Use the full 47 bits of address space on Windows >= 8.1.

base/allocator/partition_allocator/address_space_randomization.cc

Index: base/allocator/partition_allocator/address_space_randomization.cc
diff --git a/base/allocator/partition_allocator/address_space_randomization.cc b/base/allocator/partition_allocator/address_space_randomization.cc
index d710e006bfdc686d9df3ab17befcbcde38f9b5a8..55ccf987c4e33765bc4a66d030a9081e81dc10c4 100644
--- a/base/allocator/partition_allocator/address_space_randomization.cc
+++ b/base/allocator/partition_allocator/address_space_randomization.cc
@@ -10,6 +10,7 @@
 
 #if defined(OS_WIN)
 #include <windows.h>
+#include <VersionHelpers.h>
 #else
 #include <sys/time.h>
 #include <unistd.h>
@@ -89,11 +90,17 @@ void* GetRandomPageBase() {
 // This address mask gives a low likelihood of address space collisions. We
 // handle the situation gracefully if there is a collision.
 #if defined(OS_WIN)
-  // 64-bit Windows has a bizarrely small 8TB user address space. Allocates in
-  // the 1-5TB region. TODO(palmer): See if Windows >= 8.1 has the full 47 bits,
-  // and use it if so. crbug.com/672219
   random &= 0x3ffffffffffUL;
-  random += 0x10000000000UL;
+  // Windows >= 8.1 has the full 47 bits. Use them where available.
+  static bool windows_81 = false;
+  static bool windows_81_initialized = false;
+  if (!windows_81_initialized) {
+    windows_81 = IsWindows8Point1OrGreater();

[Will Harris, 2017/03/28 16:42:16]

can this code use base::win::GetVersion - by convention we prefer to use these
base apis rather than the windows ones.

Also, it takes care of the lazy initialization.

[palmer, 2017/03/28 19:05:35]

Thanks. Using it now, although I might have to go back to
IsWindows8Point1OrGreater if we end up needing to further reduces PA's base
dependencies.

+    windows_81_initialized = true;
+  }
+  if (!windows_81) {
+    random += 0x10000000000UL;
+  }
 #elif defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
   // This range is copied from the TSan source, but works for all tools.
   random &= 0x007fffffffffUL;