[wasm] Minor optimization for parallel compilation

Skip parallel compilation if less than two functions need to be
compiled, and skip over imported functions instead of adding nullptrs
for them to the compilation_units_ vector.

R=titzer@chromium.org, ahaas@chromium.org

Review-Url: https://codereview.chromium.org/2718053002
Cr-Commit-Position: refs/heads/master@{#43449}

src/flag-definitions.h

@@ -532,7 +532,7 @@ DEFINE_INT(trace_wasm_text_start, 0,
            "start function for WASM text generation (inclusive)")
 DEFINE_INT(trace_wasm_text_end, 0,
            "end function for WASM text generation (exclusive)")
-DEFINE_INT(skip_compiling_wasm_funcs, 0, "start compiling at function N")
+DEFINE_UINT(skip_compiling_wasm_funcs, 0, "start compiling at function N")
 DEFINE_BOOL(wasm_break_on_decoder_error, false,
             "debug break when wasm decoder encounters an error")
 DEFINE_BOOL(wasm_loop_assignment_analysis, true,

src/wasm/wasm-module.cc

@@ -246,25 +246,24 @@ class CompilationHelper {
     }
 
     compiler::WasmCompilationUnit* unit = compilation_units_.at(index);
-    if (unit != nullptr) {
-      unit->ExecuteCompilation();
-      base::LockGuard<base::Mutex> guard(&result_mutex_);
-      executed_units_.push(unit);
-    }
+    unit->ExecuteCompilation();
+    base::LockGuard<base::Mutex> guard(&result_mutex_);
+    executed_units_.push(unit);
     return true;
   }
 
   void InitializeParallelCompilation(const std::vector<WasmFunction>& functions,
                                      ModuleBytesEnv& module_env,
                                      ErrorThrower* thrower) {
-    compilation_units_.reserve(functions.size());
-    for (uint32_t i = FLAG_skip_compiling_wasm_funcs; i < functions.size();
-         ++i) {
+    uint32_t start = module_env.module_env.module->num_imported_functions +
+                     FLAG_skip_compiling_wasm_funcs;
+    uint32_t num_funcs = static_cast<uint32_t>(functions.size());
+    uint32_t funcs_to_compile = start > num_funcs ? 0 : num_funcs - start;
+    compilation_units_.reserve(funcs_to_compile);
+    for (uint32_t i = start; i < num_funcs; ++i) {
       const WasmFunction* func = &functions[i];
-      compilation_units_.push_back(
-          func->imported ? nullptr
-                         : new compiler::WasmCompilationUnit(
-                               thrower, isolate_, &module_env, func, i));
+      compilation_units_.push_back(new compiler::WasmCompilationUnit(
+          thrower, isolate_, &module_env, func, i));
     }
   }
 
@@ -449,7 +448,10 @@ class CompilationHelper {
     isolate_->counters()->wasm_functions_per_module()->AddSample(
         static_cast<int>(module_->functions.size()));
     CompilationHelper helper(isolate_, module_);
-    if (!FLAG_trace_wasm_decoder && FLAG_wasm_num_compilation_tasks != 0) {
+    size_t funcs_to_compile =
+        module_->functions.size() - module_->num_imported_functions;
+    if (!FLAG_trace_wasm_decoder && FLAG_wasm_num_compilation_tasks != 0 &&
+        funcs_to_compile > 1) {
       // Avoid a race condition by collecting results into a second vector.
       std::vector<Handle<Code>> results(temp_instance.function_code);
       helper.CompileInParallel(&module_env, results, thrower);