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reduce the size of arcs from 2 words to 1 #8867

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reduce the size of arcs from 2 words to 1 #8867

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10 changes: 9 additions & 1 deletion src/libstd/unstable/sync.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,7 @@ use vec;
/// An atomically reference counted pointer.
///
/// Enforces no shared-memory safety.
#[unsafe_no_drop_flag]
pub struct UnsafeArc<T> {
data: *mut ArcData<T>,
}
Expand Down Expand Up @@ -221,8 +222,9 @@ impl<T: Send> Clone for UnsafeArc<T> {
impl<T> Drop for UnsafeArc<T>{
fn drop(&self) {
unsafe {
// Happens when destructing an unwrapper's handle and from `#[unsafe_no_drop_flag]`
if self.data.is_null() {
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Looking at this, where is self.data set to null? I don't remember if destructors zero out the whole structure or just the drop flag, but it seems like without the drop flag this could be run twice and self.data is non-null both times?

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If you use #[unsafe_no_drop_flag], it zeroes the whole structure instead of the drop flag. That's how it's defined, or it wouldn't be useful.

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Ah that makes sense.

return; // Happens when destructing an unwrapper's handle.
return
}
let mut data: ~ArcData<T> = cast::transmute(self.data);
// Must be acquire+release, not just release, to make sure this
Expand Down Expand Up @@ -440,6 +442,12 @@ mod tests {
use super::{Exclusive, UnsafeArc, atomically};
use task;
use util;
use sys::size_of;

#[test]
fn test_size() {
assert_eq!(size_of::<UnsafeArc<[int, ..10]>>(), size_of::<*[int, ..10]>());
}

#[test]
fn test_atomically() {
Expand Down