diff options
Diffstat (limited to 'library/std/src/sys/common/thread_local/mod.rs')
-rw-r--r-- | library/std/src/sys/common/thread_local/mod.rs | 109 |
1 files changed, 109 insertions, 0 deletions
diff --git a/library/std/src/sys/common/thread_local/mod.rs b/library/std/src/sys/common/thread_local/mod.rs new file mode 100644 index 000000000..1fee84a04 --- /dev/null +++ b/library/std/src/sys/common/thread_local/mod.rs @@ -0,0 +1,109 @@ +//! The following module declarations are outside cfg_if because the internal +//! `__thread_local_internal` macro does not seem to be exported properly when using cfg_if +#![unstable(feature = "thread_local_internals", reason = "should not be necessary", issue = "none")] + +#[cfg(all(target_thread_local, not(all(target_family = "wasm", not(target_feature = "atomics")))))] +mod fast_local; +#[cfg(all( + not(target_thread_local), + not(all(target_family = "wasm", not(target_feature = "atomics"))) +))] +mod os_local; +#[cfg(all(target_family = "wasm", not(target_feature = "atomics")))] +mod static_local; + +#[cfg(not(test))] +cfg_if::cfg_if! { + if #[cfg(all(target_family = "wasm", not(target_feature = "atomics")))] { + #[doc(hidden)] + pub use static_local::statik::Key; + } else if #[cfg(all(target_thread_local, not(all(target_family = "wasm", not(target_feature = "atomics")))))] { + #[doc(hidden)] + pub use fast_local::fast::Key; + } else if #[cfg(all(not(target_thread_local), not(all(target_family = "wasm", not(target_feature = "atomics")))))] { + #[doc(hidden)] + pub use os_local::os::Key; + } +} + +#[doc(hidden)] +#[cfg(test)] +pub use realstd::thread::__LocalKeyInner as Key; + +mod lazy { + use crate::cell::UnsafeCell; + use crate::hint; + use crate::mem; + + pub struct LazyKeyInner<T> { + inner: UnsafeCell<Option<T>>, + } + + impl<T> LazyKeyInner<T> { + pub const fn new() -> LazyKeyInner<T> { + LazyKeyInner { inner: UnsafeCell::new(None) } + } + + pub unsafe fn get(&self) -> Option<&'static T> { + // SAFETY: The caller must ensure no reference is ever handed out to + // the inner cell nor mutable reference to the Option<T> inside said + // cell. This make it safe to hand a reference, though the lifetime + // of 'static is itself unsafe, making the get method unsafe. + unsafe { (*self.inner.get()).as_ref() } + } + + /// The caller must ensure that no reference is active: this method + /// needs unique access. + pub unsafe fn initialize<F: FnOnce() -> T>(&self, init: F) -> &'static T { + // Execute the initialization up front, *then* move it into our slot, + // just in case initialization fails. + let value = init(); + let ptr = self.inner.get(); + + // SAFETY: + // + // note that this can in theory just be `*ptr = Some(value)`, but due to + // the compiler will currently codegen that pattern with something like: + // + // ptr::drop_in_place(ptr) + // ptr::write(ptr, Some(value)) + // + // Due to this pattern it's possible for the destructor of the value in + // `ptr` (e.g., if this is being recursively initialized) to re-access + // TLS, in which case there will be a `&` and `&mut` pointer to the same + // value (an aliasing violation). To avoid setting the "I'm running a + // destructor" flag we just use `mem::replace` which should sequence the + // operations a little differently and make this safe to call. + // + // The precondition also ensures that we are the only one accessing + // `self` at the moment so replacing is fine. + unsafe { + let _ = mem::replace(&mut *ptr, Some(value)); + } + + // SAFETY: With the call to `mem::replace` it is guaranteed there is + // a `Some` behind `ptr`, not a `None` so `unreachable_unchecked` + // will never be reached. + unsafe { + // After storing `Some` we want to get a reference to the contents of + // what we just stored. While we could use `unwrap` here and it should + // always work it empirically doesn't seem to always get optimized away, + // which means that using something like `try_with` can pull in + // panicking code and cause a large size bloat. + match *ptr { + Some(ref x) => x, + None => hint::unreachable_unchecked(), + } + } + } + + /// The other methods hand out references while taking &self. + /// As such, callers of this method must ensure no `&` and `&mut` are + /// available and used at the same time. + #[allow(unused)] + pub unsafe fn take(&mut self) -> Option<T> { + // SAFETY: See doc comment for this method. + unsafe { (*self.inner.get()).take() } + } + } +} |