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style: Implement ArcSlice::default().

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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
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23
components/servo_arc/lib.rs
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@ -612,16 +612,15 @@ impl<H, T> Arc<HeaderSlice<H, [T]>> {
use std::mem::{align_of, size_of};
assert_ne!(size_of::<T>(), 0, "Need to think about ZST");
let inner_align = align_of::<ArcInner<HeaderSlice<H, [T; 0]>>>();
debug_assert!(inner_align >= align_of::<T>());
// 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; 0]>>>();
// 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;
let fake_slice_ptr = inner_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.
@ -641,13 +640,13 @@ impl<H, T> Arc<HeaderSlice<H, [T]>> {
let ptr: *mut ArcInner<HeaderSlice<H, [T]>>;
unsafe {
// Allocate the buffer.
let layout = if mem::align_of::<T>() <= mem::align_of::<usize>() {
Layout::from_size_align_unchecked(size, mem::align_of::<usize>())
} 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.
let layout = if inner_align <= align_of::<usize>() {
Layout::from_size_align_unchecked(size, align_of::<usize>())
} else if inner_align <= 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.
Layout::from_size_align_unchecked(size, mem::align_of::<u64>())
Layout::from_size_align_unchecked(size, align_of::<u64>())
} else {
panic!("Over-aligned type not handled");
};
@ -689,7 +688,7 @@ impl<H, T> Arc<HeaderSlice<H, [T]>> {
// for some padding from the alignment.
debug_assert!(
(buffer.offset(size as isize) as usize - current as *mut u8 as usize) <
align_of::<Self>()
inner_align
);
}
assert!(