From 7f66191805e87de02fec7c43fc14066e2577cb7a Mon Sep 17 00:00:00 2001
From: Mert Can Altin <mertgold60@gmail.com>
Date: Thu, 5 Feb 2026 22:09:04 +0300
Subject: [PATCH] src: use simdutf for one-byte string UTF-8 write in
 stringBytes

---
 src/string_bytes.cc | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/src/string_bytes.cc b/src/string_bytes.cc
index 6341346da07bec..28432a82ad8835 100644
--- a/src/string_bytes.cc
+++ b/src/string_bytes.cc
@@ -266,8 +266,17 @@ size_t StringBytes::Write(Isolate* isolate,
 
     case BUFFER:
     case UTF8:
-      nbytes = str->WriteUtf8V2(
-          isolate, buf, buflen, String::WriteFlags::kReplaceInvalidUtf8);
+      if (input_view.is_one_byte()) {
+        // Use simdutf for one-byte strings instead of V8's WriteUtf8V2.
+        nbytes = simdutf::convert_latin1_to_utf8_safe(
+            reinterpret_cast<const char*>(input_view.data8()),
+            input_view.length(),
+            buf,
+            buflen);
+      } else {
+        nbytes = str->WriteUtf8V2(
+            isolate, buf, buflen, String::WriteFlags::kReplaceInvalidUtf8);
+      }
       break;
 
     case UCS2: {