Use conditional synchronization for Lock

compiler/rustc_data_structures/src/sync/lock.rs

@@ -0,0 +1,276 @@
+//! This module implements a lock which only uses synchronization if `might_be_dyn_thread_safe` is true.
+//! It implements `DynSend` and `DynSync` instead of the typical `Send` and `Sync` traits.
+//!
+//! When `cfg(parallel_compiler)` is not set, the lock is instead a wrapper around `RefCell`.
+
+#[cfg(not(parallel_compiler))]
+use std::cell::RefCell;
+#[cfg(parallel_compiler)]
+use {
+    crate::cold_path,
+    crate::sync::DynSend,
+    crate::sync::DynSync,
+    parking_lot::lock_api::RawMutex,
+    std::cell::Cell,
+    std::cell::UnsafeCell,
+    std::fmt,
+    std::intrinsics::{likely, unlikely},
+    std::marker::PhantomData,
+    std::mem::ManuallyDrop,
+    std::ops::{Deref, DerefMut},
+};
+
+#[cfg(not(parallel_compiler))]
+pub use std::cell::RefMut as LockGuard;
+
+#[cfg(not(parallel_compiler))]
+#[derive(Debug)]
+pub struct Lock<T>(RefCell<T>);
+
+#[cfg(not(parallel_compiler))]
+impl<T> Lock<T> {
+    #[inline(always)]
+    pub fn new(inner: T) -> Self {
+        Lock(RefCell::new(inner))
+    }
+
+    #[inline(always)]
+    pub fn into_inner(self) -> T {
+        self.0.into_inner()
+    }
+
+    #[inline(always)]
+    pub fn get_mut(&mut self) -> &mut T {
+        self.0.get_mut()
+    }
+
+    #[inline(always)]
+    pub fn try_lock(&self) -> Option<LockGuard<'_, T>> {
+        self.0.try_borrow_mut().ok()
+    }
+
+    #[inline(always)]
+    #[track_caller]
+    pub fn lock(&self) -> LockGuard<'_, T> {
+        self.0.borrow_mut()
+    }
+}
+
+/// A guard holding mutable access to a `Lock` which is in a locked state.
+#[cfg(parallel_compiler)]
+#[must_use = "if unused the Lock will immediately unlock"]
+pub struct LockGuard<'a, T> {
+    lock: &'a Lock<T>,
+    marker: PhantomData<&'a mut T>,
+}
+
+#[cfg(parallel_compiler)]
+impl<'a, T: 'a> Deref for LockGuard<'a, T> {
+    type Target = T;
+    #[inline]
+    fn deref(&self) -> &T {
+        // SAFETY: We have shared access to the mutable access owned by this type,
+        // so we can give out a shared reference.
+        unsafe { &*self.lock.data.get() }
+    }
+}
+
+#[cfg(parallel_compiler)]
+impl<'a, T: 'a> DerefMut for LockGuard<'a, T> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut T {
+        // SAFETY: We have mutable access to the data so we can give out a mutable reference.
+        unsafe { &mut *self.lock.data.get() }
+    }
+}
+
+#[cfg(parallel_compiler)]
+impl<'a, T: 'a> Drop for LockGuard<'a, T> {
+    #[inline]
+    fn drop(&mut self) {
+        // SAFETY: We know that the lock is in a locked
+        // state because it is a invariant of this type.
+        unsafe { self.lock.raw.unlock() };
+    }
+}
+
+#[cfg(parallel_compiler)]
+union LockRawUnion {
+    /// Indicates if the cell is locked. Only used if `LockRaw.sync` is false.
+    cell: ManuallyDrop<Cell<bool>>,
+
+    /// A lock implementation that's only used if `LockRaw.sync` is true.
+    lock: ManuallyDrop<parking_lot::RawMutex>,
+}
+
+/// A raw lock which only uses synchronization if `might_be_dyn_thread_safe` is true.
+/// It contains no associated data and is used in the implementation of `Lock` which does have such data.
+///
+/// A manual implementation of a tagged union is used with the `sync` field and the `LockRawUnion` instead
+/// of using enums as it results in better code generation.
+#[cfg(parallel_compiler)]
+struct LockRaw {
+    /// Indicates if synchronization is used via `opt.lock` if true,
+    /// or if a non-thread safe cell is used via `opt.cell`. This is set on initialization and never changed.
+    sync: bool,
+    opt: LockRawUnion,
+}
+
+#[cfg(parallel_compiler)]
+impl LockRaw {
+    fn new() -> Self {
+        if unlikely(super::mode::might_be_dyn_thread_safe()) {
+            // Create the lock with synchronization enabled using the `RawMutex` type.
+            LockRaw {
+                sync: true,
+                opt: LockRawUnion { lock: ManuallyDrop::new(parking_lot::RawMutex::INIT) },
+            }
+        } else {
+            // Create the lock with synchronization disabled.
+            LockRaw { sync: false, opt: LockRawUnion { cell: ManuallyDrop::new(Cell::new(false)) } }
+        }
+    }
+
+    #[inline(always)]
+    fn try_lock(&self) -> bool {
+        // SAFETY: This is safe since the union fields are used in accordance with `self.sync`.
+        unsafe {
+            if likely(!self.sync) {
+                if self.opt.cell.get() {
+                    false
+                } else {
+                    self.opt.cell.set(true);
+                    true
+                }
+            } else {
+                self.opt.lock.try_lock()
+            }
+        }
+    }
+
+    #[inline(always)]
+    fn lock(&self) {
+        if super::ERROR_CHECKING {
+            // We're in the debugging mode, so assert that the lock is not held so we
+            // get a panic instead of waiting for the lock.
+            assert_eq!(self.try_lock(), true, "lock must not be hold");
+        } else {
+            // SAFETY: This is safe since the union fields are used in accordance with `self.sync`.
+            unsafe {
+                if likely(!self.sync) {
+                    if unlikely(self.opt.cell.replace(true)) {
+                        cold_path(|| panic!("lock was already held"))
+                    }
+                } else {
+                    self.opt.lock.lock();
+                }
+            }
+        }
+    }
+
+    /// This unlocks the lock.
+    ///
+    /// Safety
+    /// This method may only be called if the lock is currently held.
+    #[inline(always)]
+    unsafe fn unlock(&self) {
+        // SAFETY: The union use is safe since the union fields are used in accordance with
+        // `self.sync` and the `unlock` method precondition is upheld by the caller.
+        unsafe {
+            if likely(!self.sync) {
+                debug_assert_eq!(self.opt.cell.get(), true);
+                self.opt.cell.set(false);
+            } else {
+                self.opt.lock.unlock();
+            }
+        }
+    }
+}
+
+/// A lock which only uses synchronization if `might_be_dyn_thread_safe` is true.
+/// It implements `DynSend` and `DynSync` instead of the typical `Send` and `Sync`.
+#[cfg(parallel_compiler)]
+pub struct Lock<T> {
+    raw: LockRaw,
+    data: UnsafeCell<T>,
+}
+
+#[cfg(parallel_compiler)]
+impl<T> Lock<T> {
+    #[inline(always)]
+    pub fn new(inner: T) -> Self {
+        Lock { raw: LockRaw::new(), data: UnsafeCell::new(inner) }
+    }
+
+    #[inline(always)]
+    pub fn into_inner(self) -> T {
+        self.data.into_inner()
+    }
+
+    #[inline(always)]
+    pub fn get_mut(&mut self) -> &mut T {
+        self.data.get_mut()
+    }
+
+    #[inline(always)]
+    pub fn try_lock(&self) -> Option<LockGuard<'_, T>> {
+        if self.raw.try_lock() { Some(LockGuard { lock: self, marker: PhantomData }) } else { None }
+    }
+
+    #[inline(always)]
+    pub fn lock(&self) -> LockGuard<'_, T> {
+        self.raw.lock();
+        LockGuard { lock: self, marker: PhantomData }
+    }
+}
+
+impl<T> Lock<T> {
+    #[inline(always)]
+    #[track_caller]
+    pub fn with_lock<F: FnOnce(&mut T) -> R, R>(&self, f: F) -> R {
+        f(&mut *self.lock())
+    }
+
+    #[inline(always)]
+    #[track_caller]
+    pub fn borrow(&self) -> LockGuard<'_, T> {
+        self.lock()
+    }
+
+    #[inline(always)]
+    #[track_caller]
+    pub fn borrow_mut(&self) -> LockGuard<'_, T> {
+        self.lock()
+    }
+}
+
+#[cfg(parallel_compiler)]
+unsafe impl<T: DynSend> DynSend for Lock<T> {}
+#[cfg(parallel_compiler)]
+unsafe impl<T: DynSend> DynSync for Lock<T> {}
+
+#[cfg(parallel_compiler)]
+impl<T: fmt::Debug> fmt::Debug for Lock<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.try_lock() {
+            Some(guard) => f.debug_struct("Lock").field("data", &&*guard).finish(),
+            None => {
+                struct LockedPlaceholder;
+                impl fmt::Debug for LockedPlaceholder {
+                    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                        f.write_str("<locked>")
+                    }
+                }
+
+                f.debug_struct("Lock").field("data", &LockedPlaceholder).finish()
+            }
+        }
+    }
+}
+
+impl<T: Default> Default for Lock<T> {
+    #[inline]
+    fn default() -> Self {
+        Lock::new(T::default())
+    }
+}

compiler/rustc_data_structures/src/sync/worker_local.rs

@@ -1,4 +1,4 @@
-use crate::sync::Lock;
+use parking_lot::Mutex;
 use std::cell::Cell;
 use std::cell::OnceCell;
 use std::ops::Deref;
@@ -35,7 +35,7 @@ impl RegistryId {
 
 struct RegistryData {
     thread_limit: usize,
-    threads: Lock<usize>,
+    threads: Mutex<usize>,
 }
 
 /// Represents a list of threads which can access worker locals.
@@ -65,7 +65,7 @@ thread_local! {
 impl Registry {
     /// Creates a registry which can hold up to `thread_limit` threads.
     pub fn new(thread_limit: usize) -> Self {
-        Registry(Arc::new(RegistryData { thread_limit, threads: Lock::new(0) }))
+        Registry(Arc::new(RegistryData { thread_limit, threads: Mutex::new(0) }))
     }
 
     /// Gets the registry associated with the current thread. Panics if there's no such registry.