[wasm] Fix --single-threaded for WebAssembly compilation

Due to the transition to the jobs API, WebAssembly compilation was using
background threads, even when --single-threaded and therefore
--wasm-num-compilation-tasks=0 was used. With this CL, the compilation
job is started with a maximum concurrency of 0 when
--wasm-num-compilation-tasks=0. To ensure compilation progress in
asynchronous compilation, the main thread waits for baseline compilation
to finish right after initializing all compilation units, and thereby
participates in the compilation.

R=clemensb@chromium.org

Bug: v8:11279
Change-Id: I85f93f82c00cdbd6afd46110599089a052101a00
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2599546Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71944}

src/wasm/module-compiler.cc

@@ -1611,10 +1611,8 @@ class BackgroundCompileJob final : public JobTask {
     if (compile_scope.cancelled()) return 0;
     // NumOutstandingCompilations() does not reflect the units that running
     // workers are processing, thus add the current worker count to that number.
-    size_t flag_limit =
-        static_cast<size_t>(std::max(1, FLAG_wasm_num_compilation_tasks));
     return std::min(
-        flag_limit,
+        static_cast<size_t>(FLAG_wasm_num_compilation_tasks),
         worker_count +
             compile_scope.compilation_state()->NumOutstandingCompilations());
   }
@@ -2319,6 +2317,13 @@ class AsyncCompileJob::PrepareAndStartCompile : public CompileStep {
 
       // Add compilation units and kick off compilation.
       InitializeCompilationUnits(job->isolate(), job->native_module_.get());
+      // We are in single-threaded mode, so there are no worker tasks that will
+      // do the compilation. We call {WaitForCompilationEvent} here so that the
+      // main thread paticipates and finishes the compilation.
+      if (FLAG_wasm_num_compilation_tasks == 0) {
+        compilation_state->WaitForCompilationEvent(
+            CompilationEvent::kFinishedBaselineCompilation);
+      }
     }
   }
 };

test/mjsunit/wasm/single-threaded-compilation.js

+// Copyright 2020 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --wasm-num-compilation-tasks=0
+
+load('test/mjsunit/wasm/wasm-module-builder.js');
+
+(function testSyncCompilation() {
+  print(arguments.callee.name);
+  const builder = new WasmModuleBuilder();
+  builder.addFunction("main", kSig_d_d)
+      .addBody([kExprLocalGet, 0])
+      .exportFunc();
+
+  const instance = builder.instantiate();
+})();
+
+(function testAsyncCompilation() {
+  print(arguments.callee.name);
+  const builder = new WasmModuleBuilder();
+  builder.addFunction("main", kSig_i_i)
+      .addBody([kExprLocalGet, 0])
+      .exportFunc();
+
+  const instance = builder.asyncInstantiate();
+})();