Rollup merge of #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

tests/pass/prefetch.rs

@@ -0,0 +1,23 @@
+#![feature(core_intrinsics)]
+
+// Test that these intrinsics work. Their behavior should be a no-op.
+
+fn main() {
+    static X: [u8; 8] = [0; 8];
+
+    ::std::intrinsics::prefetch_read_data::<_, 1>(::std::ptr::null::<u8>());
+    ::std::intrinsics::prefetch_read_data::<_, 2>(::std::ptr::dangling::<u8>());
+    ::std::intrinsics::prefetch_read_data::<_, 3>(X.as_ptr());
+
+    ::std::intrinsics::prefetch_write_data::<_, 1>(::std::ptr::null::<u8>());
+    ::std::intrinsics::prefetch_write_data::<_, 2>(::std::ptr::dangling::<u8>());
+    ::std::intrinsics::prefetch_write_data::<_, 3>(X.as_ptr());
+
+    ::std::intrinsics::prefetch_read_instruction::<_, 1>(::std::ptr::null::<u8>());
+    ::std::intrinsics::prefetch_read_instruction::<_, 2>(::std::ptr::dangling::<u8>());
+    ::std::intrinsics::prefetch_read_instruction::<_, 3>(X.as_ptr());
+
+    ::std::intrinsics::prefetch_write_instruction::<_, 1>(::std::ptr::null::<u8>());
+    ::std::intrinsics::prefetch_write_instruction::<_, 2>(::std::ptr::dangling::<u8>());
+    ::std::intrinsics::prefetch_write_instruction::<_, 3>(X.as_ptr());
+}