[wasm] Fix write-protection performance for lazy asm.js

Similar to https://crrev.com/c/2912786, this fixes a high number of
page permission switches (incuring mprotect syscall and lock contention
overhead) by pulling a {NativeModuleModificationScope} outside of a
loop (and across a function boundary).

R=clemensb@chromium.org
CC=​​​jkummerow@chromium.org

Cq-Include-Trybots: luci.v8.try:v8_linux64_fyi_rel_ng
Bug: v8:11663, chromium:932033
Change-Id: I2ec47f3eeeb2ab9624d2eaea9b4e776738871c97
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2928504Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Daniel Lehmann <dlehmann@google.com>
Cr-Commit-Position: refs/heads/master@{#74906}

src/wasm/module-compiler.cc

@@ -1427,6 +1427,8 @@ void InitializeCompilationUnits(Isolate* isolate, NativeModule* native_module) {
 
   uint32_t start = module->num_imported_functions;
   uint32_t end = start + module->num_declared_functions;
+  base::Optional<NativeModuleModificationScope>
+      lazy_native_module_modification_scope;
   for (uint32_t func_index = start; func_index < end; func_index++) {
     if (prefer_liftoff) {
       builder.AddRecompilationUnit(func_index, ExecutionTier::kLiftoff);
@@ -1434,14 +1436,21 @@ void InitializeCompilationUnits(Isolate* isolate, NativeModule* native_module) {
     }
     CompileStrategy strategy = GetCompileStrategy(
         module, native_module->enabled_features(), func_index, lazy_module);
-    if (strategy == CompileStrategy::kLazy) {
-      native_module->UseLazyStub(func_index);
-    } else if (strategy == CompileStrategy::kLazyBaselineEagerTopTier) {
-      builder.AddTopTierUnit(func_index);
-      native_module->UseLazyStub(func_index);
-    } else {
-      DCHECK_EQ(strategy, CompileStrategy::kEager);
+    if (strategy == CompileStrategy::kEager) {
       builder.AddUnits(func_index);
+    } else {
+      // Open a single scope for all following calls to {UseLazyStub()}, instead
+      // of flipping page permissions for each {func_index} individually.
+      if (!lazy_native_module_modification_scope.has_value()) {
+        lazy_native_module_modification_scope.emplace(native_module);
+      }
+      if (strategy == CompileStrategy::kLazy) {
+        native_module->UseLazyStub(func_index);
+      } else {
+        DCHECK_EQ(strategy, CompileStrategy::kLazyBaselineEagerTopTier);
+        builder.AddTopTierUnit(func_index);
+        native_module->UseLazyStub(func_index);
+      }
     }
   }
   int num_import_wrappers = AddImportWrapperUnits(native_module, &builder);

src/wasm/wasm-code-manager.h

@@ -974,6 +974,12 @@ class V8_NODISCARD NativeModuleModificationScope final {
   explicit NativeModuleModificationScope(NativeModule* native_module);
   ~NativeModuleModificationScope();
 
+  // Disable copy constructor and copy-assignment operator, since this manages
+  // a resource and implicit copying of the scope can yield surprising errors.
+  NativeModuleModificationScope(const NativeModuleModificationScope&) = delete;
+  NativeModuleModificationScope& operator=(
+      const NativeModuleModificationScope&) = delete;
+
  private:
   NativeModule* native_module_;
 };