rustc: Prepare the atomics feature for wasm

src/librustc_codegen_llvm/back/write.rs

@@ -171,13 +171,22 @@ pub fn target_machine_factory(sess: &Session, find_features: bool)
         None => llvm::CodeModel::None,
     };
 
-    let singlethread = sess.target.target.options.singlethread;
+    let features = attributes::llvm_target_features(sess).collect::<Vec<_>>();
+    let mut singlethread = sess.target.target.options.singlethread;
+
+    // On the wasm target once the `atomics` feature is enabled that means that
+    // we're no longer single-threaded, or otherwise we don't want LLVM to
+    // lower atomic operations to single-threaded operations.
+    if singlethread &&
+        sess.target.target.llvm_target.contains("wasm32") &&
+        features.iter().any(|s| *s == "+atomics")
+    {
+        singlethread = false;
+    }
 
     let triple = SmallCStr::new(&sess.target.target.llvm_target);
     let cpu = SmallCStr::new(llvm_util::target_cpu(sess));
-    let features = attributes::llvm_target_features(sess)
-        .collect::<Vec<_>>()
-        .join(",");
+    let features = features.join(",");
     let features = CString::new(features).unwrap();
     let is_pie_binary = !find_features && is_pie_binary(sess);
     let trap_unreachable = sess.target.target.options.trap_unreachable;

src/librustc_codegen_llvm/llvm_util.rs

@@ -173,6 +173,7 @@ const MIPS_WHITELIST: &[(&str, Option<&str>)] = &[
 
 const WASM_WHITELIST: &[(&str, Option<&str>)] = &[
     ("simd128", Some("wasm_target_feature")),
+    ("atomics", Some("wasm_target_feature")),
 ];
 
 /// When rustdoc is running, provide a list of all known features so that all their respective

src/librustc_target/spec/wasm32_unknown_unknown.rs

@@ -36,8 +36,7 @@ pub fn target() -> Result<Target, String> {
         dll_suffix: ".wasm".to_string(),
         linker_is_gnu: false,
 
-        // A bit of a lie, but "eh"
-        max_atomic_width: Some(32),
+        max_atomic_width: Some(64),
 
         // Unwinding doesn't work right now, so the whole target unconditionally
         // defaults to panic=abort. Note that this is guaranteed to change in