Rollup merge of #141507 - RalfJung:atomic-intrinsics, r=bjorn3

atomic_load intrinsic: use const generic parameter for ordering

We have a gazillion intrinsics for the atomics because we encode the ordering into the intrinsic name rather than making it a parameter. This is particularly bad for those operations that take two orderings. Let's fix that!

This PR only converts `load`, to see if there's any feedback that would fundamentally change the strategy we pursue for the const generic intrinsics.

The first two commits are preparation and could be a separate PR if you prefer.

`@BoxyUwU` -- I hope this is a use of const generics that is unlikely to explode? All we need is a const generic of enum type. We could funnel it through an integer if we had to but an enum is obviously nicer...

`@bjorn3` it seems like the cranelift backend entirely ignores the ordering?

2 files changed, 46 insertions, 2 deletions

diff --git a/library/core/src/intrinsics/mod.rs b/library/core/src/intrinsics/mod.rs
index f4dba79666a..f89baef76f0 100644
--- a/library/core/src/intrinsics/mod.rs
+++ b/library/core/src/intrinsics/mod.rs
@@ -30,7 +30,7 @@
 //!
 //! The atomic intrinsics provide common atomic operations on machine
 //! words, with multiple possible memory orderings. See the
-//! [atomic types][crate::sync::atomic] docs for details.
+//! [atomic types][atomic] docs for details.
 //!
 //! # Unwinding
 //!
@@ -50,7 +50,7 @@
 )]
 #![allow(missing_docs)]
 
-use crate::marker::{DiscriminantKind, Tuple};
+use crate::marker::{ConstParamTy, DiscriminantKind, Tuple};
 use crate::ptr;
 
 pub mod fallback;
@@ -62,6 +62,20 @@ pub mod simd;
 #[cfg(all(target_has_atomic = "8", target_has_atomic = "32", target_has_atomic = "ptr"))]
 use crate::sync::atomic::{self, AtomicBool, AtomicI32, AtomicIsize, AtomicU32, Ordering};
 
+/// A type for atomic ordering parameters for intrinsics. This is a separate type from
+/// `atomic::Ordering` so that we can make it `ConstParamTy` and fix the values used here without a
+/// risk of leaking that to stable code.
+#[derive(Debug, ConstParamTy, PartialEq, Eq)]
+pub enum AtomicOrdering {
+    // These values must match the compiler's `AtomicOrdering` defined in
+    // `rustc_middle/src/ty/consts/int.rs`!
+    Relaxed = 0,
+    Release = 1,
+    Acquire = 2,
+    AcqRel = 3,
+    SeqCst = 4,
+}
+
 // N.B., these intrinsics take raw pointers because they mutate aliased
 // memory, which is not valid for either `&` or `&mut`.
 
@@ -395,10 +409,20 @@ pub unsafe fn atomic_cxchgweak_seqcst_seqcst<T: Copy>(dst: *mut T, old: T, src:
 /// `T` must be an integer or pointer type.
 ///
 /// The stabilized version of this intrinsic is available on the
+/// [`atomic`] types via the `load` method. For example, [`AtomicBool::load`].
+#[rustc_intrinsic]
+#[rustc_nounwind]
+#[cfg(not(bootstrap))]
+pub unsafe fn atomic_load<T: Copy, const ORD: AtomicOrdering>(src: *const T) -> T;
+/// Loads the current value of the pointer.
+/// `T` must be an integer or pointer type.
+///
+/// The stabilized version of this intrinsic is available on the
 /// [`atomic`] types via the `load` method by passing
 /// [`Ordering::SeqCst`] as the `order`. For example, [`AtomicBool::load`].
 #[rustc_intrinsic]
 #[rustc_nounwind]
+#[cfg(bootstrap)]
 pub unsafe fn atomic_load_seqcst<T: Copy>(src: *const T) -> T;
 /// Loads the current value of the pointer.
 /// `T` must be an integer or pointer type.
@@ -408,6 +432,7 @@ pub unsafe fn atomic_load_seqcst<T: Copy>(src: *const T) -> T;
 /// [`Ordering::Acquire`] as the `order`. For example, [`AtomicBool::load`].
 #[rustc_intrinsic]
 #[rustc_nounwind]
+#[cfg(bootstrap)]
 pub unsafe fn atomic_load_acquire<T: Copy>(src: *const T) -> T;
 /// Loads the current value of the pointer.
 /// `T` must be an integer or pointer type.
@@ -417,6 +442,7 @@ pub unsafe fn atomic_load_acquire<T: Copy>(src: *const T) -> T;
 /// [`Ordering::Relaxed`] as the `order`. For example, [`AtomicBool::load`].
 #[rustc_intrinsic]
 #[rustc_nounwind]
+#[cfg(bootstrap)]
 pub unsafe fn atomic_load_relaxed<T: Copy>(src: *const T) -> T;
 
 /// Stores the value at the specified memory location.
diff --git a/library/core/src/sync/atomic.rs b/library/core/src/sync/atomic.rs
index 6bc6bee682d..b43f3bad6e2 100644
--- a/library/core/src/sync/atomic.rs
+++ b/library/core/src/sync/atomic.rs
@@ -3822,6 +3822,7 @@ unsafe fn atomic_store<T: Copy>(dst: *mut T, val: T, order: Ordering) {
 
 #[inline]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
+#[cfg(bootstrap)]
 unsafe fn atomic_load<T: Copy>(dst: *const T, order: Ordering) -> T {
     // SAFETY: the caller must uphold the safety contract for `atomic_load`.
     unsafe {
@@ -3836,6 +3837,23 @@ unsafe fn atomic_load<T: Copy>(dst: *const T, order: Ordering) -> T {
 }
 
 #[inline]
+#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
+#[cfg(not(bootstrap))]
+unsafe fn atomic_load<T: Copy>(dst: *const T, order: Ordering) -> T {
+    use intrinsics::AtomicOrdering;
+    // SAFETY: the caller must uphold the safety contract for `atomic_load`.
+    unsafe {
+        match order {
+            Relaxed => intrinsics::atomic_load::<T, { AtomicOrdering::Relaxed }>(dst),
+            Acquire => intrinsics::atomic_load::<T, { AtomicOrdering::Acquire }>(dst),
+            SeqCst => intrinsics::atomic_load::<T, { AtomicOrdering::SeqCst }>(dst),
+            Release => panic!("there is no such thing as a release load"),
+            AcqRel => panic!("there is no such thing as an acquire-release load"),
+        }
+    }
+}
+
+#[inline]
 #[cfg(target_has_atomic)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 unsafe fn atomic_swap<T: Copy>(dst: *mut T, val: T, order: Ordering) -> T {