Auto merge of rust-lang#123550 - GnomedDev:remove-initial-arc, r=<try>

Remove last rt::init allocation for thread info

Removes the last allocation pre-main by just not storing anything in std::thread::Thread for the main thread.
- The thread name can just be a hard coded literal, as was done in rust-lang#123433.
- The ThreadId is always the `1` value, so `ThreadId::new` now starts at `2` and can fabricate the `1` value when needed.
- Storing Parker in a static that is initialized once at startup. This uses SyncUnsafeCell and MaybeUninit as this is quite performance critical and we don't need synchronization or to store a tag value and possibly leave in a panic.

This currently does not have a regression test to prevent future changes from re-adding allocations pre-main as I'm [having trouble](GnomedDev@6f7be53) implementing it, but if wanted I can draft this PR until that test is ready.

library/std/src/lib.rs

@@ -353,6 +353,7 @@
 #![feature(str_internals)]
 #![feature(strict_provenance)]
 #![feature(strict_provenance_atomic_ptr)]
+#![feature(sync_unsafe_cell)]
 // tidy-alphabetical-end
 //
 // Library features (alloc):

library/std/src/thread/mod.rs

@@ -159,11 +159,13 @@
 mod tests;
 
 use crate::any::Any;
+use crate::cell::SyncUnsafeCell;
 use crate::cell::{OnceCell, UnsafeCell};
 use crate::ffi::{CStr, CString};
 use crate::fmt;
 use crate::io;
 use crate::marker::PhantomData;
+use crate::mem::MaybeUninit;
 use crate::mem::{self, forget};
 use crate::num::NonZero;
 use crate::panic;
@@ -511,7 +513,7 @@ impl Builder {
 
         let f = MaybeDangling::new(f);
         let main = move || {
-            if let Some(name) = their_thread.cname() {
+            if let Some(name) = their_thread.0.name() {
                 imp::Thread::set_name(name);
             }
 
@@ -1143,7 +1145,7 @@ pub fn park_timeout(dur: Duration) {
     let guard = PanicGuard;
     // SAFETY: park_timeout is called on the parker owned by this thread.
     unsafe {
-        current().inner.as_ref().parker().park_timeout(dur);
+        current().0.parker().park_timeout(dur);
     }
     // No panic occurred, do not abort.
     forget(guard);
@@ -1182,7 +1184,12 @@ pub fn park_timeout(dur: Duration) {
 pub struct ThreadId(NonZero<u64>);
 
 impl ThreadId {
-    // Generate a new unique thread ID.
+    /// Generate a new unique thread ID.
+    ///
+    /// The current implementation starts at 2 and increments from there.
+    ///
+    /// This is as `1` is the value for the main thread, so std::thread::Thread does not
+    /// have to store this value when creating the main thread's information.
     fn new() -> ThreadId {
         #[cold]
         fn exhausted() -> ! {
@@ -1193,7 +1200,7 @@ impl ThreadId {
             if #[cfg(target_has_atomic = "64")] {
                 use crate::sync::atomic::{AtomicU64, Ordering::Relaxed};
 
-                static COUNTER: AtomicU64 = AtomicU64::new(0);
+                static COUNTER: AtomicU64 = AtomicU64::new(1);
 
                 let mut last = COUNTER.load(Relaxed);
                 loop {
@@ -1209,7 +1216,7 @@ impl ThreadId {
             } else {
                 use crate::sync::{Mutex, PoisonError};
 
-                static COUNTER: Mutex<u64> = Mutex::new(0);
+                static COUNTER: Mutex<u64> = Mutex::new(1);
 
                 let mut counter = COUNTER.lock().unwrap_or_else(PoisonError::into_inner);
                 let Some(id) = counter.checked_add(1) else {
@@ -1226,6 +1233,11 @@ impl ThreadId {
         }
     }
 
+    /// Creates a ThreadId with the ID of the main thread.
+    fn new_main() -> Self {
+        Self(NonZero::<u64>::MIN)
+    }
+
     /// This returns a numeric identifier for the thread identified by this
     /// `ThreadId`.
     ///
@@ -1245,23 +1257,54 @@ impl ThreadId {
 // Thread
 ////////////////////////////////////////////////////////////////////////////////
 
-/// The internal representation of a `Thread`'s name.
-enum ThreadName {
-    Main,
-    Other(CString),
-    Unnamed,
-}
+/// The parker for the main thread. This avoids having to allocate an Arc in `fn main() {}`.
+static MAIN_PARKER: SyncUnsafeCell<MaybeUninit<Parker>> =
+    SyncUnsafeCell::new(MaybeUninit::uninit());
 
-/// The internal representation of a `Thread` handle
-struct Inner {
-    name: ThreadName, // Guaranteed to be UTF-8
+/// The internal representation of a `Thread` that is not the main thread.
+struct OtherInner {
+    name: Option<CString>, // Guaranteed to be UTF-8
     id: ThreadId,
     parker: Parker,
 }
 
+/// The internal representation of a `Thread` handle.
+#[derive(Clone)]
+enum Inner {
+    Main,
+    Other(Pin<Arc<OtherInner>>),
+}
+
 impl Inner {
-    fn parker(self: Pin<&Self>) -> Pin<&Parker> {
-        unsafe { Pin::map_unchecked(self, |inner| &inner.parker) }
+    fn id(&self) -> ThreadId {
+        match self {
+            Self::Main => ThreadId::new_main(),
+            Self::Other(other) => other.id,
+        }
+    }
+
+    fn name(&self) -> Option<&CStr> {
+        match self {
+            Self::Main => Some(c"main"),
+            Self::Other(other) => other.name.as_deref(),
+        }
+    }
+
+    fn parker(&self) -> Pin<&Parker> {
+        match self {
+            Self::Main => {
+                // Safety: MAIN_PARKER only ever has a mutable reference when Inner::Main is initialised.
+                let uninit_ref: &'static MaybeUninit<Parker> = unsafe { &*MAIN_PARKER.get() };
+
+                // Safety: MAIN_PARKER is initialised when Inner::Main is initialised.
+                let parker_ref = unsafe { uninit_ref.assume_init_ref() };
+
+                Pin::static_ref(parker_ref)
+            }
+            Self::Other(inner) => unsafe {
+                Pin::map_unchecked(inner.as_ref(), |inner| &inner.parker)
+            },
+        }
     }
 }
 
@@ -1285,41 +1328,37 @@ impl Inner {
 /// docs of [`Builder`] and [`spawn`] for more details.
 ///
 /// [`thread::current`]: current
-pub struct Thread {
-    inner: Pin<Arc<Inner>>,
-}
+pub struct Thread(Inner);
 
 impl Thread {
     // Used only internally to construct a thread object without spawning
     pub(crate) fn new(name: Option<CString>) -> Thread {
-        if let Some(name) = name {
-            Self::new_inner(ThreadName::Other(name))
-        } else {
-            Self::new_inner(ThreadName::Unnamed)
-        }
-    }
-
-    // Used in runtime to construct main thread
-    pub(crate) fn new_main() -> Thread {
-        Self::new_inner(ThreadName::Main)
-    }
-
-    fn new_inner(name: ThreadName) -> Thread {
         // We have to use `unsafe` here to construct the `Parker` in-place,
         // which is required for the UNIX implementation.
         //
         // SAFETY: We pin the Arc immediately after creation, so its address never
         // changes.
         let inner = unsafe {
-            let mut arc = Arc::<Inner>::new_uninit();
+            let mut arc = Arc::<OtherInner>::new_uninit();
             let ptr = Arc::get_mut_unchecked(&mut arc).as_mut_ptr();
             addr_of_mut!((*ptr).name).write(name);
             addr_of_mut!((*ptr).id).write(ThreadId::new());
             Parker::new_in_place(addr_of_mut!((*ptr).parker));
             Pin::new_unchecked(arc.assume_init())
         };
 
-        Thread { inner }
+        Self(Inner::Other(inner))
+    }
+
+    /// # Safety
+    ///
+    /// This must only ever be called once, and must be called on the main thread.
+    pub(crate) unsafe fn new_main() -> Thread {
+        // Safety: Caller responsible for holding the mutable invariant and
+        // Parker::new_in_place does not read from the uninit value.
+        unsafe { Parker::new_in_place(MAIN_PARKER.get().cast()) }
+
+        Self(Inner::Main)
     }
 
     /// Like the public [`park`], but callable on any handle. This is used to
@@ -1328,7 +1367,7 @@ impl Thread {
     /// # Safety
     /// May only be called from the thread to which this handle belongs.
     pub(crate) unsafe fn park(&self) {
-        unsafe { self.inner.as_ref().parker().park() }
+        unsafe { self.0.parker().park() }
     }
 
     /// Atomically makes the handle's token available if it is not already.
@@ -1364,7 +1403,7 @@ impl Thread {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn unpark(&self) {
-        self.inner.as_ref().parker().unpark();
+        self.0.parker().unpark();
     }
 
     /// Gets the thread's unique identifier.
@@ -1384,7 +1423,7 @@ impl Thread {
     #[stable(feature = "thread_id", since = "1.19.0")]
     #[must_use]
     pub fn id(&self) -> ThreadId {
-        self.inner.id
+        self.0.id()
     }
 
     /// Gets the thread's name.
@@ -1427,15 +1466,7 @@ impl Thread {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
     pub fn name(&self) -> Option<&str> {
-        self.cname().map(|s| unsafe { str::from_utf8_unchecked(s.to_bytes()) })
-    }
-
-    fn cname(&self) -> Option<&CStr> {
-        match &self.inner.name {
-            ThreadName::Main => Some(c"main"),
-            ThreadName::Other(other) => Some(&other),
-            ThreadName::Unnamed => None,
-        }
+        self.0.name().map(|s| unsafe { str::from_utf8_unchecked(s.to_bytes()) })
     }
 }
 

tests/rustdoc/demo-allocator-54478.rs

@@ -40,6 +40,7 @@
 //! }
 //!
 //! fn main() {
+//!     drop(std::env::var("PWD"));
 //!     assert!(unsafe { HIT });
 //! }
 //! ```