buffer: improve concat() performance

benchmark/buffers/buffer-concat-fill.js

@@ -0,0 +1,23 @@
+'use strict';
+const common = require('../common.js');
+
+const bench = common.createBenchmark(main, {
+  extraSize: [1, 256, 4 * 256],
+  n: [8e5]
+});
+
+function main({ n, extraSize }) {
+  const pieces = 4;
+  const pieceSize = 256;
+
+  const list = Array.from({ length: pieces })
+    .fill(Buffer.allocUnsafe(pieceSize));
+
+  const totalLength = (pieces * pieceSize) + extraSize;
+
+  bench.start();
+  for (let i = 0; i < n; i++) {
+    Buffer.concat(list, totalLength);
+  }
+  bench.end(n);
+}

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;
 
@@ -568,16 +572,15 @@ 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
   if (pos < 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;

test/benchmark/test-benchmark-buffer.js

@@ -15,6 +15,7 @@ runBenchmark('buffers',
                'difflen=false',
                'encoding=utf8',
                'endian=BE',
+               'extraSize=1',
                'len=256',
                'linesCount=1',
                'method=',