Rollup merge of #109758 - nnethercote:parallel-cleanups, r=cjgillot

Parallel compiler cleanups

A few small improvements I found while looking closely at this code.

r? `@cjgillot`

cc `@Zoxc,` `@SparrowLii`

4 files changed, 45 insertions, 36 deletions

diff --git a/compiler/rustc_data_structures/src/sharded.rs b/compiler/rustc_data_structures/src/sharded.rs
index 01d292dde8d..f88c055a9b5 100644
--- a/compiler/rustc_data_structures/src/sharded.rs
+++ b/compiler/rustc_data_structures/src/sharded.rs
@@ -5,7 +5,7 @@ use std::collections::hash_map::RawEntryMut;
 use std::hash::{Hash, Hasher};
 use std::mem;
 
-#[derive(Clone, Default)]
+#[derive(Default)]
 #[cfg_attr(parallel_compiler, repr(align(64)))]
 struct CacheAligned<T>(T);
 
@@ -21,7 +21,6 @@ const SHARD_BITS: usize = 0;
 pub const SHARDS: usize = 1 << SHARD_BITS;
 
 /// An array of cache-line aligned inner locked structures with convenience methods.
-#[derive(Clone)]
 pub struct Sharded<T> {
     shards: [CacheAligned<Lock<T>>; SHARDS],
 }
diff --git a/compiler/rustc_data_structures/src/sync.rs b/compiler/rustc_data_structures/src/sync.rs
index 31323c21df0..4e2126fff7b 100644
--- a/compiler/rustc_data_structures/src/sync.rs
+++ b/compiler/rustc_data_structures/src/sync.rs
@@ -1,21 +1,46 @@
-//! This module defines types which are thread safe if cfg!(parallel_compiler) is true.
+//! This module defines various operations and types that are implemented in
+//! one way for the serial compiler, and another way the parallel compiler.
 //!
-//! `Lrc` is an alias of `Arc` if cfg!(parallel_compiler) is true, `Rc` otherwise.
+//! Operations
+//! ----------
+//! The parallel versions of operations use Rayon to execute code in parallel,
+//! while the serial versions degenerate straightforwardly to serial execution.
+//! The operations include `join`, `parallel`, `par_iter`, and `par_for_each`.
 //!
-//! `Lock` is a mutex.
-//! It internally uses `parking_lot::Mutex` if cfg!(parallel_compiler) is true,
-//! `RefCell` otherwise.
+//! `rustc_erase_owner!` erases an `OwningRef` owner into `Erased` for the
+//! serial version and `Erased + Send + Sync` for the parallel version.
 //!
-//! `RwLock` is a read-write lock.
-//! It internally uses `parking_lot::RwLock` if cfg!(parallel_compiler) is true,
-//! `RefCell` otherwise.
+//! Types
+//! -----
+//! The parallel versions of types provide various kinds of synchronization,
+//! while the serial compiler versions do not.
 //!
-//! `MTLock` is a mutex which disappears if cfg!(parallel_compiler) is false.
+//! The following table shows how the types are implemented internally. Except
+//! where noted otherwise, the type in column one is defined as a
+//! newtype around the type from column two or three.
 //!
-//! `MTRef` is an immutable reference if cfg!(parallel_compiler), and a mutable reference otherwise.
+//! | Type                    | Serial version      | Parallel version                |
+//! | ----------------------- | ------------------- | ------------------------------- |
+//! | `Lrc<T>`                | `rc::Rc<T>`         | `sync::Arc<T>`                  |
+//! |` Weak<T>`               | `rc::Weak<T>`       | `sync::Weak<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>`        | `parking_lot::Mutex<T>`         |
+//! | `RwLock<T>`             | `RefCell<T>`        | `parking_lot::RwLock<T>`        |
+//! | `MTLock<T>`        [^1] | `T`                 | `Lock<T>`                       |
+//! | `MTLockRef<'a, T>` [^2] | `&'a mut MTLock<T>` | `&'a MTLock<T>`                 |
+//! |                         |                     |                                 |
+//! | `ParallelIterator`      | `Iterator`          | `rayon::iter::ParallelIterator` |
 //!
-//! `rustc_erase_owner!` erases an OwningRef owner into Erased or Erased + Send + Sync
-//! depending on the value of cfg!(parallel_compiler).
+//! [^1] `MTLock` is similar to `Lock`, but the serial version avoids the cost
+//! of a `RefCell`. This is appropriate when interior mutability is not
+//! required.
+//!
+//! [^2] `MTLockRef` is a typedef.
 
 use crate::owning_ref::{Erased, OwningRef};
 use std::collections::HashMap;
@@ -209,7 +234,7 @@ cfg_if! {
             }
         }
 
-        pub type MTRef<'a, T> = &'a mut T;
+        pub type MTLockRef<'a, T> = &'a mut MTLock<T>;
 
         #[derive(Debug, Default)]
         pub struct MTLock<T>(T);
@@ -267,7 +292,7 @@ cfg_if! {
         pub use std::sync::Arc as Lrc;
         pub use std::sync::Weak as Weak;
 
-        pub type MTRef<'a, T> = &'a T;
+        pub type MTLockRef<'a, T> = &'a MTLock<T>;
 
         #[derive(Debug, Default)]
         pub struct MTLock<T>(Lock<T>);
@@ -555,18 +580,6 @@ impl<T> RwLock<T> {
 
     #[cfg(not(parallel_compiler))]
     #[inline(always)]
-    pub fn clone_guard<'a>(rg: &ReadGuard<'a, T>) -> ReadGuard<'a, T> {
-        ReadGuard::clone(rg)
-    }
-
-    #[cfg(parallel_compiler)]
-    #[inline(always)]
-    pub fn clone_guard<'a>(rg: &ReadGuard<'a, T>) -> ReadGuard<'a, T> {
-        ReadGuard::rwlock(&rg).read()
-    }
-
-    #[cfg(not(parallel_compiler))]
-    #[inline(always)]
     pub fn leak(&self) -> &T {
         ReadGuard::leak(self.read())
     }
diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index af0222c8172..7ca18fca18c 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -174,7 +174,7 @@
 //! regardless of whether it is actually needed or not.
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
-use rustc_data_structures::sync::{par_for_each_in, MTLock, MTRef};
+use rustc_data_structures::sync::{par_for_each_in, MTLock, MTLockRef};
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, DefIdMap, LocalDefId};
@@ -341,8 +341,8 @@ pub fn collect_crate_mono_items(
     let recursion_limit = tcx.recursion_limit();
 
     {
-        let visited: MTRef<'_, _> = &mut visited;
-        let inlining_map: MTRef<'_, _> = &mut inlining_map;
+        let visited: MTLockRef<'_, _> = &mut visited;
+        let inlining_map: MTLockRef<'_, _> = &mut inlining_map;
 
         tcx.sess.time("monomorphization_collector_graph_walk", || {
             par_for_each_in(roots, |root| {
@@ -407,10 +407,10 @@ fn collect_roots(tcx: TyCtxt<'_>, mode: MonoItemCollectionMode) -> Vec<MonoItem<
 fn collect_items_rec<'tcx>(
     tcx: TyCtxt<'tcx>,
     starting_point: Spanned<MonoItem<'tcx>>,
-    visited: MTRef<'_, MTLock<FxHashSet<MonoItem<'tcx>>>>,
+    visited: MTLockRef<'_, FxHashSet<MonoItem<'tcx>>>,
     recursion_depths: &mut DefIdMap<usize>,
     recursion_limit: Limit,
-    inlining_map: MTRef<'_, MTLock<InliningMap<'tcx>>>,
+    inlining_map: MTLockRef<'_, InliningMap<'tcx>>,
 ) {
     if !visited.lock_mut().insert(starting_point.node) {
         // We've been here already, no need to search again.
diff --git a/compiler/rustc_query_system/src/query/caches.rs b/compiler/rustc_query_system/src/query/caches.rs
index 5f554a54dea..d3efc22a194 100644
--- a/compiler/rustc_query_system/src/query/caches.rs
+++ b/compiler/rustc_query_system/src/query/caches.rs
@@ -21,9 +21,6 @@ pub trait QueryCache: Sized {
     type Value: Copy + Debug;
 
     /// Checks if the query is already computed and in the cache.
-    /// It returns the shard index and a lock guard to the shard,
-    /// which will be used if the query is not in the cache and we need
-    /// to compute it.
     fn lookup(&self, key: &Self::Key) -> Option<(Self::Value, DepNodeIndex)>;
 
     fn complete(&self, key: Self::Key, value: Self::Value, index: DepNodeIndex);