Rollup merge of rust-lang#128149 - RalfJung:nontemporal_store, r=jiey…

…ouxu,Amanieu,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#114582
Cc `@Amanieu` `@workingjubilee`

src/intrinsics/mod.rs

@@ -725,7 +725,8 @@ fn codegen_regular_intrinsic_call<'tcx>(
 
  // Cranelift treats stores as volatile by default
  // FIXME correctly handle unaligned_volatile_store
- // FIXME actually do nontemporal stores if requested
+ // FIXME actually do nontemporal stores if requested (but do not just emit MOVNT on x86;
+ // see the LLVM backend for details)
  let dest = CPlace::for_ptr(Pointer::new(ptr), val.layout());
  dest.write_cvalue(fx, val);
  }