buffer: even faster atob

PR-URL: #52443
Reviewed-By: Yagiz Nizipli <yagiz.nizipli@sentry.io>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

src/node_buffer.cc

@@ -1286,19 +1286,31 @@ static void Atob(const FunctionCallbackInfo<Value>& args) {
     auto ext = input->GetExternalOneByteStringResource();
     size_t expected_length =
         simdutf::maximal_binary_length_from_base64(ext->data(), ext->length());
-    buffer.AllocateSufficientStorage(expected_length + 1);
-    buffer.SetLengthAndZeroTerminate(expected_length);
+    buffer.AllocateSufficientStorage(expected_length);
+    buffer.SetLength(expected_length);
     result = simdutf::base64_to_binary(
         ext->data(), ext->length(), buffer.out(), simdutf::base64_default);
+  } else if (input->IsOneByte()) {
+    MaybeStackBuffer<uint8_t> stack_buf(input->Length());
+    input->WriteOneByte(args.GetIsolate(),
+                        stack_buf.out(),
+                        0,
+                        input->Length(),
+                        String::NO_NULL_TERMINATION);
+    const char* data = reinterpret_cast<const char*>(*stack_buf);
+    size_t expected_length =
+        simdutf::maximal_binary_length_from_base64(data, input->Length());
+    buffer.AllocateSufficientStorage(expected_length);
+    buffer.SetLength(expected_length);
+    result = simdutf::base64_to_binary(data, input->Length(), buffer.out());
   } else {  // 16-bit case
     String::Value value(env->isolate(), input);
     auto data = reinterpret_cast<const char16_t*>(*value);
     size_t expected_length =
         simdutf::maximal_binary_length_from_base64(data, value.length());
-    buffer.AllocateSufficientStorage(expected_length + 1);
-    buffer.SetLengthAndZeroTerminate(expected_length);
-    result = simdutf::base64_to_binary(
-        data, value.length(), buffer.out(), simdutf::base64_default);
+    buffer.AllocateSufficientStorage(expected_length);
+    buffer.SetLength(expected_length);
+    result = simdutf::base64_to_binary(data, value.length(), buffer.out());
   }
 
   if (result.error == simdutf::error_code::SUCCESS) {