[wasm] Reduce number of background compile scopes

This reduces the number of {BackgroundCompileScope}s per compilation
unit from 2 to 1. This reduces the number of concurrent accesses to the
{SharedMutex}.

R=ahaas@chromium.org

Bug: v8:8916
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Change-Id: I872683101dfc9361aa7054ea67f16e3f3f2b82b1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1495989
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60034}

src/wasm/module-compiler.cc

@@ -646,67 +646,76 @@ class BackgroundCompileTask : public CancelableTask {
     TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.wasm"),
                  "BackgroundCompileTask::RunInternal");
 
-    // These fields are initialized before getting the first unit of work.
+    double deadline = MonotonicallyIncreasingTimeInMs() + 50.0;
+
+    // These fields are initialized in a {BackgroundCompileScope} before
+    // starting compilation.
     base::Optional<CompilationEnv> env;
     std::shared_ptr<WireBytesStorage> wire_bytes;
     std::shared_ptr<const WasmModule> module;
-
+    std::unique_ptr<WasmCompilationUnit> unit;
     WasmFeatures detected_features = kNoWasmFeatures;
-    double deadline = MonotonicallyIncreasingTimeInMs() + 50.0;
-    while (true) {
-      // Step 1 (synchronized): Get a WasmCompilationUnit, and initialize some
-      // fields if this is the first unit executed by this task.
-      std::unique_ptr<WasmCompilationUnit> unit;
-      {
-        BackgroundCompileScope compile_scope(token_);
-        if (compile_scope.cancelled()) return;
-        if (!env.has_value()) {
-          env.emplace(compile_scope.native_module()->CreateCompilationEnv());
-          wire_bytes = compile_scope.compilation_state()->GetWireBytesStorage();
-          module = compile_scope.native_module()->shared_module();
-        }
-        unit = compile_scope.compilation_state()->GetNextCompilationUnit();
-        if (unit == nullptr) {
-          compile_scope.compilation_state()->OnBackgroundTaskStopped(
-              detected_features);
-          return;
-        }
+
+    // Preparation (synchronized): Initialize the fields above and get the first
+    // compilation unit.
+    {
+      BackgroundCompileScope compile_scope(token_);
+      if (compile_scope.cancelled()) return;
+      env.emplace(compile_scope.native_module()->CreateCompilationEnv());
+      wire_bytes = compile_scope.compilation_state()->GetWireBytesStorage();
+      module = compile_scope.native_module()->shared_module();
+      unit = compile_scope.compilation_state()->GetNextCompilationUnit();
+      if (unit == nullptr) {
+        compile_scope.compilation_state()->OnBackgroundTaskStopped(
+            detected_features);
+        return;
       }
+    }
 
-      // Step 2: Execute the compilation.
+    bool compilation_failed = false;
+    while (true) {
+      // (asynchronous): Execute the compilation.
 
       WasmCompilationResult result = unit->ExecuteCompilation(
           &env.value(), wire_bytes, async_counters_.get(), &detected_features);
 
-      // Step 3 (synchronized): Publish the compilation result.
-      bool cancel_compilation = false;
+      // (synchronized): Publish the compilation result and get the next unit.
       {
         BackgroundCompileScope compile_scope(token_);
         if (compile_scope.cancelled()) return;
         WasmCode* code =
             unit->Publish(std::move(result), compile_scope.native_module());
         if (code == nullptr) {
+          // Compile error.
           compile_scope.compilation_state()->OnBackgroundTaskStopped(
               detected_features);
-          // Also, cancel all remaining compilation.
-          cancel_compilation = true;
-        } else {
-          compile_scope.compilation_state()->OnFinishedUnit(
-              unit->requested_tier(), code);
-          if (deadline < MonotonicallyIncreasingTimeInMs()) {
-            compile_scope.compilation_state()->ReportDetectedFeatures(
-                detected_features);
-            compile_scope.compilation_state()->RestartBackgroundCompileTask();
-            return;
-          }
+          compilation_failed = true;
+          break;
+        }
+
+        // Successfully finished one unit.
+        compile_scope.compilation_state()->OnFinishedUnit(
+            unit->requested_tier(), code);
+        if (deadline < MonotonicallyIncreasingTimeInMs()) {
+          compile_scope.compilation_state()->ReportDetectedFeatures(
+              detected_features);
+          compile_scope.compilation_state()->RestartBackgroundCompileTask();
+          return;
+        }
+
+        // Get next unit.
+        unit = compile_scope.compilation_state()->GetNextCompilationUnit();
+        if (unit == nullptr) {
+          compile_scope.compilation_state()->OnBackgroundTaskStopped(
+              detected_features);
+          return;
         }
-      }
-      if (cancel_compilation) {
-        token_->Cancel();
-        return;
       }
     }
-    UNREACHABLE();  // Loop exits via explicit return.
+    // We only get here if compilation failed. Other exits return directly.
+    DCHECK(compilation_failed);
+    USE(compilation_failed);
+    token_->Cancel();
   }
 
  private: