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 40 matching lines @@
 #elif OS(WIN)
 
 #include <windows.h>
 
 #else
 #error Unknown OS
 #endif // OS(POSIX)
 
 namespace WTF {
 
+#if OS(WIN)
+
+static bool shouldUseAddressHint()
+{
+#if !CPU(X86_64)

[Chris Evans, 2014/07/21 18:02:53]

Nit: probably clearer with the new-ish:
#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.
+    static BOOL bIsWow64 = -1;
+    if (bIsWow64 == -1) {
+        IsWow64Process(GetCurrentProcess(), &bIsWow64);
+    }
+    return !!bIsWow64;
+#else // !CPU(X86_64)
+    return true;
+#endif // !CPU(X86_64)
+}
+
+#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.
 static void* systemAllocPages(void* addr, size_t len)
 {
     ASSERT(!(len & kPageAllocationGranularityOffsetMask));
     ASSERT(!(reinterpret_cast<uintptr_t>(addr) & kPageAllocationGranularityOffsetMask));
-    void* ret;
+    void* ret = 0;
 #if OS(WIN)
-    ret = VirtualAlloc(addr, len, MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE);
+    if (shouldUseAddressHint())
+        ret = VirtualAlloc(addr, len, MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE);
     if (!ret)
         ret = VirtualAlloc(0, len, MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE);
 #else
     ret = mmap(addr, len, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
     if (ret == MAP_FAILED)
         ret = 0;
 #endif
     return ret;
 }
 
@@ skipping 135 matching lines @@
     ASSERT(!(len & kSystemPageOffsetMask));
 #if OS(POSIX)
     (void) addr;
 #else
     setSystemPagesAccessible(addr, len);
 #endif
 }
 
 } // namespace WTF