style: Implement ArcSlice::default().

Share a singleton to avoid allocating for empty lists.

Differential Revision: https://phabricator.services.mozilla.com/D30543
This commit is contained in:
Emilio Cobos Álvarez 2019-05-16 23:22:04 +00:00
parent ab8776a144
commit 5e4fdf647d
4 changed files with 52 additions and 21 deletions

View file

@ -5,7 +5,7 @@
//! A thin atomically-reference-counted slice.
use servo_arc::ThinArc;
use std::mem;
use std::{iter, mem};
use std::ops::Deref;
use std::ptr::NonNull;
@ -14,7 +14,10 @@ use std::ptr::NonNull;
/// Given we cannot use a zero-sized-type for the header, since well, C++
/// doesn't have zsts, and we want to use cbindgen for this type, we may as well
/// assert some sanity at runtime.
const ARC_SLICE_CANARY: u32 = 0xf3f3f3f3;
///
/// We use an u64, to guarantee that we can use a single singleton for every
/// empty slice, even if the types they hold are aligned differently.
const ARC_SLICE_CANARY: u64 = 0xf3f3f3f3f3f3f3f3;
/// A wrapper type for a refcounted slice using ThinArc.
///
@ -22,7 +25,7 @@ const ARC_SLICE_CANARY: u32 = 0xf3f3f3f3;
/// cbindgen:derive-neq=false
#[repr(C)]
#[derive(Clone, Debug, Eq, PartialEq, ToShmem)]
pub struct ArcSlice<T>(#[shmem(field_bound)] ThinArc<u32, T>);
pub struct ArcSlice<T>(#[shmem(field_bound)] ThinArc<u64, T>);
impl<T> Deref for ArcSlice<T> {
type Target = [T];
@ -34,12 +37,28 @@ impl<T> Deref for ArcSlice<T> {
}
}
/// The inner pointer of an ArcSlice<T>, to be sent via FFI.
/// The type of the pointer is a bit of a lie, we just want to preserve the type
/// but these pointers cannot be constructed outside of this crate, so we're
/// good.
#[repr(C)]
pub struct ForgottenArcSlicePtr<T>(NonNull<T>);
lazy_static! {
// ThinArc doesn't support alignments greater than align_of::<u64>.
static ref EMPTY_ARC_SLICE: ArcSlice<u64> = {
ArcSlice(ThinArc::from_header_and_iter(ARC_SLICE_CANARY, iter::empty()))
};
}
impl<T> Default for ArcSlice<T> {
#[allow(unsafe_code)]
fn default() -> Self {
debug_assert!(
mem::align_of::<T>() <= mem::align_of::<u64>(),
"Need to increase the alignment of EMPTY_ARC_SLICE"
);
unsafe {
let empty: ArcSlice<_> = EMPTY_ARC_SLICE.clone();
let empty: Self = mem::transmute(empty);
debug_assert_eq!(empty.len(), 0);
empty
}
}
}
impl<T> ArcSlice<T> {
/// Creates an Arc for a slice using the given iterator to generate the
@ -49,6 +68,9 @@ impl<T> ArcSlice<T> {
where
I: Iterator<Item = T> + ExactSizeIterator,
{
if items.len() == 0 {
return Self::default();
}
ArcSlice(ThinArc::from_header_and_iter(ARC_SLICE_CANARY, items))
}
@ -64,3 +86,10 @@ impl<T> ArcSlice<T> {
ret
}
}
/// The inner pointer of an ArcSlice<T>, to be sent via FFI.
/// The type of the pointer is a bit of a lie, we just want to preserve the type
/// but these pointers cannot be constructed outside of this crate, so we're
/// good.
#[repr(C)]
pub struct ForgottenArcSlicePtr<T>(NonNull<T>);