arrays. /// /// This is provided to allow for creation of scratchpads backed by large /// static arrays while guaranteeing that both arrays and the created /// `Scratchpad` are never accidentally stored on the stack, avoiding /// possible stack overflow. /// /// It is strongly recommended to use arrays of [`MaybeUninit`] elements /// for both allocation and tracking storage if possible when using this /// function. Even though [`ByteData`] types can safely store any pattern /// of bits without causing undefined behavior, the rules in Rust for /// using integers whose memory is specifically uninitialized have not /// been finalized. /// /// # Safety /// /// This function is unsafe because it operates on a raw pointer. /// /// It does not drop any existing contents of `dst` before writing, nor /// does it check for whether `dst` is a valid pointer. /// /// `dst` must be properly aligned for storage of an instance of /// `Scratchpad`. /// /// After returning, the contents of `dst` will need to be dropped when /// the scratchpad is no longer in use before the memory pointed to by /// `dst` is freed. /// /// # Examples /// /// ``` /// # #[macro_use] /// # extern crate scratchpad; /// use scratchpad::{CacheAligned, Scratchpad}; /// # #[cfg(any(stable_maybe_uninit, feature = "unstable"))] /// use std::mem::MaybeUninit; /// /// // Scratchpad that can hold up to 1 MB of data and up to 16 allocation /// // markers. /// # #[cfg(not(any(stable_maybe_uninit, feature = "unstable")))] /// # type LargeScratchpad = Scratchpad< /// # array_type_for_bytes!(CacheAligned, 1024 * 1024), /// # array_type_for_markers!(usize, 16), /// # >; /// # #[cfg(any(stable_maybe_uninit, feature = "unstable"))] /// type LargeScratchpad = Scratchpad< /// array_type_for_bytes!(MaybeUninit<CacheAligned>, 1024 * 1024), /// array_type_for_markers!(MaybeUninit<usize>, 16), /// >; /// /// # fn main() { /// unsafe { /// // The `Vec` here represents any region of memory in which a /// // `Scratchpad` needs to be initialized at runtime, whether /// // allocated from the heap or elsewhere. /// let mut memory = Vec::with_capacity(1); /// memory.set_len(1); /// /// LargeScratchpad::static_new_in_place(memory.as_mut_ptr()); /// /// let scratchpad = &memory[0]; /// let marker = scratchpad.mark_front().unwrap(); /// let allocation = marker.allocate(12).unwrap(); /// assert_eq!(*allocation, 12); /// } /// # } /// ``` /// /// [`MaybeUninit`]: https://doc.rust-lang.org/std/mem/union.MaybeUninit.html /// [`ByteData`]: trait.ByteData.html #[inline] pub unsafe fn static_new_in_place(dst: *mut Self) { // `UnsafeCell<T>` simply wraps a `T` value, so we don't need to do // any special initialization for the `buffer` or `MarkerStacks::data` // fields. RefCell::new_uninitialized_value_in_place(&mut (*dst).markers); let markers = (*dst).markers.get_mut(); ptr::write(&mut markers.front, 0); ptr::write(&mut markers.back, markers.data.capacity()); } } impl<BufferT, TrackingT> Scratchpad<BufferT, TrackingT> where BufferT: Buffer, TrackingT: Tracking, { /// Creates a marker at the front of the allocation buffer for subsequent /// allocations. /// /// # Examples /// /// ``` /// use scratchpad::Scratchpad; /// /// let scratchpad = Scratchpad::<[u64; 1], [usize; 1]>::new([0], [0]); /// /// let marker = scratchpad.mark_front().unwrap(); /// // `marker` can now be used for allocations... /// ``` pub fn mark_front( &self, ) -> Result<MarkerFront<BufferT, TrackingT>, Error<()>> { let mut markers = self.markers.borrow_mut(); // `markers.back` is lazy-initialized when the "unstable" feature is // enabled so that `Scratchpad::new()` can be a `const` function. #[cfg(feature = "unstable")] { if markers.back == ::core::usize::MAX { markers.back = markers.data.capacity(); } } let index = markers.front; if index == markers.back { return Err(Error::new(ErrorKind::MarkerLimit, ())); } let buffer_offset = if index == 0 { 0 } else { markers.data.get(index - 1) }; markers.data.set(index, buffer_offset); markers.front = index + 1; Ok(MarkerFront { scratchpad: self, index, }) } /// Creates a marker at the back of the allocation buffer for subsequent /// allocations. /// /// # Examples /// /// ``` /// use scratchpad::Scratchpad; /// /// let scratchpad = Scratchpad::<[u64; 1], [usize; 1]>::new([0], [0]); /// /// let marker = scratchpad.mark_back().unwrap(); /// // `marker` can now be used for allocations... /// ``` pub fn mark_back( &self, ) -> Result<MarkerBack<BufferT, TrackingT>, Error<()>> { let mut markers = self.markers.borrow_mut(); // `markers.back` is lazy-initialized when the "unstable" feature is // enabled so that `Scratchpad::new()` can be a `const` function. #[cfg(feature = "unstable")] { if markers.back == ::core::usize::MAX { markers.back = markers.data.capacity(); } } let mut index = markers.back; if index == markers.front { return Err(Error::new(ErrorKind::MarkerLimit, ())); } let buffer_offset = if index == markers.data.capacity() { unsafe { (*self.buffer.get()).as_bytes().len() } } else { markers.data.get(index) }; index -= 1; markers.data.set(index, buffer_offset); markers.back = index; Ok(MarkerBack { scratchpad: self, index, }) } } impl<BufferT, TrackingT> fmt::Debug for Scratchpad<BufferT, TrackingT> where BufferT: Buffer, TrackingT: Tracking, { #[inline] fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!( f, "Scratchpad {{ buffer.len = {}, markers: {:?} }}", unsafe { &*self.buffer.get() }.as_bytes().len(), self.markers.borrow(), ) } }