forked from mirrors/gecko-dev
1205 lines
39 KiB
Rust
1205 lines
39 KiB
Rust
// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! Fork of Arc for Servo. This has the following advantages over std::sync::Arc:
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//!
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//! * We don't waste storage on the weak reference count.
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//! * We don't do extra RMU operations to handle the possibility of weak references.
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//! * We can experiment with arena allocation (todo).
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//! * We can add methods to support our custom use cases [1].
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//! * We have support for dynamically-sized types (see from_header_and_iter).
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//! * We have support for thin arcs to unsized types (see ThinArc).
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//!
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//! [1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1360883
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// The semantics of `Arc` are alread documented in the Rust docs, so we don't
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// duplicate those here.
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#![allow(missing_docs)]
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extern crate nodrop;
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#[cfg(feature = "servo")]
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extern crate serde;
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extern crate stable_deref_trait;
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use nodrop::NoDrop;
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#[cfg(feature = "servo")]
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use serde::{Deserialize, Serialize};
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use stable_deref_trait::{CloneStableDeref, StableDeref};
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use std::{isize, usize};
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use std::borrow;
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use std::cmp::Ordering;
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use std::convert::From;
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use std::fmt;
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use std::hash::{Hash, Hasher};
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use std::iter::{ExactSizeIterator, Iterator};
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use std::marker::PhantomData;
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use std::mem;
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use std::ops::{Deref, DerefMut};
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use std::os::raw::c_void;
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use std::process;
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use std::ptr;
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use std::slice;
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use std::sync::atomic;
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use std::sync::atomic::Ordering::{Acquire, Relaxed, Release};
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// Private macro to get the offset of a struct field in bytes from the address of the struct.
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macro_rules! offset_of {
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($container:path, $field:ident) => {{
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// Make sure the field actually exists. This line ensures that a compile-time error is
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// generated if $field is accessed through a Deref impl.
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let $container { $field: _, .. };
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// Create an (invalid) instance of the container and calculate the offset to its
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// field. Using a null pointer might be UB if `&(*(0 as *const T)).field` is interpreted to
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// be nullptr deref.
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let invalid: $container = ::std::mem::uninitialized();
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let offset = &invalid.$field as *const _ as usize - &invalid as *const _ as usize;
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// Do not run destructors on the made up invalid instance.
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::std::mem::forget(invalid);
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offset as isize
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}};
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}
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/// A soft limit on the amount of references that may be made to an `Arc`.
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///
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/// Going above this limit will abort your program (although not
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/// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
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const MAX_REFCOUNT: usize = (isize::MAX) as usize;
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/// An atomically reference counted shared pointer
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///
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/// See the documentation for [`Arc`] in the standard library. Unlike the
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/// standard library `Arc`, this `Arc` does not support weak reference counting.
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///
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/// [`Arc`]: https://doc.rust-lang.org/stable/std/sync/struct.Arc.html
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#[repr(C)]
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pub struct Arc<T: ?Sized> {
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p: ptr::NonNull<ArcInner<T>>,
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}
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/// An `Arc` that is known to be uniquely owned
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///
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/// When `Arc`s are constructed, they are known to be
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/// uniquely owned. In such a case it is safe to mutate
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/// the contents of the `Arc`. Normally, one would just handle
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/// this by mutating the data on the stack before allocating the
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/// `Arc`, however it's possible the data is large or unsized
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/// and you need to heap-allocate it earlier in such a way
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/// that it can be freely converted into a regular `Arc` once you're
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/// done.
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///
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/// `UniqueArc` exists for this purpose, when constructed it performs
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/// the same allocations necessary for an `Arc`, however it allows mutable access.
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/// Once the mutation is finished, you can call `.shareable()` and get a regular `Arc`
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/// out of it.
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///
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/// ```rust
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/// # use servo_arc::UniqueArc;
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/// let data = [1, 2, 3, 4, 5];
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/// let mut x = UniqueArc::new(data);
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/// x[4] = 7; // mutate!
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/// let y = x.shareable(); // y is an Arc<T>
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/// ```
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pub struct UniqueArc<T: ?Sized>(Arc<T>);
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impl<T> UniqueArc<T> {
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#[inline]
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/// Construct a new UniqueArc
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pub fn new(data: T) -> Self {
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UniqueArc(Arc::new(data))
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}
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#[inline]
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/// Convert to a shareable Arc<T> once we're done mutating it
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pub fn shareable(self) -> Arc<T> {
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self.0
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}
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}
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impl<T> Deref for UniqueArc<T> {
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type Target = T;
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fn deref(&self) -> &T {
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&*self.0
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}
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}
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impl<T> DerefMut for UniqueArc<T> {
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fn deref_mut(&mut self) -> &mut T {
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// We know this to be uniquely owned
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unsafe { &mut (*self.0.ptr()).data }
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}
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}
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unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
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unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
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/// The object allocated by an Arc<T>
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#[repr(C)]
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struct ArcInner<T: ?Sized> {
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count: atomic::AtomicUsize,
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data: T,
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}
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unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
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unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
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impl<T> Arc<T> {
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/// Construct an `Arc<T>`
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#[inline]
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pub fn new(data: T) -> Self {
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let x = Box::new(ArcInner {
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count: atomic::AtomicUsize::new(1),
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data: data,
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});
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unsafe {
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Arc {
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p: ptr::NonNull::new_unchecked(Box::into_raw(x)),
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}
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}
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}
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/// Convert the Arc<T> to a raw pointer, suitable for use across FFI
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///
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/// Note: This returns a pointer to the data T, which is offset in the allocation.
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///
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/// It is recommended to use RawOffsetArc for this.
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#[inline]
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fn into_raw(this: Self) -> *const T {
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let ptr = unsafe { &((*this.ptr()).data) as *const _ };
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mem::forget(this);
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ptr
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}
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/// Reconstruct the Arc<T> from a raw pointer obtained from into_raw()
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///
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/// Note: This raw pointer will be offset in the allocation and must be preceded
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/// by the atomic count.
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///
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/// It is recommended to use RawOffsetArc for this
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#[inline]
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unsafe fn from_raw(ptr: *const T) -> Self {
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// To find the corresponding pointer to the `ArcInner` we need
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// to subtract the offset of the `data` field from the pointer.
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let ptr = (ptr as *const u8).offset(-offset_of!(ArcInner<T>, data));
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Arc {
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p: ptr::NonNull::new_unchecked(ptr as *mut ArcInner<T>),
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}
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}
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/// Produce a pointer to the data that can be converted back
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/// to an Arc. This is basically an `&Arc<T>`, without the extra indirection.
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/// It has the benefits of an `&T` but also knows about the underlying refcount
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/// and can be converted into more `Arc<T>`s if necessary.
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#[inline]
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pub fn borrow_arc<'a>(&'a self) -> ArcBorrow<'a, T> {
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ArcBorrow(&**self)
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}
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/// Temporarily converts |self| into a bonafide RawOffsetArc and exposes it to the
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/// provided callback. The refcount is not modified.
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#[inline(always)]
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pub fn with_raw_offset_arc<F, U>(&self, f: F) -> U
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where
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F: FnOnce(&RawOffsetArc<T>) -> U,
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{
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// Synthesize transient Arc, which never touches the refcount of the ArcInner.
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let transient = unsafe { NoDrop::new(Arc::into_raw_offset(ptr::read(self))) };
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// Expose the transient Arc to the callback, which may clone it if it wants.
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let result = f(&transient);
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// Forget the transient Arc to leave the refcount untouched.
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mem::forget(transient);
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// Forward the result.
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result
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}
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/// Returns the address on the heap of the Arc itself -- not the T within it -- for memory
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/// reporting.
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pub fn heap_ptr(&self) -> *const c_void {
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self.p.as_ptr() as *const ArcInner<T> as *const c_void
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}
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}
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impl<T: ?Sized> Arc<T> {
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#[inline]
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fn inner(&self) -> &ArcInner<T> {
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// This unsafety is ok because while this arc is alive we're guaranteed
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// that the inner pointer is valid. Furthermore, we know that the
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// `ArcInner` structure itself is `Sync` because the inner data is
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// `Sync` as well, so we're ok loaning out an immutable pointer to these
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// contents.
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unsafe { &*self.ptr() }
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}
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// Non-inlined part of `drop`. Just invokes the destructor.
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#[inline(never)]
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unsafe fn drop_slow(&mut self) {
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let _ = Box::from_raw(self.ptr());
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}
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/// Test pointer equality between the two Arcs, i.e. they must be the _same_
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/// allocation
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#[inline]
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pub fn ptr_eq(this: &Self, other: &Self) -> bool {
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this.ptr() == other.ptr()
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}
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fn ptr(&self) -> *mut ArcInner<T> {
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self.p.as_ptr()
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}
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}
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impl<T: ?Sized> Clone for Arc<T> {
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#[inline]
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fn clone(&self) -> Self {
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// Using a relaxed ordering is alright here, as knowledge of the
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// original reference prevents other threads from erroneously deleting
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// the object.
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//
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// As explained in the [Boost documentation][1], Increasing the
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// reference counter can always be done with memory_order_relaxed: New
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// references to an object can only be formed from an existing
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// reference, and passing an existing reference from one thread to
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// another must already provide any required synchronization.
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//
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// [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
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let old_size = self.inner().count.fetch_add(1, Relaxed);
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// However we need to guard against massive refcounts in case someone
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// is `mem::forget`ing Arcs. If we don't do this the count can overflow
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// and users will use-after free. We racily saturate to `isize::MAX` on
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// the assumption that there aren't ~2 billion threads incrementing
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// the reference count at once. This branch will never be taken in
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// any realistic program.
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//
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// We abort because such a program is incredibly degenerate, and we
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// don't care to support it.
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if old_size > MAX_REFCOUNT {
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process::abort();
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}
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unsafe {
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Arc {
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p: ptr::NonNull::new_unchecked(self.ptr()),
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}
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}
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}
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}
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impl<T: ?Sized> Deref for Arc<T> {
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type Target = T;
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#[inline]
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fn deref(&self) -> &T {
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&self.inner().data
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}
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}
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impl<T: Clone> Arc<T> {
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/// Makes a mutable reference to the `Arc`, cloning if necessary
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///
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/// This is functionally equivalent to [`Arc::make_mut`][mm] from the standard library.
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///
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/// If this `Arc` is uniquely owned, `make_mut()` will provide a mutable
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/// reference to the contents. If not, `make_mut()` will create a _new_ `Arc`
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/// with a copy of the contents, update `this` to point to it, and provide
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/// a mutable reference to its contents.
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///
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/// This is useful for implementing copy-on-write schemes where you wish to
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/// avoid copying things if your `Arc` is not shared.
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///
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/// [mm]: https://doc.rust-lang.org/stable/std/sync/struct.Arc.html#method.make_mut
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#[inline]
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pub fn make_mut(this: &mut Self) -> &mut T {
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if !this.is_unique() {
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// Another pointer exists; clone
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*this = Arc::new((**this).clone());
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}
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unsafe {
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// This unsafety is ok because we're guaranteed that the pointer
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// returned is the *only* pointer that will ever be returned to T. Our
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// reference count is guaranteed to be 1 at this point, and we required
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// the Arc itself to be `mut`, so we're returning the only possible
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// reference to the inner data.
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&mut (*this.ptr()).data
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}
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}
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}
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impl<T: ?Sized> Arc<T> {
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/// Provides mutable access to the contents _if_ the `Arc` is uniquely owned.
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#[inline]
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pub fn get_mut(this: &mut Self) -> Option<&mut T> {
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if this.is_unique() {
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unsafe {
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// See make_mut() for documentation of the threadsafety here.
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Some(&mut (*this.ptr()).data)
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}
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} else {
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None
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}
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}
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/// Whether or not the `Arc` is uniquely owned (is the refcount 1?)
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#[inline]
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pub fn is_unique(&self) -> bool {
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// See the extensive discussion in [1] for why this needs to be Acquire.
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//
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// [1] https://github.com/servo/servo/issues/21186
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self.inner().count.load(Acquire) == 1
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}
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}
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impl<T: ?Sized> Drop for Arc<T> {
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#[inline]
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fn drop(&mut self) {
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// Because `fetch_sub` is already atomic, we do not need to synchronize
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// with other threads unless we are going to delete the object.
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if self.inner().count.fetch_sub(1, Release) != 1 {
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return;
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}
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// FIXME(bholley): Use the updated comment when [2] is merged.
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//
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// This load is needed to prevent reordering of use of the data and
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// deletion of the data. Because it is marked `Release`, the decreasing
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// of the reference count synchronizes with this `Acquire` load. This
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// means that use of the data happens before decreasing the reference
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// count, which happens before this load, which happens before the
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// deletion of the data.
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//
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// As explained in the [Boost documentation][1],
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//
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// > It is important to enforce any possible access to the object in one
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// > thread (through an existing reference) to *happen before* deleting
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// > the object in a different thread. This is achieved by a "release"
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// > operation after dropping a reference (any access to the object
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// > through this reference must obviously happened before), and an
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// > "acquire" operation before deleting the object.
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//
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// [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
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// [2]: https://github.com/rust-lang/rust/pull/41714
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self.inner().count.load(Acquire);
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unsafe {
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self.drop_slow();
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}
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}
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}
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impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
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fn eq(&self, other: &Arc<T>) -> bool {
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Self::ptr_eq(self, other) || *(*self) == *(*other)
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}
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fn ne(&self, other: &Arc<T>) -> bool {
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!Self::ptr_eq(self, other) && *(*self) != *(*other)
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}
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}
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impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
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fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
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(**self).partial_cmp(&**other)
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}
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fn lt(&self, other: &Arc<T>) -> bool {
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*(*self) < *(*other)
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}
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fn le(&self, other: &Arc<T>) -> bool {
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*(*self) <= *(*other)
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}
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fn gt(&self, other: &Arc<T>) -> bool {
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*(*self) > *(*other)
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}
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fn ge(&self, other: &Arc<T>) -> bool {
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*(*self) >= *(*other)
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}
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}
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impl<T: ?Sized + Ord> Ord for Arc<T> {
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fn cmp(&self, other: &Arc<T>) -> Ordering {
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(**self).cmp(&**other)
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}
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}
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impl<T: ?Sized + Eq> Eq for Arc<T> {}
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impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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fmt::Display::fmt(&**self, f)
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}
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}
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impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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fmt::Debug::fmt(&**self, f)
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}
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}
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impl<T: ?Sized> fmt::Pointer for Arc<T> {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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fmt::Pointer::fmt(&self.ptr(), f)
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}
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}
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impl<T: Default> Default for Arc<T> {
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fn default() -> Arc<T> {
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Arc::new(Default::default())
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}
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}
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impl<T: ?Sized + Hash> Hash for Arc<T> {
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fn hash<H: Hasher>(&self, state: &mut H) {
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(**self).hash(state)
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}
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}
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impl<T> From<T> for Arc<T> {
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#[inline]
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fn from(t: T) -> Self {
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Arc::new(t)
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}
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}
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impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
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#[inline]
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fn borrow(&self) -> &T {
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&**self
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}
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}
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impl<T: ?Sized> AsRef<T> for Arc<T> {
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#[inline]
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fn as_ref(&self) -> &T {
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&**self
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}
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}
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unsafe impl<T: ?Sized> StableDeref for Arc<T> {}
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unsafe impl<T: ?Sized> CloneStableDeref for Arc<T> {}
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|
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#[cfg(feature = "servo")]
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impl<'de, T: Deserialize<'de>> Deserialize<'de> for Arc<T> {
|
|
fn deserialize<D>(deserializer: D) -> Result<Arc<T>, D::Error>
|
|
where
|
|
D: ::serde::de::Deserializer<'de>,
|
|
{
|
|
T::deserialize(deserializer).map(Arc::new)
|
|
}
|
|
}
|
|
|
|
#[cfg(feature = "servo")]
|
|
impl<T: Serialize> Serialize for Arc<T> {
|
|
fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
|
|
where
|
|
S: ::serde::ser::Serializer,
|
|
{
|
|
(**self).serialize(serializer)
|
|
}
|
|
}
|
|
|
|
/// Structure to allow Arc-managing some fixed-sized data and a variably-sized
|
|
/// slice in a single allocation.
|
|
#[derive(Debug, Eq, PartialEq, PartialOrd)]
|
|
pub struct HeaderSlice<H, T: ?Sized> {
|
|
/// The fixed-sized data.
|
|
pub header: H,
|
|
|
|
/// The dynamically-sized data.
|
|
pub slice: T,
|
|
}
|
|
|
|
#[inline(always)]
|
|
fn divide_rounding_up(dividend: usize, divisor: usize) -> usize {
|
|
(dividend + divisor - 1) / divisor
|
|
}
|
|
|
|
impl<H, T> Arc<HeaderSlice<H, [T]>> {
|
|
/// Creates an Arc for a HeaderSlice using the given header struct and
|
|
/// iterator to generate the slice. The resulting Arc will be fat.
|
|
#[inline]
|
|
pub fn from_header_and_iter<I>(header: H, mut items: I) -> Self
|
|
where
|
|
I: Iterator<Item = T> + ExactSizeIterator,
|
|
{
|
|
use std::mem::size_of;
|
|
assert_ne!(size_of::<T>(), 0, "Need to think about ZST");
|
|
|
|
// Compute the required size for the allocation.
|
|
let num_items = items.len();
|
|
let size = {
|
|
// First, determine the alignment of a hypothetical pointer to a
|
|
// HeaderSlice.
|
|
let fake_slice_ptr_align: usize = mem::align_of::<ArcInner<HeaderSlice<H, [T; 1]>>>();
|
|
|
|
// Next, synthesize a totally garbage (but properly aligned) pointer
|
|
// to a sequence of T.
|
|
let fake_slice_ptr = fake_slice_ptr_align as *const T;
|
|
|
|
// Convert that sequence to a fat pointer. The address component of
|
|
// the fat pointer will be garbage, but the length will be correct.
|
|
let fake_slice = unsafe { slice::from_raw_parts(fake_slice_ptr, num_items) };
|
|
|
|
// Pretend the garbage address points to our allocation target (with
|
|
// a trailing sequence of T), rather than just a sequence of T.
|
|
let fake_ptr = fake_slice as *const [T] as *const ArcInner<HeaderSlice<H, [T]>>;
|
|
let fake_ref: &ArcInner<HeaderSlice<H, [T]>> = unsafe { &*fake_ptr };
|
|
|
|
// Use size_of_val, which will combine static information about the
|
|
// type with the length from the fat pointer. The garbage address
|
|
// will not be used.
|
|
mem::size_of_val(fake_ref)
|
|
};
|
|
|
|
let ptr: *mut ArcInner<HeaderSlice<H, [T]>>;
|
|
unsafe {
|
|
// Allocate the buffer. We use Vec because the underlying allocation
|
|
// machinery isn't available in stable Rust.
|
|
//
|
|
// To avoid alignment issues, we allocate words rather than bytes,
|
|
// rounding up to the nearest word size.
|
|
let buffer = if mem::align_of::<T>() <= mem::align_of::<usize>() {
|
|
Self::allocate_buffer::<usize>(size)
|
|
} else if mem::align_of::<T>() <= mem::align_of::<u64>() {
|
|
// On 32-bit platforms <T> may have 8 byte alignment while usize has 4 byte aligment.
|
|
// Use u64 to avoid over-alignment.
|
|
// This branch will compile away in optimized builds.
|
|
Self::allocate_buffer::<u64>(size)
|
|
} else {
|
|
panic!("Over-aligned type not handled");
|
|
};
|
|
|
|
// Synthesize the fat pointer. We do this by claiming we have a direct
|
|
// pointer to a [T], and then changing the type of the borrow. The key
|
|
// point here is that the length portion of the fat pointer applies
|
|
// only to the number of elements in the dynamically-sized portion of
|
|
// the type, so the value will be the same whether it points to a [T]
|
|
// or something else with a [T] as its last member.
|
|
let fake_slice: &mut [T] = slice::from_raw_parts_mut(buffer as *mut T, num_items);
|
|
ptr = fake_slice as *mut [T] as *mut ArcInner<HeaderSlice<H, [T]>>;
|
|
|
|
// Write the data.
|
|
//
|
|
// Note that any panics here (i.e. from the iterator) are safe, since
|
|
// we'll just leak the uninitialized memory.
|
|
ptr::write(&mut ((*ptr).count), atomic::AtomicUsize::new(1));
|
|
ptr::write(&mut ((*ptr).data.header), header);
|
|
let mut current: *mut T = &mut (*ptr).data.slice[0];
|
|
for _ in 0..num_items {
|
|
ptr::write(
|
|
current,
|
|
items
|
|
.next()
|
|
.expect("ExactSizeIterator over-reported length"),
|
|
);
|
|
current = current.offset(1);
|
|
}
|
|
assert!(
|
|
items.next().is_none(),
|
|
"ExactSizeIterator under-reported length"
|
|
);
|
|
|
|
// We should have consumed the buffer exactly.
|
|
debug_assert_eq!(current as *mut u8, buffer.offset(size as isize));
|
|
}
|
|
|
|
// Return the fat Arc.
|
|
assert_eq!(
|
|
size_of::<Self>(),
|
|
size_of::<usize>() * 2,
|
|
"The Arc will be fat"
|
|
);
|
|
unsafe {
|
|
Arc {
|
|
p: ptr::NonNull::new_unchecked(ptr),
|
|
}
|
|
}
|
|
}
|
|
|
|
#[inline]
|
|
unsafe fn allocate_buffer<W>(size: usize) -> *mut u8 {
|
|
let words_to_allocate = divide_rounding_up(size, mem::size_of::<W>());
|
|
let mut vec = Vec::<W>::with_capacity(words_to_allocate);
|
|
vec.set_len(words_to_allocate);
|
|
Box::into_raw(vec.into_boxed_slice()) as *mut W as *mut u8
|
|
}
|
|
}
|
|
|
|
/// Header data with an inline length. Consumers that use HeaderWithLength as the
|
|
/// Header type in HeaderSlice can take advantage of ThinArc.
|
|
#[derive(Debug, Eq, PartialEq, PartialOrd)]
|
|
pub struct HeaderWithLength<H> {
|
|
/// The fixed-sized data.
|
|
pub header: H,
|
|
|
|
/// The slice length.
|
|
length: usize,
|
|
}
|
|
|
|
impl<H> HeaderWithLength<H> {
|
|
/// Creates a new HeaderWithLength.
|
|
pub fn new(header: H, length: usize) -> Self {
|
|
HeaderWithLength {
|
|
header: header,
|
|
length: length,
|
|
}
|
|
}
|
|
}
|
|
|
|
type HeaderSliceWithLength<H, T> = HeaderSlice<HeaderWithLength<H>, T>;
|
|
|
|
/// A "thin" `Arc` containing dynamically sized data
|
|
///
|
|
/// This is functionally equivalent to Arc<(H, [T])>
|
|
///
|
|
/// When you create an `Arc` containing a dynamically sized type
|
|
/// like `HeaderSlice<H, [T]>`, the `Arc` is represented on the stack
|
|
/// as a "fat pointer", where the length of the slice is stored
|
|
/// alongside the `Arc`'s pointer. In some situations you may wish to
|
|
/// have a thin pointer instead, perhaps for FFI compatibility
|
|
/// or space efficiency.
|
|
///
|
|
/// `ThinArc` solves this by storing the length in the allocation itself,
|
|
/// via `HeaderSliceWithLength`.
|
|
#[repr(C)]
|
|
pub struct ThinArc<H, T> {
|
|
ptr: *mut ArcInner<HeaderSliceWithLength<H, [T; 1]>>,
|
|
}
|
|
|
|
unsafe impl<H: Sync + Send, T: Sync + Send> Send for ThinArc<H, T> {}
|
|
unsafe impl<H: Sync + Send, T: Sync + Send> Sync for ThinArc<H, T> {}
|
|
|
|
// Synthesize a fat pointer from a thin pointer.
|
|
//
|
|
// See the comment around the analogous operation in from_header_and_iter.
|
|
fn thin_to_thick<H, T>(
|
|
thin: *mut ArcInner<HeaderSliceWithLength<H, [T; 1]>>,
|
|
) -> *mut ArcInner<HeaderSliceWithLength<H, [T]>> {
|
|
let len = unsafe { (*thin).data.header.length };
|
|
let fake_slice: *mut [T] = unsafe { slice::from_raw_parts_mut(thin as *mut T, len) };
|
|
|
|
fake_slice as *mut ArcInner<HeaderSliceWithLength<H, [T]>>
|
|
}
|
|
|
|
impl<H, T> ThinArc<H, T> {
|
|
/// Temporarily converts |self| into a bonafide Arc and exposes it to the
|
|
/// provided callback. The refcount is not modified.
|
|
#[inline]
|
|
pub fn with_arc<F, U>(&self, f: F) -> U
|
|
where
|
|
F: FnOnce(&Arc<HeaderSliceWithLength<H, [T]>>) -> U,
|
|
{
|
|
// Synthesize transient Arc, which never touches the refcount of the ArcInner.
|
|
let transient = unsafe {
|
|
NoDrop::new(Arc {
|
|
p: ptr::NonNull::new_unchecked(thin_to_thick(self.ptr)),
|
|
})
|
|
};
|
|
|
|
// Expose the transient Arc to the callback, which may clone it if it wants.
|
|
let result = f(&transient);
|
|
|
|
// Forget the transient Arc to leave the refcount untouched.
|
|
// XXXManishearth this can be removed when unions stabilize,
|
|
// since then NoDrop becomes zero overhead
|
|
mem::forget(transient);
|
|
|
|
// Forward the result.
|
|
result
|
|
}
|
|
|
|
/// Creates a `ThinArc` for a HeaderSlice using the given header struct and
|
|
/// iterator to generate the slice.
|
|
pub fn from_header_and_iter<I>(header: H, items: I) -> Self
|
|
where
|
|
I: Iterator<Item = T> + ExactSizeIterator,
|
|
{
|
|
let header = HeaderWithLength::new(header, items.len());
|
|
Arc::into_thin(Arc::from_header_and_iter(header, items))
|
|
}
|
|
|
|
/// Returns the address on the heap of the ThinArc itself -- not the T
|
|
/// within it -- for memory reporting.
|
|
#[inline]
|
|
pub fn heap_ptr(&self) -> *const c_void {
|
|
self.ptr as *const ArcInner<T> as *const c_void
|
|
}
|
|
}
|
|
|
|
impl<H, T> Deref for ThinArc<H, T> {
|
|
type Target = HeaderSliceWithLength<H, [T]>;
|
|
|
|
#[inline]
|
|
fn deref(&self) -> &Self::Target {
|
|
unsafe { &(*thin_to_thick(self.ptr)).data }
|
|
}
|
|
}
|
|
|
|
impl<H, T> Clone for ThinArc<H, T> {
|
|
#[inline]
|
|
fn clone(&self) -> Self {
|
|
ThinArc::with_arc(self, |a| Arc::into_thin(a.clone()))
|
|
}
|
|
}
|
|
|
|
impl<H, T> Drop for ThinArc<H, T> {
|
|
#[inline]
|
|
fn drop(&mut self) {
|
|
let _ = Arc::from_thin(ThinArc { ptr: self.ptr });
|
|
}
|
|
}
|
|
|
|
impl<H, T> Arc<HeaderSliceWithLength<H, [T]>> {
|
|
/// Converts an `Arc` into a `ThinArc`. This consumes the `Arc`, so the refcount
|
|
/// is not modified.
|
|
#[inline]
|
|
pub fn into_thin(a: Self) -> ThinArc<H, T> {
|
|
assert_eq!(
|
|
a.header.length,
|
|
a.slice.len(),
|
|
"Length needs to be correct for ThinArc to work"
|
|
);
|
|
let fat_ptr: *mut ArcInner<HeaderSliceWithLength<H, [T]>> = a.ptr();
|
|
mem::forget(a);
|
|
let thin_ptr = fat_ptr as *mut [usize] as *mut usize;
|
|
ThinArc {
|
|
ptr: thin_ptr as *mut ArcInner<HeaderSliceWithLength<H, [T; 1]>>,
|
|
}
|
|
}
|
|
|
|
/// Converts a `ThinArc` into an `Arc`. This consumes the `ThinArc`, so the refcount
|
|
/// is not modified.
|
|
#[inline]
|
|
pub fn from_thin(a: ThinArc<H, T>) -> Self {
|
|
let ptr = thin_to_thick(a.ptr);
|
|
mem::forget(a);
|
|
unsafe {
|
|
Arc {
|
|
p: ptr::NonNull::new_unchecked(ptr),
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
impl<H: PartialEq, T: PartialEq> PartialEq for ThinArc<H, T> {
|
|
#[inline]
|
|
fn eq(&self, other: &ThinArc<H, T>) -> bool {
|
|
ThinArc::with_arc(self, |a| ThinArc::with_arc(other, |b| *a == *b))
|
|
}
|
|
}
|
|
|
|
impl<H: Eq, T: Eq> Eq for ThinArc<H, T> {}
|
|
|
|
/// An `Arc`, except it holds a pointer to the T instead of to the
|
|
/// entire ArcInner. This struct is FFI-compatible.
|
|
///
|
|
/// ```text
|
|
/// Arc<T> RawOffsetArc<T>
|
|
/// | |
|
|
/// v v
|
|
/// ---------------------
|
|
/// | RefCount | T (data) | [ArcInner<T>]
|
|
/// ---------------------
|
|
/// ```
|
|
///
|
|
/// This means that this is a direct pointer to
|
|
/// its contained data (and can be read from by both C++ and Rust),
|
|
/// but we can also convert it to a "regular" Arc<T> by removing the offset.
|
|
///
|
|
/// This is very useful if you have an Arc-containing struct shared between Rust and C++,
|
|
/// and wish for C++ to be able to read the data behind the `Arc` without incurring
|
|
/// an FFI call overhead.
|
|
#[derive(Eq)]
|
|
#[repr(C)]
|
|
pub struct RawOffsetArc<T> {
|
|
ptr: ptr::NonNull<T>,
|
|
}
|
|
|
|
unsafe impl<T: Sync + Send> Send for RawOffsetArc<T> {}
|
|
unsafe impl<T: Sync + Send> Sync for RawOffsetArc<T> {}
|
|
|
|
impl<T> Deref for RawOffsetArc<T> {
|
|
type Target = T;
|
|
fn deref(&self) -> &Self::Target {
|
|
unsafe { &*self.ptr.as_ptr() }
|
|
}
|
|
}
|
|
|
|
impl<T> Clone for RawOffsetArc<T> {
|
|
#[inline]
|
|
fn clone(&self) -> Self {
|
|
Arc::into_raw_offset(self.clone_arc())
|
|
}
|
|
}
|
|
|
|
impl<T> Drop for RawOffsetArc<T> {
|
|
fn drop(&mut self) {
|
|
let _ = Arc::from_raw_offset(RawOffsetArc {
|
|
ptr: self.ptr.clone(),
|
|
});
|
|
}
|
|
}
|
|
|
|
impl<T: fmt::Debug> fmt::Debug for RawOffsetArc<T> {
|
|
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
|
|
fmt::Debug::fmt(&**self, f)
|
|
}
|
|
}
|
|
|
|
impl<T: PartialEq> PartialEq for RawOffsetArc<T> {
|
|
fn eq(&self, other: &RawOffsetArc<T>) -> bool {
|
|
*(*self) == *(*other)
|
|
}
|
|
|
|
fn ne(&self, other: &RawOffsetArc<T>) -> bool {
|
|
*(*self) != *(*other)
|
|
}
|
|
}
|
|
|
|
impl<T> RawOffsetArc<T> {
|
|
/// Temporarily converts |self| into a bonafide Arc and exposes it to the
|
|
/// provided callback. The refcount is not modified.
|
|
#[inline]
|
|
pub fn with_arc<F, U>(&self, f: F) -> U
|
|
where
|
|
F: FnOnce(&Arc<T>) -> U,
|
|
{
|
|
// Synthesize transient Arc, which never touches the refcount of the ArcInner.
|
|
let transient = unsafe { NoDrop::new(Arc::from_raw(self.ptr.as_ptr())) };
|
|
|
|
// Expose the transient Arc to the callback, which may clone it if it wants.
|
|
let result = f(&transient);
|
|
|
|
// Forget the transient Arc to leave the refcount untouched.
|
|
// XXXManishearth this can be removed when unions stabilize,
|
|
// since then NoDrop becomes zero overhead
|
|
mem::forget(transient);
|
|
|
|
// Forward the result.
|
|
result
|
|
}
|
|
|
|
/// If uniquely owned, provide a mutable reference
|
|
/// Else create a copy, and mutate that
|
|
///
|
|
/// This is functionally the same thing as `Arc::make_mut`
|
|
#[inline]
|
|
pub fn make_mut(&mut self) -> &mut T
|
|
where
|
|
T: Clone,
|
|
{
|
|
unsafe {
|
|
// extract the RawOffsetArc as an owned variable
|
|
let this = ptr::read(self);
|
|
// treat it as a real Arc
|
|
let mut arc = Arc::from_raw_offset(this);
|
|
// obtain the mutable reference. Cast away the lifetime
|
|
// This may mutate `arc`
|
|
let ret = Arc::make_mut(&mut arc) as *mut _;
|
|
// Store the possibly-mutated arc back inside, after converting
|
|
// it to a RawOffsetArc again
|
|
ptr::write(self, Arc::into_raw_offset(arc));
|
|
&mut *ret
|
|
}
|
|
}
|
|
|
|
/// Clone it as an `Arc`
|
|
#[inline]
|
|
pub fn clone_arc(&self) -> Arc<T> {
|
|
RawOffsetArc::with_arc(self, |a| a.clone())
|
|
}
|
|
|
|
/// Produce a pointer to the data that can be converted back
|
|
/// to an `Arc`
|
|
#[inline]
|
|
pub fn borrow_arc<'a>(&'a self) -> ArcBorrow<'a, T> {
|
|
ArcBorrow(&**self)
|
|
}
|
|
}
|
|
|
|
impl<T> Arc<T> {
|
|
/// Converts an `Arc` into a `RawOffsetArc`. This consumes the `Arc`, so the refcount
|
|
/// is not modified.
|
|
#[inline]
|
|
pub fn into_raw_offset(a: Self) -> RawOffsetArc<T> {
|
|
unsafe {
|
|
RawOffsetArc {
|
|
ptr: ptr::NonNull::new_unchecked(Arc::into_raw(a) as *mut T),
|
|
}
|
|
}
|
|
}
|
|
|
|
/// Converts a `RawOffsetArc` into an `Arc`. This consumes the `RawOffsetArc`, so the refcount
|
|
/// is not modified.
|
|
#[inline]
|
|
pub fn from_raw_offset(a: RawOffsetArc<T>) -> Self {
|
|
let ptr = a.ptr.as_ptr();
|
|
mem::forget(a);
|
|
unsafe { Arc::from_raw(ptr) }
|
|
}
|
|
}
|
|
|
|
/// A "borrowed `Arc`". This is a pointer to
|
|
/// a T that is known to have been allocated within an
|
|
/// `Arc`.
|
|
///
|
|
/// This is equivalent in guarantees to `&Arc<T>`, however it is
|
|
/// a bit more flexible. To obtain an `&Arc<T>` you must have
|
|
/// an `Arc<T>` instance somewhere pinned down until we're done with it.
|
|
/// It's also a direct pointer to `T`, so using this involves less pointer-chasing
|
|
///
|
|
/// However, C++ code may hand us refcounted things as pointers to T directly,
|
|
/// so we have to conjure up a temporary `Arc` on the stack each time. The
|
|
/// same happens for when the object is managed by a `RawOffsetArc`.
|
|
///
|
|
/// `ArcBorrow` lets us deal with borrows of known-refcounted objects
|
|
/// without needing to worry about where the `Arc<T>` is.
|
|
#[derive(Debug, Eq, PartialEq)]
|
|
pub struct ArcBorrow<'a, T: 'a>(&'a T);
|
|
|
|
impl<'a, T> Copy for ArcBorrow<'a, T> {}
|
|
impl<'a, T> Clone for ArcBorrow<'a, T> {
|
|
#[inline]
|
|
fn clone(&self) -> Self {
|
|
*self
|
|
}
|
|
}
|
|
|
|
impl<'a, T> ArcBorrow<'a, T> {
|
|
/// Clone this as an `Arc<T>`. This bumps the refcount.
|
|
#[inline]
|
|
pub fn clone_arc(&self) -> Arc<T> {
|
|
let arc = unsafe { Arc::from_raw(self.0) };
|
|
// addref it!
|
|
mem::forget(arc.clone());
|
|
arc
|
|
}
|
|
|
|
/// For constructing from a reference known to be Arc-backed,
|
|
/// e.g. if we obtain such a reference over FFI
|
|
#[inline]
|
|
pub unsafe fn from_ref(r: &'a T) -> Self {
|
|
ArcBorrow(r)
|
|
}
|
|
|
|
/// Compare two `ArcBorrow`s via pointer equality. Will only return
|
|
/// true if they come from the same allocation
|
|
pub fn ptr_eq(this: &Self, other: &Self) -> bool {
|
|
this.0 as *const T == other.0 as *const T
|
|
}
|
|
|
|
/// Temporarily converts |self| into a bonafide Arc and exposes it to the
|
|
/// provided callback. The refcount is not modified.
|
|
#[inline]
|
|
pub fn with_arc<F, U>(&self, f: F) -> U
|
|
where
|
|
F: FnOnce(&Arc<T>) -> U,
|
|
T: 'static,
|
|
{
|
|
// Synthesize transient Arc, which never touches the refcount.
|
|
let transient = unsafe { NoDrop::new(Arc::from_raw(self.0)) };
|
|
|
|
// Expose the transient Arc to the callback, which may clone it if it wants.
|
|
let result = f(&transient);
|
|
|
|
// Forget the transient Arc to leave the refcount untouched.
|
|
// XXXManishearth this can be removed when unions stabilize,
|
|
// since then NoDrop becomes zero overhead
|
|
mem::forget(transient);
|
|
|
|
// Forward the result.
|
|
result
|
|
}
|
|
|
|
/// Similar to deref, but uses the lifetime |a| rather than the lifetime of
|
|
/// self, which is incompatible with the signature of the Deref trait.
|
|
#[inline]
|
|
pub fn get(&self) -> &'a T {
|
|
self.0
|
|
}
|
|
}
|
|
|
|
impl<'a, T> Deref for ArcBorrow<'a, T> {
|
|
type Target = T;
|
|
|
|
#[inline]
|
|
fn deref(&self) -> &T {
|
|
self.0
|
|
}
|
|
}
|
|
|
|
/// A tagged union that can represent `Arc<A>` or `Arc<B>` while only consuming a
|
|
/// single word. The type is also `NonNull`, and thus can be stored in an Option
|
|
/// without increasing size.
|
|
///
|
|
/// This is functionally equivalent to
|
|
/// `enum ArcUnion<A, B> { First(Arc<A>), Second(Arc<B>)` but only takes up
|
|
/// up a single word of stack space.
|
|
///
|
|
/// This could probably be extended to support four types if necessary.
|
|
pub struct ArcUnion<A, B> {
|
|
p: ptr::NonNull<()>,
|
|
phantom_a: PhantomData<A>,
|
|
phantom_b: PhantomData<B>,
|
|
}
|
|
|
|
unsafe impl<A: Sync + Send, B: Send + Sync> Send for ArcUnion<A, B> {}
|
|
unsafe impl<A: Sync + Send, B: Send + Sync> Sync for ArcUnion<A, B> {}
|
|
|
|
impl<A: PartialEq, B: PartialEq> PartialEq for ArcUnion<A, B> {
|
|
fn eq(&self, other: &Self) -> bool {
|
|
use ArcUnionBorrow::*;
|
|
match (self.borrow(), other.borrow()) {
|
|
(First(x), First(y)) => x == y,
|
|
(Second(x), Second(y)) => x == y,
|
|
(_, _) => false,
|
|
}
|
|
}
|
|
}
|
|
|
|
/// This represents a borrow of an `ArcUnion`.
|
|
#[derive(Debug)]
|
|
pub enum ArcUnionBorrow<'a, A: 'a, B: 'a> {
|
|
First(ArcBorrow<'a, A>),
|
|
Second(ArcBorrow<'a, B>),
|
|
}
|
|
|
|
impl<A, B> ArcUnion<A, B> {
|
|
unsafe fn new(ptr: *mut ()) -> Self {
|
|
ArcUnion {
|
|
p: ptr::NonNull::new_unchecked(ptr),
|
|
phantom_a: PhantomData,
|
|
phantom_b: PhantomData,
|
|
}
|
|
}
|
|
|
|
/// Returns true if the two values are pointer-equal.
|
|
pub fn ptr_eq(this: &Self, other: &Self) -> bool {
|
|
this.p == other.p
|
|
}
|
|
|
|
/// Returns an enum representing a borrow of either A or B.
|
|
pub fn borrow(&self) -> ArcUnionBorrow<A, B> {
|
|
if self.is_first() {
|
|
let ptr = self.p.as_ptr() as *const A;
|
|
let borrow = unsafe { ArcBorrow::from_ref(&*ptr) };
|
|
ArcUnionBorrow::First(borrow)
|
|
} else {
|
|
let ptr = ((self.p.as_ptr() as usize) & !0x1) as *const B;
|
|
let borrow = unsafe { ArcBorrow::from_ref(&*ptr) };
|
|
ArcUnionBorrow::Second(borrow)
|
|
}
|
|
}
|
|
|
|
/// Creates an `ArcUnion` from an instance of the first type.
|
|
pub fn from_first(other: Arc<A>) -> Self {
|
|
unsafe { Self::new(Arc::into_raw(other) as *mut _) }
|
|
}
|
|
|
|
/// Creates an `ArcUnion` from an instance of the second type.
|
|
pub fn from_second(other: Arc<B>) -> Self {
|
|
unsafe { Self::new(((Arc::into_raw(other) as usize) | 0x1) as *mut _) }
|
|
}
|
|
|
|
/// Returns true if this `ArcUnion` contains the first type.
|
|
pub fn is_first(&self) -> bool {
|
|
self.p.as_ptr() as usize & 0x1 == 0
|
|
}
|
|
|
|
/// Returns true if this `ArcUnion` contains the second type.
|
|
pub fn is_second(&self) -> bool {
|
|
!self.is_first()
|
|
}
|
|
|
|
/// Returns a borrow of the first type if applicable, otherwise `None`.
|
|
pub fn as_first(&self) -> Option<ArcBorrow<A>> {
|
|
match self.borrow() {
|
|
ArcUnionBorrow::First(x) => Some(x),
|
|
ArcUnionBorrow::Second(_) => None,
|
|
}
|
|
}
|
|
|
|
/// Returns a borrow of the second type if applicable, otherwise None.
|
|
pub fn as_second(&self) -> Option<ArcBorrow<B>> {
|
|
match self.borrow() {
|
|
ArcUnionBorrow::First(_) => None,
|
|
ArcUnionBorrow::Second(x) => Some(x),
|
|
}
|
|
}
|
|
}
|
|
|
|
impl<A, B> Clone for ArcUnion<A, B> {
|
|
fn clone(&self) -> Self {
|
|
match self.borrow() {
|
|
ArcUnionBorrow::First(x) => ArcUnion::from_first(x.clone_arc()),
|
|
ArcUnionBorrow::Second(x) => ArcUnion::from_second(x.clone_arc()),
|
|
}
|
|
}
|
|
}
|
|
|
|
impl<A, B> Drop for ArcUnion<A, B> {
|
|
fn drop(&mut self) {
|
|
match self.borrow() {
|
|
ArcUnionBorrow::First(x) => unsafe {
|
|
let _ = Arc::from_raw(&*x);
|
|
},
|
|
ArcUnionBorrow::Second(x) => unsafe {
|
|
let _ = Arc::from_raw(&*x);
|
|
},
|
|
}
|
|
}
|
|
}
|
|
|
|
impl<A: fmt::Debug, B: fmt::Debug> fmt::Debug for ArcUnion<A, B> {
|
|
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
|
|
fmt::Debug::fmt(&self.borrow(), f)
|
|
}
|
|
}
|
|
|
|
#[cfg(test)]
|
|
mod tests {
|
|
use std::clone::Clone;
|
|
use std::ops::Drop;
|
|
use std::sync::atomic;
|
|
use std::sync::atomic::Ordering::{Acquire, SeqCst};
|
|
use super::{Arc, HeaderWithLength, ThinArc};
|
|
|
|
#[derive(PartialEq)]
|
|
struct Canary(*mut atomic::AtomicUsize);
|
|
|
|
impl Drop for Canary {
|
|
fn drop(&mut self) {
|
|
unsafe {
|
|
(*self.0).fetch_add(1, SeqCst);
|
|
}
|
|
}
|
|
}
|
|
|
|
#[test]
|
|
fn slices_and_thin() {
|
|
let mut canary = atomic::AtomicUsize::new(0);
|
|
let c = Canary(&mut canary as *mut atomic::AtomicUsize);
|
|
let v = vec![5, 6];
|
|
let header = HeaderWithLength::new(c, v.len());
|
|
{
|
|
let x = Arc::into_thin(Arc::from_header_and_iter(header, v.into_iter()));
|
|
let y = ThinArc::with_arc(&x, |q| q.clone());
|
|
let _ = y.clone();
|
|
let _ = x == x;
|
|
Arc::from_thin(x.clone());
|
|
}
|
|
assert_eq!(canary.load(Acquire), 1);
|
|
}
|
|
}
|