iOS: Cache the result of IsRunningOnIOS10OrLater()

base/ios/ios_util.mm

 // Copyright 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/ios/ios_util.h"
 
 #import <Foundation/Foundation.h>
 #include <stddef.h>
 
 #include "base/macros.h"
 #include "base/sys_info.h"
 
 namespace {
 
 // Return a 3 elements array containing the major, minor and bug fix version of
 // the OS.
 const int32_t* OSVersionAsArray() {
   int32_t* digits = new int32_t[3];
   base::SysInfo::OperatingSystemVersionNumbers(
       &digits[0], &digits[1], &digits[2]);
   return digits;
 }
 
+// Caches the result of IsRunningOnIOS10OrLater().

[rohitrao (ping after 24h), 2017/06/14 01:10:13]

We'll be adding IsRunningOnIOS11OrLater() shortly, so it might be worth either
adding this now or considering how we would add it later.  For example, the
ternary operation on line 37 wouldn't work.

+enum { kUnknown, kIos9orEarlier, kIos10orLater } g_ios_version = kUnknown;

[Eugene But (OOO till 7-30), 2017/06/14 00:59:48]

Should we cache the result of OSVersionAsArray instead? Otherwise the issue may
show up again later.

[rohitrao (ping after 24h), 2017/06/14 01:10:13]

Can you clarify what issue might "show up again later"?  Is the problem that
some code calls IsRunningOnOrLater() directly?

[Eugene But (OOO till 7-30), 2017/06/14 01:31:53]

Sorry, I meant IsRunningOnIOS11(12/13/etc..)OrLater may become a bottleneck in
the future in some other place. So if improving performance for
IsRunningOnOrLater is an option, then we should probably do that.

+
 std::string* g_icudtl_path_override = nullptr;
 
 }  // namespace
 
 namespace base {
 namespace ios {
 
 bool IsRunningOnIOS10OrLater() {
-  return IsRunningOnOrLater(10, 0, 0);
+  if (g_ios_version == kUnknown) {

[Eugene But (OOO till 7-30), 2017/06/14 00:59:48]

This makes the function non-thread safe, which you can fix by using
dispatch_once API.

+    g_ios_version =
+        IsRunningOnOrLater(10, 0, 0) ? kIos10orLater : kIos9orEarlier;
+  }
+  return g_ios_version == kIos10orLater;
 }
 
 bool IsRunningOnOrLater(int32_t major, int32_t minor, int32_t bug_fix) {
   static const int32_t* current_version = OSVersionAsArray();
   int32_t version[] = {major, minor, bug_fix};
   for (size_t i = 0; i < arraysize(version); i++) {
     if (current_version[i] != version[i])
       return current_version[i] > version[i];
   }
   return true;
@@ skipping 11 matching lines @@
 
 FilePath FilePathOfEmbeddedICU() {
   if (g_icudtl_path_override) {
     return FilePath(*g_icudtl_path_override);
   }
   return FilePath();
 }
 
 }  // namespace ios
 }  // namespace base