LibJS: Use non-copying GetUint8ArrayBytes() in Uint8Array.toBase64()

If the ArrayBuffer we're looking at is non-shared, we can simply get a
view onto the underlying bytes and pass that to the Base64 encoder.

Author: awesomekling

Libraries/LibJS/Runtime/Uint8Array.cpp

@@ -333,22 +333,33 @@ JS_DEFINE_NATIVE_FUNCTION(Uint8ArrayPrototypeHelpers::to_base64)
     }
 
     // 8. Let toEncode be ? GetUint8ArrayBytes(O).
-    auto to_encode = TRY(get_uint8_array_bytes(vm, typed_array));
-
     String out_ascii;
 
-    // 9. If alphabet is "base64", then
-    if (alphabet == Alphabet::Base64) {
-        // a. Let outAscii be the sequence of code points which results from encoding toEncode according to the base64
-        //    encoding specified in section 4 of RFC 4648. Padding is included if and only if omitPadding is false.
-        out_ascii = MUST(encode_base64(to_encode, omit_padding));
-    }
-    // 10. Else,
-    else {
-        // a. Assert: alphabet is "base64url".
-        // b. Let outAscii be the sequence of code points which results from encoding toEncode according to the base64url
-        //    encoding specified in section 5 of RFC 4648. Padding is included if and only if omitPadding is false.
-        out_ascii = MUST(encode_base64url(to_encode, omit_padding));
+    // OPTIMIZATION: If the ArrayBuffer is not shared, we can avoid copying the bytes.
+    if (!typed_array->viewed_array_buffer()->is_shared_array_buffer()
+        && !typed_array->viewed_array_buffer()->is_detached()) {
+        auto to_encode = TRY(get_uint8_array_bytes_view(vm, typed_array));
+        if (alphabet == Alphabet::Base64) {
+            out_ascii = MUST(encode_base64(to_encode, omit_padding));
+        } else {
+            out_ascii = MUST(encode_base64url(to_encode, omit_padding));
+        }
+    } else {
+        auto to_encode = TRY(get_uint8_array_bytes(vm, typed_array));
+
+        // 9. If alphabet is "base64", then
+        if (alphabet == Alphabet::Base64) {
+            // a. Let outAscii be the sequence of code points which results from encoding toEncode according to the base64
+            //    encoding specified in section 4 of RFC 4648. Padding is included if and only if omitPadding is false.
+            out_ascii = MUST(encode_base64(to_encode, omit_padding));
+        }
+        // 10. Else,
+        else {
+            // a. Assert: alphabet is "base64url".
+            // b. Let outAscii be the sequence of code points which results from encoding toEncode according to the base64url
+            //    encoding specified in section 5 of RFC 4648. Padding is included if and only if omitPadding is false.
+            out_ascii = MUST(encode_base64url(to_encode, omit_padding));
+        }
     }
 
     // 11. Return CodePointsToString(outAscii).
@@ -438,6 +449,26 @@ ThrowCompletionOr<ByteBuffer> get_uint8_array_bytes(VM& vm, TypedArrayBase const
     return bytes;
 }
 
+// 23.3.3.2 GetUint8ArrayBytes ( ta ), https://tc39.es/ecma262/#sec-getuint8arraybytes
+// NOTE: This is an optimized version that returns a view into the underlying buffer when possible.
+//       It's only safe to use when the ArrayBuffer is known to not be shared or detached.
+ThrowCompletionOr<ReadonlyBytes> get_uint8_array_bytes_view(VM& vm, TypedArrayBase const& typed_array)
+{
+    VERIFY(typed_array.kind() == TypedArrayBase::Kind::Uint8Array);
+    VERIFY(!typed_array.viewed_array_buffer()->is_shared_array_buffer());
+    VERIFY(!typed_array.viewed_array_buffer()->is_detached());
+    auto typed_array_record = make_typed_array_with_buffer_witness_record(typed_array, ArrayBuffer::Order::SeqCst);
+    if (is_typed_array_out_of_bounds(typed_array_record))
+        return vm.throw_completion<TypeError>(ErrorType::BufferOutOfBounds, "TypedArray"sv);
+    auto length = typed_array_length(typed_array_record);
+    auto byte_offset = typed_array.byte_offset();
+
+    return ReadonlyBytes {
+        typed_array.viewed_array_buffer()->buffer().data() + byte_offset,
+        length
+    };
+}
+
 // 23.3.3.3 SetUint8ArrayBytes ( into, bytes ), https://tc39.es/ecma262/#sec-setuint8arraybytes
 void set_uint8_array_bytes(TypedArrayBase& into, ReadonlyBytes bytes)
 {

Libraries/LibJS/Runtime/Uint8Array.h

@@ -50,6 +50,7 @@ struct DecodeResult {
 
 ThrowCompletionOr<GC::Ref<TypedArrayBase>> validate_uint8_array(VM&);
 ThrowCompletionOr<ByteBuffer> get_uint8_array_bytes(VM&, TypedArrayBase const&);
+ThrowCompletionOr<ReadonlyBytes> get_uint8_array_bytes_view(VM&, TypedArrayBase const&);
 void set_uint8_array_bytes(TypedArrayBase&, ReadonlyBytes);
 DecodeResult from_base64(VM&, StringView string, Alphabet alphabet, AK::LastChunkHandling last_chunk_handling, Optional<size_t> max_length = {});
 DecodeResult from_hex(VM&, StringView string, Optional<size_t> max_length = {});