buffer: faster case for create Buffer from new Buffer(0)

When create Buffer from a Buffer will copy data
from old to new even though length is zero.

This patch can improve edge case 4x faster.
following is benchmark results.

new: buffers/buffer_zero.js n=1024: 2463.53891
old: buffers/buffer_zero.js n=1024: 618.70801

PR-URL: https://github.com/nodejs/node/pull/4326
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/benchmark/buffers/buffer_zero.js b/benchmark/buffers/buffer_zero.js
new file mode 100644
index 0000000000..461378758b
--- /dev/null
+++ b/benchmark/buffers/buffer_zero.js
@@ -0,0 +1,18 @@
+'use strict';
+
+const common = require('../common.js');
+
+const bench = common.createBenchmark(main, {
+  n: [1024]
+});
+
+const zero = new Buffer(0);
+
+function main(conf) {
+  var n = +conf.n;
+  bench.start();
+  for (let i = 0; i < n * 1024; i++) {
+    new Buffer(zero);
+  }
+  bench.end(n);
+}