improve stringwrapper bytestring unwrap

src/workerd/jsg/BUILD.bazel

@@ -52,7 +52,6 @@ wd_cc_library(
  "web-idl.h",
  "wrappable.h",
  ],
- implementation_deps = ["@simdutf"],
  visibility = ["//visibility:public"],
  deps = [
  ":exception",
@@ -65,6 +64,7 @@ wd_cc_library(
  "//src/workerd/util:thread-scopes",
  "//src/workerd/util:uuid",
  "@capnp-cpp//src/kj",
+ "@simdutf",
  "@workerd-v8//:v8",
  ],
 )

src/workerd/jsg/value.h

@@ -8,6 +8,7 @@
 // Handling of various basic value types: numbers, booleans, strings, optionals, maybes, variants,
 // arrays, buffers, dicts.
 
+#include "simdutf.h"
 #include "util.h"
 #include "web-idl.h"
 #include "wrappable.h"
@@ -468,20 +469,25 @@ class StringWrapper {
  kj::Maybe<v8::Local<v8::Object>> parentObject) {
  // TODO(cleanup): Move to a HeaderStringWrapper in the api directory.
  v8::Local<v8::String> str = check(handle->ToString(context));
- auto result =
- ByteString(KJ_ASSERT_NONNULL(tryUnwrap(context, str, (kj::String*)nullptr, parentObject)));
- if (str->ContainsOnlyOneByte()) {
- auto buf = kj::heapArray<kj::byte>(str->Length() + 1);
- str->WriteOneByte(context->GetIsolate(), buf.begin());
- for (auto b: buf) {
- if (b >= 128) {
- result.warning = ByteString::Warning::CONTAINS_EXTENDED_ASCII;
- break;
- }
+ auto result = ByteString(KJ_ASSERT_NONNULL(
+ tryUnwrap(context, str, static_cast<kj::String*>(nullptr), parentObject)));
+
+ if (!simdutf::validate_ascii(result.begin(), result.size())) {
+ v8::String::ValueView str_view(context->GetIsolate(), str);
+
+ // If storage is one-byte or the string contains only one-byte
+ // characters, we know that it contains extended ASCII characters.
+ //
+ // The order of execution matters, since ContainsOnlyOneByte()
+ // will scan the whole string for two-byte storage.
+ if (str_view.is_one_byte() || str->ContainsOnlyOneByte()) {
+ result.warning = ByteString::Warning::CONTAINS_EXTENDED_ASCII;
+ } else {
+ // Storage is two-bytes and it contains two-byte characters.
+ result.warning = ByteString::Warning::CONTAINS_UNICODE;
  }
- } else {
- result.warning = ByteString::Warning::CONTAINS_UNICODE;
  }
+
  return kj::mv(result);
  }
 };