Rollup merge of rust-lang#145626 - folkertdev:prefetch-fallback, r=Amanieu

add a fallback implementation for the `prefetch_*` intrinsics

related ACP: rust-lang/libs-team#638

The fallback is to just ignore the arguments. That is a valid implementation because this intrinsic is just a hint.

I also added the `miri::intrinsic_fallback_is_spec` annotation, so that miri now supports these operations. A prefetch intrinsic call is valid on any pointer. (specifically LLVM guarantees this https://llvm.org/docs/LangRef.html#llvm-prefetch-intrinsic)

Next, I made the `LOCALITY` argument a const generic. That argument must be const (otherwise LLVM crashes), but that was not reflected in the type.

Finally, with these changes, the intrinsic can be safe and `const` (a prefetch at const evaluation time is just a no-op).

cc `@Amanieu`
r? `@RalfJung`

Author: jhpratt

core/src/intrinsics/mod.rs

@@ -261,53 +261,72 @@ pub unsafe fn atomic_fence<const ORD: AtomicOrdering>();
 pub unsafe fn atomic_singlethreadfence<const ORD: AtomicOrdering>();
 
 /// The `prefetch` intrinsic is a hint to the code generator to insert a prefetch instruction
-/// if supported; otherwise, it is a no-op.
+/// for the given address if supported; otherwise, it is a no-op.
 /// Prefetches have no effect on the behavior of the program but can change its performance
 /// characteristics.
 ///
-/// The `locality` argument must be a constant integer and is a temporal locality specifier
-/// ranging from (0) - no locality, to (3) - extremely local keep in cache.
+/// The `LOCALITY` argument is a temporal locality specifier ranging from (0) - no locality,
+/// to (3) - extremely local keep in cache.
 ///
 /// This intrinsic does not have a stable counterpart.
 #[rustc_intrinsic]
 #[rustc_nounwind]
-pub unsafe fn prefetch_read_data<T>(data: *const T, locality: i32);
+#[miri::intrinsic_fallback_is_spec]
+pub const fn prefetch_read_data<T, const LOCALITY: i32>(data: *const T) {
+    // This operation is a no-op, unless it is overridden by the backend.
+    let _ = data;
+}
+
 /// The `prefetch` intrinsic is a hint to the code generator to insert a prefetch instruction
-/// if supported; otherwise, it is a no-op.
+/// for the given address if supported; otherwise, it is a no-op.
 /// Prefetches have no effect on the behavior of the program but can change its performance
 /// characteristics.
 ///
-/// The `locality` argument must be a constant integer and is a temporal locality specifier
-/// ranging from (0) - no locality, to (3) - extremely local keep in cache.
+/// The `LOCALITY` argument is a temporal locality specifier ranging from (0) - no locality,
+/// to (3) - extremely local keep in cache.
 ///
 /// This intrinsic does not have a stable counterpart.
 #[rustc_intrinsic]
 #[rustc_nounwind]
-pub unsafe fn prefetch_write_data<T>(data: *const T, locality: i32);
+#[miri::intrinsic_fallback_is_spec]
+pub const fn prefetch_write_data<T, const LOCALITY: i32>(data: *const T) {
+    // This operation is a no-op, unless it is overridden by the backend.
+    let _ = data;
+}
+
 /// The `prefetch` intrinsic is a hint to the code generator to insert a prefetch instruction
-/// if supported; otherwise, it is a no-op.
+/// for the given address if supported; otherwise, it is a no-op.
 /// Prefetches have no effect on the behavior of the program but can change its performance
 /// characteristics.
 ///
-/// The `locality` argument must be a constant integer and is a temporal locality specifier
-/// ranging from (0) - no locality, to (3) - extremely local keep in cache.
+/// The `LOCALITY` argument is a temporal locality specifier ranging from (0) - no locality,
+/// to (3) - extremely local keep in cache.
 ///
 /// This intrinsic does not have a stable counterpart.
 #[rustc_intrinsic]
 #[rustc_nounwind]
-pub unsafe fn prefetch_read_instruction<T>(data: *const T, locality: i32);
+#[miri::intrinsic_fallback_is_spec]
+pub const fn prefetch_read_instruction<T, const LOCALITY: i32>(data: *const T) {
+    // This operation is a no-op, unless it is overridden by the backend.
+    let _ = data;
+}
+
 /// The `prefetch` intrinsic is a hint to the code generator to insert a prefetch instruction
-/// if supported; otherwise, it is a no-op.
+/// for the given address if supported; otherwise, it is a no-op.
 /// Prefetches have no effect on the behavior of the program but can change its performance
 /// characteristics.
 ///
-/// The `locality` argument must be a constant integer and is a temporal locality specifier
-/// ranging from (0) - no locality, to (3) - extremely local keep in cache.
+/// The `LOCALITY` argument is a temporal locality specifier ranging from (0) - no locality,
+/// to (3) - extremely local keep in cache.
 ///
 /// This intrinsic does not have a stable counterpart.
 #[rustc_intrinsic]
 #[rustc_nounwind]
-pub unsafe fn prefetch_write_instruction<T>(data: *const T, locality: i32);
+#[miri::intrinsic_fallback_is_spec]
+pub const fn prefetch_write_instruction<T, const LOCALITY: i32>(data: *const T) {
+    // This operation is a no-op, unless it is overridden by the backend.
+    let _ = data;
+}
 
 /// Executes a breakpoint trap, for inspection by a debugger.
 ///