Rollup merge of #128149 - RalfJung:nontemporal_store, r=jieyouxu,Aman…

…ieu,Jubilee

nontemporal_store: make sure that the intrinsic is truly just a hint

The `!nontemporal` flag for stores in LLVM *sounds* like it is just a hint, but actually, it is not -- at least on x86, non-temporal stores need very special treatment by the programmer or else the Rust memory model breaks down. LLVM still treats these stores as-if they were normal stores for optimizations, which is [highly dubious](llvm/llvm-project#64521). Let's avoid all that dubiousness by making our own non-temporal stores be truly just a hint, which is possible on some targets (e.g. ARM). On all other targets, non-temporal stores become regular stores.

~~Blocked on rust-lang/stdarch#1541 propagating to the rustc repo, to make sure the `_mm_stream` intrinsics are unaffected by this change.~~

Fixes rust-lang/rust#114582
Cc `@Amanieu` `@workingjubilee`

src/builder.rs

@@ -1127,6 +1127,8 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
         self.llbb().add_assignment(self.location, aligned_destination, val);
         // TODO(antoyo): handle align and flags.
         // NOTE: dummy value here since it's never used. FIXME(antoyo): API should not return a value here?
+        // When adding support for NONTEMPORAL, make sure to not just emit MOVNT on x86; see the
+        // LLVM backend for details.
         self.cx.context.new_rvalue_zero(self.type_i32())
     }