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"),