make the codegen test also cover an ill-behaved arch, and add links

Author: RalfJung

compiler/rustc_codegen_llvm/src/builder.rs

@@ -737,6 +737,8 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
                 // a hint, and use regular stores everywhere else.
                 // (In the future, we could alternatively ensure that an sfence gets emitted after a sequence of movnt
                 // before any kind of synchronizing operation. But it's not clear how to do that with LLVM.)
+                // For more context, see <https://github.com/rust-lang/rust/issues/114582> and
+                // <https://github.com/llvm/llvm-project/issues/64521>.
                 const WELL_BEHAVED_NONTEMPORAL_ARCHS: &[&str] =
                     &["aarch64", "arm", "riscv32", "riscv64"];
 

tests/codegen/intrinsics/nontemporal.rs

@@ -1,6 +1,14 @@
 //@ compile-flags: -O
-//@ compile-flags: --target aarch64-unknown-linux-gnu
-//@ needs-llvm-components: aarch64
+//@revisions: with_nontemporal without_nontemporal
+//@[with_nontemporal] compile-flags: --target aarch64-unknown-linux-gnu
+//@[with_nontemporal] needs-llvm-components: aarch64
+//@[without_nontemporal] compile-flags: --target x86_64-unknown-linux-gnu
+//@[without_nontemporal] needs-llvm-components: x86
+
+// Ensure that we *do* omit the `!nontemporal` flag on architectures where it
+// is well-behaved, but do *not* emit it on architectures where it is ill-behaved.
+// For more context, see <https://github.com/rust-lang/rust/issues/114582> and
+// <https://github.com/llvm/llvm-project/issues/64521>.
 
 #![feature(no_core, lang_items, intrinsics)]
 #![no_core]
@@ -21,7 +29,8 @@ extern "rust-intrinsic" {
 #[no_mangle]
 pub fn a(a: &mut u32, b: u32) {
     // CHECK-LABEL: define{{.*}}void @a
-    // CHECK: store i32 %b, ptr %a, align 4, !nontemporal
+    // with_nontemporal: store i32 %b, ptr %a, align 4, !nontemporal
+    // without_nontemporal-NOT: nontemporal
     unsafe {
         nontemporal_store(a, b);
     }