Rollup merge of #136858 - safinaskar:parallel-cleanup-2025-02-11-07-54, r=SparrowLii

Parallel-compiler-related cleanup

Parallel-compiler-related cleanup

I carefully split changes into commits. Commit messages are self-explanatory. Squashing is not recommended.

cc "Parallel Rustc Front-end" https://github.com/rust-lang/rust/issues/113349

r? SparrowLii

``@rustbot`` label: +WG-compiler-parallel

compiler/rustc_codegen_gcc/src/back/lto.rs

@@ -710,10 +710,6 @@ pub struct ThinBuffer {
     context: Arc<SyncContext>,
 }
 
-// TODO: check if this makes sense to make ThinBuffer Send and Sync.
-unsafe impl Send for ThinBuffer {}
-unsafe impl Sync for ThinBuffer {}
-
 impl ThinBuffer {
     pub(crate) fn new(context: &Arc<SyncContext>) -> Self {
         Self { context: Arc::clone(context) }

compiler/rustc_codegen_llvm/src/lib.rs

@@ -245,9 +245,6 @@ impl WriteBackendMethods for LlvmCodegenBackend {
     }
 }
 
-unsafe impl Send for LlvmCodegenBackend {} // Llvm is on a per-thread basis
-unsafe impl Sync for LlvmCodegenBackend {}
-
 impl LlvmCodegenBackend {
     pub fn new() -> Box<dyn CodegenBackend> {
         Box::new(LlvmCodegenBackend(()))

compiler/rustc_data_structures/src/owned_slice.rs

@@ -2,11 +2,6 @@ use std::borrow::Borrow;
 use std::ops::Deref;
 use std::sync::Arc;
 
-// Use our fake Send/Sync traits when on not parallel compiler,
-// so that `OwnedSlice` only implements/requires Send/Sync
-// for parallel compiler builds.
-use crate::sync;
-
 /// An owned slice.
 ///
 /// This is similar to `Arc<[u8]>` but allows slicing and using anything as the
@@ -34,7 +29,7 @@ pub struct OwnedSlice {
     //       \/
     //      ⊂(´･◡･⊂ )∘˚˳° (I am the phantom remnant of #97770)
     #[expect(dead_code)]
-    owner: Arc<dyn sync::Send + sync::Sync>,
+    owner: Arc<dyn Send + Sync>,
 }
 
 /// Makes an [`OwnedSlice`] out of an `owner` and a `slicer` function.
@@ -61,7 +56,7 @@ pub struct OwnedSlice {
 /// ```
 pub fn slice_owned<O, F>(owner: O, slicer: F) -> OwnedSlice
 where
-    O: sync::Send + sync::Sync + 'static,
+    O: Send + Sync + 'static,
     F: FnOnce(&O) -> &[u8],
 {
     try_slice_owned(owner, |x| Ok::<_, !>(slicer(x))).into_ok()
@@ -72,7 +67,7 @@ where
 /// See [`slice_owned`] for the infallible version.
 pub fn try_slice_owned<O, F, E>(owner: O, slicer: F) -> Result<OwnedSlice, E>
 where
-    O: sync::Send + sync::Sync + 'static,
+    O: Send + Sync + 'static,
     F: FnOnce(&O) -> Result<&[u8], E>,
 {
     // We wrap the owner of the bytes in, so it doesn't move.
@@ -139,10 +134,10 @@ impl Borrow<[u8]> for OwnedSlice {
 }
 
 // Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Arc<dyn Send + Sync>)`, which is `Send`
-unsafe impl sync::Send for OwnedSlice {}
+unsafe impl Send for OwnedSlice {}
 
 // Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Arc<dyn Send + Sync>)`, which is `Sync`
-unsafe impl sync::Sync for OwnedSlice {}
+unsafe impl Sync for OwnedSlice {}
 
 #[cfg(test)]
 mod tests;

compiler/rustc_data_structures/src/sync.rs

@@ -18,14 +18,8 @@
 //!
 //! | Type                    | Serial version      | Parallel version                |
 //! | ----------------------- | ------------------- | ------------------------------- |
-//! |` Weak<T>`               | `rc::Weak<T>`       | `sync::Weak<T>`                 |
 //! | `LRef<'a, T>` [^2]      | `&'a mut T`         | `&'a T`                         |
 //! |                         |                     |                                 |
-//! | `AtomicBool`            | `Cell<bool>`        | `atomic::AtomicBool`            |
-//! | `AtomicU32`             | `Cell<u32>`         | `atomic::AtomicU32`             |
-//! | `AtomicU64`             | `Cell<u64>`         | `atomic::AtomicU64`             |
-//! | `AtomicUsize`           | `Cell<usize>`       | `atomic::AtomicUsize`           |
-//! |                         |                     |                                 |
 //! | `Lock<T>`               | `RefCell<T>`        | `RefCell<T>` or                 |
 //! |                         |                     | `parking_lot::Mutex<T>`         |
 //! | `RwLock<T>`             | `RefCell<T>`        | `parking_lot::RwLock<T>`        |
@@ -103,18 +97,15 @@ mod mode {
 
 // FIXME(parallel_compiler): Get rid of these aliases across the compiler.
 
-pub use std::marker::{Send, Sync};
+pub use std::sync::OnceLock;
 // Use portable AtomicU64 for targets without native 64-bit atomics
 #[cfg(target_has_atomic = "64")]
 pub use std::sync::atomic::AtomicU64;
-pub use std::sync::atomic::{AtomicBool, AtomicU32, AtomicUsize};
-pub use std::sync::{OnceLock, Weak};
 
 pub use mode::{is_dyn_thread_safe, set_dyn_thread_safe_mode};
 pub use parking_lot::{
-    MappedMutexGuard as MappedLockGuard, MappedRwLockReadGuard as MappedReadGuard,
-    MappedRwLockWriteGuard as MappedWriteGuard, RwLockReadGuard as ReadGuard,
-    RwLockWriteGuard as WriteGuard,
+    MappedRwLockReadGuard as MappedReadGuard, MappedRwLockWriteGuard as MappedWriteGuard,
+    RwLockReadGuard as ReadGuard, RwLockWriteGuard as WriteGuard,
 };
 #[cfg(not(target_has_atomic = "64"))]
 pub use portable_atomic::AtomicU64;
@@ -203,12 +194,6 @@ impl<T> RwLock<T> {
         }
     }
 
-    #[inline(always)]
-    #[track_caller]
-    pub fn with_read_lock<F: FnOnce(&T) -> R, R>(&self, f: F) -> R {
-        f(&*self.read())
-    }
-
     #[inline(always)]
     pub fn try_write(&self) -> Result<WriteGuard<'_, T>, ()> {
         self.0.try_write().ok_or(())
@@ -223,12 +208,6 @@ impl<T> RwLock<T> {
         }
     }
 
-    #[inline(always)]
-    #[track_caller]
-    pub fn with_write_lock<F: FnOnce(&mut T) -> R, R>(&self, f: F) -> R {
-        f(&mut *self.write())
-    }
-
     #[inline(always)]
     #[track_caller]
     pub fn borrow(&self) -> ReadGuard<'_, T> {
@@ -240,20 +219,4 @@ impl<T> RwLock<T> {
     pub fn borrow_mut(&self) -> WriteGuard<'_, T> {
         self.write()
     }
-
-    #[inline(always)]
-    pub fn leak(&self) -> &T {
-        let guard = self.read();
-        let ret = unsafe { &*(&raw const *guard) };
-        std::mem::forget(guard);
-        ret
-    }
-}
-
-// FIXME: Probably a bad idea
-impl<T: Clone> Clone for RwLock<T> {
-    #[inline]
-    fn clone(&self) -> Self {
-        RwLock::new(self.borrow().clone())
-    }
 }

compiler/rustc_data_structures/src/sync/freeze.rs

@@ -3,9 +3,9 @@ use std::intrinsics::likely;
 use std::marker::PhantomData;
 use std::ops::{Deref, DerefMut};
 use std::ptr::NonNull;
-use std::sync::atomic::Ordering;
+use std::sync::atomic::{AtomicBool, Ordering};
 
-use crate::sync::{AtomicBool, DynSend, DynSync, ReadGuard, RwLock, WriteGuard};
+use crate::sync::{DynSend, DynSync, ReadGuard, RwLock, WriteGuard};
 
 /// A type which allows mutation using a lock until
 /// the value is frozen and can be accessed lock-free.

compiler/rustc_data_structures/src/sync/worker_local.rs

@@ -106,12 +106,6 @@ pub struct WorkerLocal<T> {
     registry: Registry,
 }
 
-// This is safe because the `deref` call will return a reference to a `T` unique to each thread
-// or it will panic for threads without an associated local. So there isn't a need for `T` to do
-// it's own synchronization. The `verify` method on `RegistryId` has an issue where the id
-// can be reused, but `WorkerLocal` has a reference to `Registry` which will prevent any reuse.
-unsafe impl<T: Send> Sync for WorkerLocal<T> {}
-
 impl<T> WorkerLocal<T> {
     /// Creates a new worker local where the `initial` closure computes the
     /// value this worker local should take for each thread in the registry.
@@ -138,6 +132,11 @@ impl<T> Deref for WorkerLocal<T> {
     fn deref(&self) -> &T {
         // This is safe because `verify` will only return values less than
         // `self.registry.thread_limit` which is the size of the `self.locals` array.
+
+        // The `deref` call will return a reference to a `T` unique to each thread
+        // or it will panic for threads without an associated local. So there isn't a need for `T` to do
+        // it's own synchronization. The `verify` method on `RegistryId` has an issue where the id
+        // can be reused, but `WorkerLocal` has a reference to `Registry` which will prevent any reuse.
         unsafe { &self.locals.get_unchecked(self.registry.id().verify()).0 }
     }
 }

compiler/rustc_query_system/src/dep_graph/graph.rs

@@ -4,14 +4,14 @@ use std::fmt::Debug;
 use std::hash::Hash;
 use std::marker::PhantomData;
 use std::sync::Arc;
-use std::sync::atomic::Ordering;
+use std::sync::atomic::{AtomicU32, Ordering};
 
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::profiling::{QueryInvocationId, SelfProfilerRef};
 use rustc_data_structures::sharded::{self, Sharded};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
-use rustc_data_structures::sync::{AtomicU32, AtomicU64, Lock};
+use rustc_data_structures::sync::{AtomicU64, Lock};
 use rustc_data_structures::unord::UnordMap;
 use rustc_index::IndexVec;
 use rustc_macros::{Decodable, Encodable};

config.example.toml

@@ -595,11 +595,6 @@
 # Whether to always use incremental compilation when building rustc
 #incremental = false
 
-# Build a multi-threaded rustc. This allows users to use parallel rustc
-# via the unstable option `-Z threads=n`.
-# This option is deprecated and always true.
-#parallel-compiler = true
-
 # The default linker that will be hard-coded into the generated
 # compiler for targets that don't specify a default linker explicitly
 # in their target specifications.  Note that this is not the linker

src/doc/rustc-dev-guide/src/parallel-rustc.md

@@ -46,8 +46,6 @@ are implemented differently depending on whether `parallel-compiler` is true.
 
 | data structure                   | parallel                                            | non-parallel |
 | -------------------------------- | --------------------------------------------------- | ------------ |
-| Weak                             | std::sync::Weak                                     | std::rc::Weak |
-| Atomic{Bool}/{Usize}/{U32}/{U64} | std::sync::atomic::Atomic{Bool}/{Usize}/{U32}/{U64} | (std::cell::Cell<bool/usize/u32/u64>) |
 | OnceCell                         | std::sync::OnceLock                                 | std::cell::OnceCell |
 | Lock\<T> | (parking_lot::Mutex\<T>) | (std::cell::RefCell) |
 | RwLock\<T> | (parking_lot::RwLock\<T>) | (std::cell::RefCell) |
@@ -58,7 +56,6 @@ are implemented differently depending on whether `parallel-compiler` is true.
 | WriteGuard | parking_lot::RwLockWriteGuard | std::cell::RefMut |
 | MappedWriteGuard | parking_lot::MappedRwLockWriteGuard | std::cell::RefMut |
 | LockGuard | parking_lot::MutexGuard | std::cell::RefMut |
-| MappedLockGuard | parking_lot::MappedMutexGuard | std::cell::RefMut |
 
 - These thread-safe data structures are interspersed during compilation which
   can cause lock contention resulting in degraded performance as the number of
@@ -173,12 +170,10 @@ Here are some resources that can be used to learn more:
 - [This list of interior mutability in the compiler by nikomatsakis][imlist]
 
 [`rayon`]: https://crates.io/crates/rayon
-[Arc]: https://doc.rust-lang.org/std/sync/struct.Arc.html
 [imlist]: https://github.com/nikomatsakis/rustc-parallelization/blob/master/interior-mutability-list.md
 [irlo0]: https://internals.rust-lang.org/t/parallelizing-rustc-using-rayon/6606
 [irlo1]: https://internals.rust-lang.org/t/help-test-parallel-rustc/11503
 [monomorphization]: backend/monomorph.md
 [parallel-rustdoc]: https://github.com/rust-lang/rust/issues/82741
-[Rc]: https://doc.rust-lang.org/std/rc/struct.Rc.html
 [rustc-rayon]: https://github.com/rust-lang/rustc-rayon
 [tracking]: https://github.com/rust-lang/rust/issues/48685