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

base/rand_util_posix.cc

Index: base/rand_util_posix.cc
diff --git a/base/rand_util_posix.cc b/base/rand_util_posix.cc
index 082d64923d5b9bf63890565bcf922eb361cc3ab3..0a72a20d6420961e75aabf82f06fff41ca5e1dc1 100644
--- a/base/rand_util_posix.cc
+++ b/base/rand_util_posix.cc
@@ -20,19 +20,16 @@ namespace {
 // we can use LazyInstance to handle opening it on the first access.
 class URandomFd {
  public:
-  URandomFd() {
-    fd_ = open("/dev/urandom", O_RDONLY);
+  URandomFd() : fd_(open("/dev/urandom", O_RDONLY)) {
     DCHECK_GE(fd_, 0) << "Cannot open /dev/urandom: " << errno;
   }
 
-  ~URandomFd() {
-    close(fd_);
-  }
+  ~URandomFd() { close(fd_); }
 
   int fd() const { return fd_; }
 
  private:
-  int fd_;
+  const int fd_;
 };
 
 base::LazyInstance<URandomFd>::Leaky g_urandom_fd = LAZY_INSTANCE_INITIALIZER;
@@ -44,13 +41,15 @@ namespace base {
 // NOTE: This function must be cryptographically secure. http://crbug.com/140076
 uint64 RandUint64() {
   uint64 number;
+  RandBytes(&number, sizeof(number));
+  return number;
+}
 
-  int urandom_fd = g_urandom_fd.Pointer()->fd();
-  bool success = ReadFromFD(urandom_fd, reinterpret_cast<char*>(&number),
-                            sizeof(number));
+void RandBytes(void* output, size_t output_length) {
+  const int urandom_fd = g_urandom_fd.Pointer()->fd();
+  const bool success =
+      ReadFromFD(urandom_fd, static_cast<char*>(output), output_length);
   CHECK(success);
-
-  return number;
 }
 
 int GetUrandomFD(void) {