Rollup merge of #98383 - m-ou-se:remove-memory-order-restrictions, r=joshtriplett

Remove restrictions on compare-exchange memory ordering.

We currently don't allow the failure memory ordering of compare-exchange operations to be stronger than the success ordering, as was the case in C++11 when its memory model was copied to Rust. However, this restriction was lifted in C++17 as part of [p0418r2](https://wg21.link/p0418r2). It's time  we lift the restriction too.

| Success | Failure | Before | After |
|---------|---------|--------|-------|
| Relaxed | Relaxed | ✔️ | ✔️ |
| Relaxed | Acquire | ❌                | ✔️ |
| Relaxed | SeqCst  | ❌                | ✔️ |
| Acquire | Relaxed | ✔️ | ✔️ |
| Acquire | Acquire | ✔️ | ✔️ |
| Acquire | SeqCst  | ❌                | ✔️ |
| Release | Relaxed | ✔️ | ✔️ |
| Release | Acquire | ❌                | ✔️ |
| Release | SeqCst  | ❌                | ✔️ |
| AcqRel  | Relaxed | ✔️ | ✔️ |
| AcqRel  | Acquire | ✔️ | ✔️ |
| AcqRel  | SeqCst  | ❌                | ✔️ |
| SeqCst  | Relaxed | ✔️ | ✔️ |
| SeqCst  | Acquire | ✔️ | ✔️ |
| SeqCst  | SeqCst  | ✔️ | ✔️ |
| \*      | Release | ❌                | ❌                |
| \*      | AcqRel  | ❌                | ❌                |

Fixes #68464

Author: JohnTitor

library/core/src/sync/atomic.rs

@@ -581,8 +581,7 @@ impl AtomicBool {
     /// `failure` describes the required ordering for the load operation that takes place when
     /// the comparison fails. Using [`Acquire`] as success ordering makes the store part
     /// of this operation [`Relaxed`], and using [`Release`] makes the successful load
-    /// [`Relaxed`]. The failure ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`]
-    /// and must be equivalent to or weaker than the success ordering.
+    /// [`Relaxed`]. The failure ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`].
     ///
     /// **Note:** This method is only available on platforms that support atomic
     /// operations on `u8`.
@@ -640,8 +639,7 @@ impl AtomicBool {
     /// `failure` describes the required ordering for the load operation that takes place when
     /// the comparison fails. Using [`Acquire`] as success ordering makes the store part
     /// of this operation [`Relaxed`], and using [`Release`] makes the successful load
-    /// [`Relaxed`]. The failure ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`]
-    /// and must be equivalent to or weaker than the success ordering.
+    /// [`Relaxed`]. The failure ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`].
     ///
     /// **Note:** This method is only available on platforms that support atomic
     /// operations on `u8`.
@@ -941,8 +939,7 @@ impl AtomicBool {
     /// Using [`Acquire`] as success ordering makes the store part of this
     /// operation [`Relaxed`], and using [`Release`] makes the final successful
     /// load [`Relaxed`]. The (failed) load ordering can only be [`SeqCst`],
-    /// [`Acquire`] or [`Relaxed`] and must be equivalent to or weaker than the
-    /// success ordering.
+    /// [`Acquire`] or [`Relaxed`].
     ///
     /// **Note:** This method is only available on platforms that support atomic
     /// operations on `u8`.
@@ -1301,8 +1298,7 @@ impl<T> AtomicPtr<T> {
     /// `failure` describes the required ordering for the load operation that takes place when
     /// the comparison fails. Using [`Acquire`] as success ordering makes the store part
     /// of this operation [`Relaxed`], and using [`Release`] makes the successful load
-    /// [`Relaxed`]. The failure ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`]
-    /// and must be equivalent to or weaker than the success ordering.
+    /// [`Relaxed`]. The failure ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`].
     ///
     /// **Note:** This method is only available on platforms that support atomic
     /// operations on pointers.
@@ -1347,8 +1343,7 @@ impl<T> AtomicPtr<T> {
     /// `failure` describes the required ordering for the load operation that takes place when
     /// the comparison fails. Using [`Acquire`] as success ordering makes the store part
     /// of this operation [`Relaxed`], and using [`Release`] makes the successful load
-    /// [`Relaxed`]. The failure ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`]
-    /// and must be equivalent to or weaker than the success ordering.
+    /// [`Relaxed`]. The failure ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`].
     ///
     /// **Note:** This method is only available on platforms that support atomic
     /// operations on pointers.
@@ -1404,8 +1399,7 @@ impl<T> AtomicPtr<T> {
     /// Using [`Acquire`] as success ordering makes the store part of this
     /// operation [`Relaxed`], and using [`Release`] makes the final successful
     /// load [`Relaxed`]. The (failed) load ordering can only be [`SeqCst`],
-    /// [`Acquire`] or [`Relaxed`] and must be equivalent to or weaker than the
-    /// success ordering.
+    /// [`Acquire`] or [`Relaxed`].
     ///
     /// **Note:** This method is only available on platforms that support atomic
     /// operations on pointers.
@@ -2227,8 +2221,7 @@ macro_rules! atomic_int {
             /// `failure` describes the required ordering for the load operation that takes place when
             /// the comparison fails. Using [`Acquire`] as success ordering makes the store part
             /// of this operation [`Relaxed`], and using [`Release`] makes the successful load
-            /// [`Relaxed`]. The failure ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`]
-            /// and must be equivalent to or weaker than the success ordering.
+            /// [`Relaxed`]. The failure ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`].
             ///
             /// **Note**: This method is only available on platforms that support atomic operations on
             #[doc = concat!("[`", $s_int_type, "`].")]
@@ -2279,8 +2272,7 @@ macro_rules! atomic_int {
             /// `failure` describes the required ordering for the load operation that takes place when
             /// the comparison fails. Using [`Acquire`] as success ordering makes the store part
             /// of this operation [`Relaxed`], and using [`Release`] makes the successful load
-            /// [`Relaxed`]. The failure ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`]
-            /// and must be equivalent to or weaker than the success ordering.
+            /// [`Relaxed`]. The failure ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`].
             ///
             /// **Note**: This method is only available on platforms that support atomic operations on
             #[doc = concat!("[`", $s_int_type, "`].")]
@@ -2517,8 +2509,7 @@ macro_rules! atomic_int {
             ///
             /// Using [`Acquire`] as success ordering makes the store part
             /// of this operation [`Relaxed`], and using [`Release`] makes the final successful load
-            /// [`Relaxed`]. The (failed) load ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`]
-            /// and must be equivalent to or weaker than the success ordering.
+            /// [`Relaxed`]. The (failed) load ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`].
             ///
             /// **Note**: This method is only available on platforms that support atomic operations on
             #[doc = concat!("[`", $s_int_type, "`].")]
@@ -3035,22 +3026,29 @@ unsafe fn atomic_compare_exchange<T: Copy>(
     let (val, ok) = unsafe {
         match (success, failure) {
             (Relaxed, Relaxed) => intrinsics::atomic_cxchg_relaxed_relaxed(dst, old, new),
-            //(Relaxed, Acquire) => intrinsics::atomic_cxchg_relaxed_acquire(dst, old, new),
-            //(Relaxed, SeqCst) => intrinsics::atomic_cxchg_relaxed_seqcst(dst, old, new),
+            #[cfg(not(bootstrap))]
+            (Relaxed, Acquire) => intrinsics::atomic_cxchg_relaxed_acquire(dst, old, new),
+            #[cfg(not(bootstrap))]
+            (Relaxed, SeqCst) => intrinsics::atomic_cxchg_relaxed_seqcst(dst, old, new),
             (Acquire, Relaxed) => intrinsics::atomic_cxchg_acquire_relaxed(dst, old, new),
             (Acquire, Acquire) => intrinsics::atomic_cxchg_acquire_acquire(dst, old, new),
-            //(Acquire, SeqCst) => intrinsics::atomic_cxchg_acquire_seqcst(dst, old, new),
+            #[cfg(not(bootstrap))]
+            (Acquire, SeqCst) => intrinsics::atomic_cxchg_acquire_seqcst(dst, old, new),
             (Release, Relaxed) => intrinsics::atomic_cxchg_release_relaxed(dst, old, new),
-            //(Release, Acquire) => intrinsics::atomic_cxchg_release_acquire(dst, old, new),
-            //(Release, SeqCst) => intrinsics::atomic_cxchg_release_seqcst(dst, old, new),
+            #[cfg(not(bootstrap))]
+            (Release, Acquire) => intrinsics::atomic_cxchg_release_acquire(dst, old, new),
+            #[cfg(not(bootstrap))]
+            (Release, SeqCst) => intrinsics::atomic_cxchg_release_seqcst(dst, old, new),
             (AcqRel, Relaxed) => intrinsics::atomic_cxchg_acqrel_relaxed(dst, old, new),
             (AcqRel, Acquire) => intrinsics::atomic_cxchg_acqrel_acquire(dst, old, new),
-            //(AcqRel, SeqCst) => intrinsics::atomic_cxchg_acqrel_seqcst(dst, old, new),
+            #[cfg(not(bootstrap))]
+            (AcqRel, SeqCst) => intrinsics::atomic_cxchg_acqrel_seqcst(dst, old, new),
             (SeqCst, Relaxed) => intrinsics::atomic_cxchg_seqcst_relaxed(dst, old, new),
             (SeqCst, Acquire) => intrinsics::atomic_cxchg_seqcst_acquire(dst, old, new),
             (SeqCst, SeqCst) => intrinsics::atomic_cxchg_seqcst_seqcst(dst, old, new),
             (_, AcqRel) => panic!("there is no such thing as an acquire-release failure ordering"),
             (_, Release) => panic!("there is no such thing as a release failure ordering"),
+            #[cfg(bootstrap)]
             _ => panic!("a failure ordering can't be stronger than a success ordering"),
         }
     };
@@ -3070,22 +3068,29 @@ unsafe fn atomic_compare_exchange_weak<T: Copy>(
     let (val, ok) = unsafe {
         match (success, failure) {
             (Relaxed, Relaxed) => intrinsics::atomic_cxchgweak_relaxed_relaxed(dst, old, new),
-            //(Relaxed, Acquire) => intrinsics::atomic_cxchgweak_relaxed_acquire(dst, old, new),
-            //(Relaxed, SeqCst) => intrinsics::atomic_cxchgweak_relaxed_seqcst(dst, old, new),
+            #[cfg(not(bootstrap))]
+            (Relaxed, Acquire) => intrinsics::atomic_cxchgweak_relaxed_acquire(dst, old, new),
+            #[cfg(not(bootstrap))]
+            (Relaxed, SeqCst) => intrinsics::atomic_cxchgweak_relaxed_seqcst(dst, old, new),
             (Acquire, Relaxed) => intrinsics::atomic_cxchgweak_acquire_relaxed(dst, old, new),
             (Acquire, Acquire) => intrinsics::atomic_cxchgweak_acquire_acquire(dst, old, new),
-            //(Acquire, SeqCst) => intrinsics::atomic_cxchgweak_acquire_seqcst(dst, old, new),
+            #[cfg(not(bootstrap))]
+            (Acquire, SeqCst) => intrinsics::atomic_cxchgweak_acquire_seqcst(dst, old, new),
             (Release, Relaxed) => intrinsics::atomic_cxchgweak_release_relaxed(dst, old, new),
-            //(Release, Acquire) => intrinsics::atomic_cxchgweak_release_acquire(dst, old, new),
-            //(Release, SeqCst) => intrinsics::atomic_cxchgweak_release_seqcst(dst, old, new),
+            #[cfg(not(bootstrap))]
+            (Release, Acquire) => intrinsics::atomic_cxchgweak_release_acquire(dst, old, new),
+            #[cfg(not(bootstrap))]
+            (Release, SeqCst) => intrinsics::atomic_cxchgweak_release_seqcst(dst, old, new),
             (AcqRel, Relaxed) => intrinsics::atomic_cxchgweak_acqrel_relaxed(dst, old, new),
             (AcqRel, Acquire) => intrinsics::atomic_cxchgweak_acqrel_acquire(dst, old, new),
-            //(AcqRel, SeqCst) => intrinsics::atomic_cxchgweak_acqrel_seqcst(dst, old, new),
+            #[cfg(not(bootstrap))]
+            (AcqRel, SeqCst) => intrinsics::atomic_cxchgweak_acqrel_seqcst(dst, old, new),
             (SeqCst, Relaxed) => intrinsics::atomic_cxchgweak_seqcst_relaxed(dst, old, new),
             (SeqCst, Acquire) => intrinsics::atomic_cxchgweak_seqcst_acquire(dst, old, new),
             (SeqCst, SeqCst) => intrinsics::atomic_cxchgweak_seqcst_seqcst(dst, old, new),
             (_, AcqRel) => panic!("there is no such thing as an acquire-release failure ordering"),
             (_, Release) => panic!("there is no such thing as a release failure ordering"),
+            #[cfg(bootstrap)]
             _ => panic!("a failure ordering can't be stronger than a success ordering"),
         }
     };