Re-land: 178586 Avoid fragmenting address space on small windows systems.

Source/wtf/PageAllocator.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 51 matching lines @@
 
 static bool shouldUseAddressHint()
 {
 #if CPU(32BIT)
     // When running 32-bit processes under 32-bit Windows, the userspace is
     // limited to 2 GB, and we risk fragmenting it badly if we allow further
     // randomization via our address hint. On the other hand, if the process
     // is running under WOW64, then it has at least 3 GB available (and likely
     // 4 GB depending upon the OS version), and we want use the additional
     // randomness.
-    // TODO(tsepez): presently disabled due to IsWow64Process() compatibility.
-    return true;
+    static BOOL bIsWow64 = -1;
+    if (bIsWow64 == -1) {
+        BOOL result = IsWow64Process(GetCurrentProcess(), &bIsWow64);
+        if (!result) {
+            ASSERT_NOT_REACHED();
+            bIsWow64 = 0;
+        }
+    }
+    return !!bIsWow64;
 #else // CPU(32BIT)
     return true;
 #endif // CPU(32BIT)
 }
 
 #endif // OS(WIN)
 
 // This simple internal function wraps the OS-specific page allocation call so
 // that it behaves consistently: the address is a hint and if it cannot be used,
 // the allocation will be placed elsewhere.
@@ skipping 153 matching lines @@
     ASSERT(!(len & kSystemPageOffsetMask));
 #if OS(POSIX)
     (void) addr;
 #else
     setSystemPagesAccessible(addr, len);
 #endif
 }
 
 } // namespace WTF