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Fix Rc/Arc allocation layout
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* Rounds allocation layout up to a multiple of alignment
* Adds a convenience method `Layout::pad_to_align` to perform rounding
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murarth committed Nov 8, 2018
1 parent 653da4f commit 317f494
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Showing 3 changed files with 25 additions and 4 deletions.
6 changes: 4 additions & 2 deletions src/liballoc/rc.rs
Original file line number Diff line number Diff line change
Expand Up @@ -672,14 +672,16 @@ impl<T: ?Sized> Rc<T> {
// Previously, layout was calculated on the expression
// `&*(ptr as *const RcBox<T>)`, but this created a misaligned
// reference (see #54908).
let (layout, _) = Layout::new::<RcBox<()>>()
.extend(Layout::for_value(&*ptr)).unwrap();
let layout = Layout::new::<RcBox<()>>()
.extend(Layout::for_value(&*ptr)).unwrap().0
.pad_to_align().unwrap();

let mem = Global.alloc(layout)
.unwrap_or_else(|_| handle_alloc_error(layout));

// Initialize the RcBox
let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut RcBox<T>;
debug_assert_eq!(Layout::for_value(&*inner), layout);

ptr::write(&mut (*inner).strong, Cell::new(1));
ptr::write(&mut (*inner).weak, Cell::new(1));
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6 changes: 4 additions & 2 deletions src/liballoc/sync.rs
Original file line number Diff line number Diff line change
Expand Up @@ -575,14 +575,16 @@ impl<T: ?Sized> Arc<T> {
// Previously, layout was calculated on the expression
// `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
// reference (see #54908).
let (layout, _) = Layout::new::<ArcInner<()>>()
.extend(Layout::for_value(&*ptr)).unwrap();
let layout = Layout::new::<ArcInner<()>>()
.extend(Layout::for_value(&*ptr)).unwrap().0
.pad_to_align().unwrap();

let mem = Global.alloc(layout)
.unwrap_or_else(|_| handle_alloc_error(layout));

// Initialize the ArcInner
let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut ArcInner<T>;
debug_assert_eq!(Layout::for_value(&*inner), layout);

ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
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17 changes: 17 additions & 0 deletions src/libcore/alloc.rs
Original file line number Diff line number Diff line change
Expand Up @@ -218,6 +218,23 @@ impl Layout {
len_rounded_up.wrapping_sub(len)
}

/// Creates a layout by rounding the size of this layout up to a multiple
/// of the layout's alignment.
///
/// Returns `Err` if the padded size would overflow.
///
/// This is equivalent to adding the result of `padding_needed_for`
/// to the layout's current size.
#[unstable(feature = "alloc_layout_extra", issue = "55724")]
#[inline]
pub fn pad_to_align(&self) -> Result<Layout, LayoutErr> {
let pad = self.padding_needed_for(self.align());
let new_size = self.size().checked_add(pad)
.ok_or(LayoutErr { private: () })?;

Layout::from_size_align(new_size, self.align())
}

/// Creates a layout describing the record for `n` instances of
/// `self`, with a suitable amount of padding between each to
/// ensure that each instance is given its requested size and
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