buffer: optimize Buffer#toString()

Break up Buffer#toString() into a fast and slow path.  The fast path
optimizes for zero-length buffers and no-arg method invocation.

The speedup for zero-length buffers is a satisfying 700%.  The no-arg
toString() operation gets faster by about 13% for a one-byte buffer.

This change exploits the fact that most Buffer#toString() calls are
plain no-arg method calls.  Rewriting the method to take no arguments
means a call doesn't go through an ArgumentsAdaptorTrampoline stack
frame in the common case.

PR-URL: nodejs#2027
Reviewed-By: Brian White <[email protected]>
Reviewed-By: Christian Tellnes <[email protected]>
Reviewed-By: Daniel Cousens <[email protected]>
Reviewed-By: Jeremiah Senkpiel <[email protected]>
Reviewed-By: Trevor Norris <[email protected]>

benchmark/buffers/buffer-tostring.js

@@ -0,0 +1,26 @@
+'use strict';
+
+const common = require('../common.js');
+
+const bench = common.createBenchmark(main, {
+  arg: [true, false],
+  len: [0, 1, 64, 1024],
+  n: [1e7]
+});
+
+function main(conf) {
+  const arg = conf.arg;
+  const len = conf.len | 0;
+  const n = conf.n | 0;
+  const buf = Buffer(len).fill(42);
+
+  bench.start();
+  if (arg) {
+    for (var i = 0; i < n; i += 1)
+      buf.toString('utf8');
+  } else {
+    for (var i = 0; i < n; i += 1)
+      buf.toString();
+  }
+  bench.end(n);
+}