Auto merge of rust-lang#116123 - joboet:rewrite_native_tls, r=m-ou-se

Rewrite native thread-local storage

(part of rust-lang#110897)

The current native thread-local storage implementation has become quite messy, uses indescriptive names and unnecessarily adds code to the macro expansion. This PR tries to fix that by using a new implementation that also allows more layout optimizations and potentially increases performance by eliminating unnecessary TLS accesses.

This does not change the recursive initialization behaviour I described in [this comment](rust-lang#110897 (comment)), so it should be a library-only change. Changing that behaviour should be quite easy now, however.

r? `@m-ou-se`
`@rustbot` label +T-libs

library/std/src/sys/thread_local/fast_local/eager.rs

@@ -0,0 +1,82 @@
+use crate::cell::{Cell, UnsafeCell};
+use crate::ptr::{self, drop_in_place};
+use crate::sys::thread_local::abort_on_dtor_unwind;
+use crate::sys::thread_local_dtor::register_dtor;
+
+#[derive(Clone, Copy)]
+enum State {
+    Initial,
+    Alive,
+    Destroyed,
+}
+
+#[allow(missing_debug_implementations)]
+pub struct Storage<T> {
+    state: Cell<State>,
+    val: UnsafeCell<T>,
+}
+
+impl<T> Storage<T> {
+    pub const fn new(val: T) -> Storage<T> {
+        Storage { state: Cell::new(State::Initial), val: UnsafeCell::new(val) }
+    }
+
+    /// Get a reference to the TLS value. If the TLS variable has been destroyed,
+    /// `None` is returned.
+    ///
+    /// # Safety
+    /// * The `self` reference must remain valid until the TLS destructor has been
+    ///   run.
+    /// * The returned reference may only be used until thread destruction occurs
+    ///   and may not be used after reentrant initialization has occurred.
+    ///
+    // FIXME(#110897): return NonNull instead of lying about the lifetime.
+    #[inline]
+    pub unsafe fn get(&self) -> Option<&'static T> {
+        match self.state.get() {
+            // SAFETY: as the state is not `Destroyed`, the value cannot have
+            // been destroyed yet. The reference fulfills the terms outlined
+            // above.
+            State::Alive => unsafe { Some(&*self.val.get()) },
+            State::Destroyed => None,
+            State::Initial => unsafe { self.initialize() },
+        }
+    }
+
+    #[cold]
+    unsafe fn initialize(&self) -> Option<&'static T> {
+        // Register the destructor
+
+        // SAFETY:
+        // * the destructor will be called at thread destruction.
+        // * the caller guarantees that `self` will be valid until that time.
+        unsafe {
+            register_dtor(ptr::from_ref(self).cast_mut().cast(), destroy::<T>);
+        }
+        self.state.set(State::Alive);
+        // SAFETY: as the state is not `Destroyed`, the value cannot have
+        // been destroyed yet. The reference fulfills the terms outlined
+        // above.
+        unsafe { Some(&*self.val.get()) }
+    }
+}
+
+/// Transition an `Alive` TLS variable into the `Destroyed` state, dropping its
+/// value.
+///
+/// # Safety
+/// * Must only be called at thread destruction.
+/// * `ptr` must point to an instance of `Storage` with `Alive` state and be
+///   valid for accessing that instance.
+unsafe extern "C" fn destroy<T>(ptr: *mut u8) {
+    // Print a nice abort message if a panic occurs.
+    abort_on_dtor_unwind(|| {
+        let storage = unsafe { &*(ptr as *const Storage<T>) };
+        // Update the state before running the destructor as it may attempt to
+        // access the variable.
+        storage.state.set(State::Destroyed);
+        unsafe {
+            drop_in_place(storage.val.get());
+        }
+    })
+}