src: implement v8 array iteration using the new callback-based API

Using this to iterate over an array can be faster than calling
Array::Get repeatedly. Local experiment shows that this is faster
once the array size is bigger than 2.

PR-URL: #51758
Reviewed-By: Yagiz Nizipli <[email protected]>
Reviewed-By: Chengzhong Wu <[email protected]>

src/util-inl.h

@@ -401,6 +401,29 @@ inline char* UncheckedCalloc(size_t n) { return UncheckedCalloc<char>(n); }
 // headers than we really need to.
 void ThrowErrStringTooLong(v8::Isolate* isolate);
 
+struct ArrayIterationData {
+  std::vector<v8::Global<v8::Value>>* out;
+  v8::Isolate* isolate = nullptr;
+};
+
+inline v8::Array::CallbackResult PushItemToVector(uint32_t index,
+                                                  v8::Local<v8::Value> element,
+                                                  void* data) {
+  auto vec = static_cast<ArrayIterationData*>(data)->out;
+  auto isolate = static_cast<ArrayIterationData*>(data)->isolate;
+  vec->push_back(v8::Global<v8::Value>(isolate, element));
+  return v8::Array::CallbackResult::kContinue;
+}
+
+v8::Maybe<void> FromV8Array(v8::Local<v8::Context> context,
+                            v8::Local<v8::Array> js_array,
+                            std::vector<v8::Global<v8::Value>>* out) {
+  uint32_t count = js_array->Length();
+  out->reserve(count);
+  ArrayIterationData data{out, context->GetIsolate()};
+  return js_array->Iterate(context, PushItemToVector, &data);
+}
+
 v8::MaybeLocal<v8::Value> ToV8Value(v8::Local<v8::Context> context,
                                     std::string_view str,
                                     v8::Isolate* isolate) {

src/util.h

@@ -697,6 +697,16 @@ struct FunctionDeleter {
 template <typename T, void (*function)(T*)>
 using DeleteFnPtr = typename FunctionDeleter<T, function>::Pointer;
 
+// Convert a v8::Array into an std::vector using the callback-based API.
+// This can be faster than calling Array::Get() repeatedly when the array
+// has more than 2 entries.
+// Note that iterating over an array in C++ and performing operations on each
+// element in a C++ loop is still slower than iterating over the array in JS
+// and calling into native in the JS loop repeatedly on each element,
+// as of V8 11.9.
+inline v8::Maybe<void> FromV8Array(v8::Local<v8::Context> context,
+                                   v8::Local<v8::Array> js_array,
+                                   std::vector<v8::Global<v8::Value>>* out);
 std::vector<std::string_view> SplitString(const std::string_view in,
                                           const std::string_view delim);