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Bug 1550554 - Implement ArcSlice::default(). r=heycam
Share a singleton to avoid allocating for empty lists. Differential Revision: https://phabricator.services.mozilla.com/D30543 --HG-- extra : moz-landing-system : lando
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Cargo.lock
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1
Cargo.lock
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@ -2818,6 +2818,7 @@ dependencies = [
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"bitflags 1.0.4 (registry+https://github.com/rust-lang/crates.io-index)",
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"cssparser 0.25.3 (registry+https://github.com/rust-lang/crates.io-index)",
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"euclid 0.19.5 (registry+https://github.com/rust-lang/crates.io-index)",
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"lazy_static 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
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"malloc_size_of 0.0.1",
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"malloc_size_of_derive 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
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"selectors 0.21.0",
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@ -23,7 +23,7 @@ static constexpr const size_t kStaticRefcount =
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std::numeric_limits<size_t>::max();
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static constexpr const size_t kMaxRefcount =
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std::numeric_limits<intptr_t>::max();
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static constexpr const uint32_t kArcSliceCanary = 0xf3f3f3f3;
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static constexpr const uint64_t kArcSliceCanary = 0xf3f3f3f3f3f3f3f3;
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#define ASSERT_CANARY \
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MOZ_DIAGNOSTIC_ASSERT(_0.ptr->data.header.header == kArcSliceCanary, "Uh?");
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@ -612,16 +612,15 @@ impl<H, T> Arc<HeaderSlice<H, [T]>> {
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use std::mem::{align_of, size_of};
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assert_ne!(size_of::<T>(), 0, "Need to think about ZST");
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let inner_align = align_of::<ArcInner<HeaderSlice<H, [T; 0]>>>();
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debug_assert!(inner_align >= align_of::<T>());
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// Compute the required size for the allocation.
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let num_items = items.len();
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let size = {
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// First, determine the alignment of a hypothetical pointer to a
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// HeaderSlice.
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let fake_slice_ptr_align: usize = mem::align_of::<ArcInner<HeaderSlice<H, [T; 0]>>>();
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// Next, synthesize a totally garbage (but properly aligned) pointer
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// to a sequence of T.
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let fake_slice_ptr = fake_slice_ptr_align as *const T;
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let fake_slice_ptr = inner_align as *const T;
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// Convert that sequence to a fat pointer. The address component of
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// the fat pointer will be garbage, but the length will be correct.
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@ -641,13 +640,13 @@ impl<H, T> Arc<HeaderSlice<H, [T]>> {
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let ptr: *mut ArcInner<HeaderSlice<H, [T]>>;
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unsafe {
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// Allocate the buffer.
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let layout = if mem::align_of::<T>() <= mem::align_of::<usize>() {
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Layout::from_size_align_unchecked(size, mem::align_of::<usize>())
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} else if mem::align_of::<T>() <= mem::align_of::<u64>() {
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// On 32-bit platforms <T> may have 8 byte alignment while usize has 4 byte aligment.
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// Use u64 to avoid over-alignment.
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let layout = if inner_align <= align_of::<usize>() {
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Layout::from_size_align_unchecked(size, align_of::<usize>())
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} else if inner_align <= align_of::<u64>() {
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// On 32-bit platforms <T> may have 8 byte alignment while usize
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// has 4 byte aligment. Use u64 to avoid over-alignment.
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// This branch will compile away in optimized builds.
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Layout::from_size_align_unchecked(size, mem::align_of::<u64>())
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Layout::from_size_align_unchecked(size, align_of::<u64>())
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} else {
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panic!("Over-aligned type not handled");
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};
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@ -689,7 +688,7 @@ impl<H, T> Arc<HeaderSlice<H, [T]>> {
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// for some padding from the alignment.
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debug_assert!(
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(buffer.offset(size as isize) as usize - current as *mut u8 as usize) <
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align_of::<Self>()
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inner_align
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);
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}
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assert!(
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@ -18,6 +18,7 @@ app_units = "0.7"
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cssparser = "0.25"
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bitflags = "1.0"
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euclid = "0.19"
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lazy_static = "1"
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malloc_size_of = { path = "../malloc_size_of" }
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malloc_size_of_derive = "0.1"
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selectors = { path = "../selectors" }
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@ -5,7 +5,7 @@
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//! A thin atomically-reference-counted slice.
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use servo_arc::ThinArc;
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use std::mem;
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use std::{iter, mem};
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use std::ops::Deref;
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use std::ptr::NonNull;
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@ -14,7 +14,10 @@ use std::ptr::NonNull;
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/// Given we cannot use a zero-sized-type for the header, since well, C++
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/// doesn't have zsts, and we want to use cbindgen for this type, we may as well
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/// assert some sanity at runtime.
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const ARC_SLICE_CANARY: u32 = 0xf3f3f3f3;
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///
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/// We use an u64, to guarantee that we can use a single singleton for every
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/// empty slice, even if the types they hold are aligned differently.
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const ARC_SLICE_CANARY: u64 = 0xf3f3f3f3f3f3f3f3;
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/// A wrapper type for a refcounted slice using ThinArc.
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///
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@ -22,7 +25,7 @@ const ARC_SLICE_CANARY: u32 = 0xf3f3f3f3;
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/// cbindgen:derive-neq=false
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#[repr(C)]
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#[derive(Debug, Clone, PartialEq, Eq, ToShmem)]
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pub struct ArcSlice<T>(#[shmem(field_bound)] ThinArc<u32, T>);
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pub struct ArcSlice<T>(#[shmem(field_bound)] ThinArc<u64, T>);
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impl<T> Deref for ArcSlice<T> {
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type Target = [T];
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@ -34,12 +37,28 @@ impl<T> Deref for ArcSlice<T> {
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}
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}
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/// The inner pointer of an ArcSlice<T>, to be sent via FFI.
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/// The type of the pointer is a bit of a lie, we just want to preserve the type
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/// but these pointers cannot be constructed outside of this crate, so we're
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/// good.
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#[repr(C)]
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pub struct ForgottenArcSlicePtr<T>(NonNull<T>);
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lazy_static! {
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// ThinArc doesn't support alignments greater than align_of::<u64>.
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static ref EMPTY_ARC_SLICE: ArcSlice<u64> = {
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ArcSlice(ThinArc::from_header_and_iter(ARC_SLICE_CANARY, iter::empty()))
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};
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}
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impl<T> Default for ArcSlice<T> {
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#[allow(unsafe_code)]
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fn default() -> Self {
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debug_assert!(
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mem::align_of::<T>() <= mem::align_of::<u64>(),
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"Need to increase the alignment of EMPTY_ARC_SLICE"
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);
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unsafe {
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let empty: ArcSlice<_> = EMPTY_ARC_SLICE.clone();
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let empty: Self = mem::transmute(empty);
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debug_assert_eq!(empty.len(), 0);
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empty
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}
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}
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}
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impl<T> ArcSlice<T> {
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/// Creates an Arc for a slice using the given iterator to generate the
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@ -49,6 +68,9 @@ impl<T> ArcSlice<T> {
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where
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I: Iterator<Item = T> + ExactSizeIterator,
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{
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if items.len() == 0 {
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return Self::default();
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}
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ArcSlice(ThinArc::from_header_and_iter(ARC_SLICE_CANARY, items))
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}
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@ -64,3 +86,10 @@ impl<T> ArcSlice<T> {
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ret
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}
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}
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/// The inner pointer of an ArcSlice<T>, to be sent via FFI.
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/// The type of the pointer is a bit of a lie, we just want to preserve the type
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/// but these pointers cannot be constructed outside of this crate, so we're
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/// good.
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#[repr(C)]
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pub struct ForgottenArcSlicePtr<T>(NonNull<T>);
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@ -16,6 +16,8 @@ extern crate bitflags;
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#[macro_use]
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extern crate cssparser;
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extern crate euclid;
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#[macro_use]
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extern crate lazy_static;
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extern crate malloc_size_of;
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#[macro_use]
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extern crate malloc_size_of_derive;
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