Improve base::RandBytes() performance by 1.75x-2.10x on POSIX.

base/rand_util_win.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/rand_util.h"
 
 #include <stdlib.h>
 
 #include "base/basictypes.h"
 #include "base/logging.h"
@@ skipping 10 matching lines @@
 
 namespace base {
 
 // NOTE: This function must be cryptographically secure. http://crbug.com/140076
 uint64 RandUint64() {
   uint32 first_half = RandUint32();
   uint32 second_half = RandUint32();
   return (static_cast<uint64>(first_half) << 32) + second_half;
 }
 
+void RandBytes(void* output, size_t output_length) {
+  uint64 random_int;

[bradn, 2014/01/22 21:56:12]

Any reason not to use:
CryptGenRandom ?
http://msdn.microsoft.com/en-us/library/aa379942.aspx

We appear to use it already in a few places:
  crypto/symmetric_key_win.cc
  third_party/tlslite/tlslite/utils/entropy.c

[wtc, 2014/01/22 22:29:09]

There is a better Windows function: RtlGenRandom. Using RtlGenRandom will be
more efficient. But it is fine to use the current code as a starting point.

+  const size_t random_int_size = sizeof(random_int);
+  for (size_t i = 0; i < output_length; i += random_int_size) {
+    random_int = base::RandUint64();
+    size_t copy_count = std::min(output_length - i, random_int_size);

[wtc, 2014/01/22 22:29:09]

|copy_count| can also be 'const'. (I don't do this myself, but since I noticed
you like to use 'const', I thought I should mention it.)

[DaleCurtis, 2014/01/22 22:54:14]

Done.

+    memcpy(((uint8*)output) + i, &random_int, copy_count);
+  }
+}
+
 }  // namespace base