buffer: improve allocation performance

assertSize() is adjusted to be inlineable according to V8's default
function size limits when determining inlineability. This results in
up to 11% performance gains when allocating any kind of Buffer.

Avoid avoids use of in, resulting in ~50% improvement when creating
a Buffer from an array-like object.

PR-URL: #10443
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

benchmark/buffers/buffer-from.js

@@ -10,7 +10,8 @@ const bench = common.createBenchmark(main, {
     'buffer',
     'uint8array',
     'string',
-    'string-base64'
+    'string-base64',
+    'object'
   ],
   len: [10, 2048],
   n: [1024]
@@ -25,6 +26,7 @@ function main(conf) {
   const str = 'a'.repeat(len);
   const buffer = Buffer.allocUnsafe(len);
   const uint8array = new Uint8Array(len);
+  const obj = { length: null }; // Results in a new, empty Buffer
 
   var i;
 
@@ -80,6 +82,13 @@ function main(conf) {
       }
       bench.end(n);
       break;
+    case 'object':
+      bench.start();
+      for (i = 0; i < n * 1024; i++) {
+        Buffer.from(obj);
+      }
+      bench.end(n);
+      break;
     default:
       assert.fail(null, null, 'Should not get here');
   }

lib/buffer.js

@@ -109,6 +109,9 @@ Buffer.from = function(value, encodingOrOffset, length) {
 
 Object.setPrototypeOf(Buffer, Uint8Array);
 
+// The 'assertSize' method will remove itself from the callstack when an error
+// occurs. This is done simply to keep the internal details of the
+// implementation from bleeding out to users.
 function assertSize(size) {
   let err = null;
 
@@ -121,9 +124,6 @@ function assertSize(size) {
                          'than ' + binding.kMaxLength);
 
   if (err) {
-    // The following hides the 'assertSize' method from the
-    // callstack. This is done simply to hide the internal
-    // details of the implementation from bleeding out to users.
     Error.captureStackTrace(err, assertSize);
     throw err;
   }
@@ -263,7 +263,7 @@ function fromObject(obj) {
   }
 
   if (obj) {
-    if ('length' in obj || isArrayBuffer(obj.buffer) ||
+    if (obj.length !== undefined || isArrayBuffer(obj.buffer) ||
         isSharedArrayBuffer(obj.buffer)) {
       if (typeof obj.length !== 'number' || obj.length !== obj.length) {
         return new FastBuffer();