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

base/rand_util_win.cc

Index: base/rand_util_win.cc
diff --git a/base/rand_util_win.cc b/base/rand_util_win.cc
index 391fe5b9e61d6f48f413b97f3f3f536dd38d5da0..35f2d2e863ab83f84e0c00dffe19872f808fa43a 100644
--- a/base/rand_util_win.cc
+++ b/base/rand_util_win.cc
@@ -28,4 +28,14 @@ uint64 RandUint64() {
   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