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?

1 files changed, 18 insertions, 0 deletions

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 {