Allow codegen backends to opt-out of parallel codegen

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -369,6 +369,10 @@ pub struct CodegenContext<B: WriteBackendMethods> {
     pub incr_comp_session_dir: Option<PathBuf>,
     /// Channel back to the main control thread to send messages to
     pub coordinator_send: Sender<Box<dyn Any + Send>>,
+    /// `true` if the codegen should be run in parallel.
+    ///
+    /// Depends on [`CodegenBackend::supports_parallel()`] and `-Zno_parallel_backend`.
+    pub parallel: bool,
 }
 
 impl<B: WriteBackendMethods> CodegenContext<B> {
@@ -1129,6 +1133,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
         target_arch: tcx.sess.target.arch.to_string(),
         split_debuginfo: tcx.sess.split_debuginfo(),
         split_dwarf_kind: tcx.sess.opts.unstable_opts.split_dwarf_kind,
+        parallel: backend.supports_parallel() && !sess.opts.unstable_opts.no_parallel_backend,
     };
 
     // This is the "main loop" of parallel work happening for parallel codegen.
@@ -1399,7 +1404,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
                             .binary_search_by_key(&cost, |&(_, cost)| cost)
                             .unwrap_or_else(|e| e);
                         work_items.insert(insertion_index, (work, cost));
-                        if !cgcx.opts.unstable_opts.no_parallel_llvm {
+                        if cgcx.parallel {
                             helper.request_token();
                         }
                     }
@@ -1522,7 +1527,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
                     };
                     work_items.insert(insertion_index, (llvm_work_item, cost));
 
-                    if !cgcx.opts.unstable_opts.no_parallel_llvm {
+                    if cgcx.parallel {
                         helper.request_token();
                     }
                     assert_eq!(main_thread_state, MainThreadState::Codegenning);

compiler/rustc_codegen_ssa/src/traits/backend.rs

@@ -111,6 +111,13 @@ pub trait CodegenBackend {
         codegen_results: CodegenResults,
         outputs: &OutputFilenames,
     ) -> Result<(), ErrorGuaranteed>;
+
+    /// Returns `true` if this backend can be safely called from multiple threads.
+    ///
+    /// Defaults to `true`.
+    fn supports_parallel(&self) -> bool {
+        true
+    }
 }
 
 pub trait ExtraBackendMethods:

compiler/rustc_interface/src/tests.rs

@@ -685,7 +685,7 @@ fn test_unstable_options_tracking_hash() {
     untracked!(nll_facts, true);
     untracked!(no_analysis, true);
     untracked!(no_leak_check, true);
-    untracked!(no_parallel_llvm, true);
+    untracked!(no_parallel_backend, true);
     untracked!(parse_only, true);
     // `pre_link_arg` is omitted because it just forwards to `pre_link_args`.
     untracked!(pre_link_args, vec![String::from("abc"), String::from("def")]);

compiler/rustc_session/src/options.rs

@@ -1753,7 +1753,7 @@ options! {
         "disable the 'leak check' for subtyping; unsound, but useful for tests"),
     no_link: bool = (false, parse_no_flag, [TRACKED],
         "compile without linking"),
-    no_parallel_llvm: bool = (false, parse_no_flag, [UNTRACKED],
+    no_parallel_backend: bool = (false, parse_no_flag, [UNTRACKED],
         "run LLVM in non-parallel mode (while keeping codegen-units and ThinLTO)"),
     no_profiler_runtime: bool = (false, parse_no_flag, [TRACKED],
         "prevent automatic injection of the profiler_builtins crate"),