buffer: improve concat() performance

PR-URL: https://github.com/nodejs/node/pull/31522
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

lib/buffer.js

@@ -238,6 +238,10 @@ function _copy(source, target, targetStart, sourceStart, sourceEnd) {
                                sourceStart);
   }
 
+  return _copyActual(source, target, targetStart, sourceStart, sourceEnd);
+}
+
+function _copyActual(source, target, targetStart, sourceStart, sourceEnd) {
   if (sourceEnd - sourceStart > target.length - targetStart)
     sourceEnd = sourceStart + target.length - targetStart;
 
@@ -568,8 +572,7 @@ Buffer.concat = function concat(list, length) {
       throw new ERR_INVALID_ARG_TYPE(
         `list[${i}]`, ['Buffer', 'Uint8Array'], list[i]);
     }
-    _copy(buf, buffer, pos);
-    pos += buf.length;
+    pos += _copyActual(buf, buffer, pos, 0, buf.length);
   }
 
   // Note: `length` is always equal to `buffer.length` at this point
@@ -577,7 +580,7 @@ Buffer.concat = function concat(list, length) {
     // Zero-fill the remaining bytes if the specified `length` was more than
     // the actual total length, i.e. if we have some remaining allocated bytes
     // there were not initialized.
-    buffer.fill(0, pos, length);
+    TypedArrayFill.call(buffer, 0, pos, length);
   }
 
   return buffer;