move 32 bit heap hint on sunos

src/platform-posix.cc

Index: src/platform-posix.cc
diff --git a/src/platform-posix.cc b/src/platform-posix.cc
index 9d3d7695f0955b1f84d0b0310dda815909aec726..3aa8587cf4b042b48b3dafd4be1f27e989d7b5c2 100644
--- a/src/platform-posix.cc
+++ b/src/platform-posix.cc
@@ -123,11 +123,26 @@ void* OS::GetRandomMmapAddr() {
     raw_addr &= V8_UINT64_C(0x3ffffffff000);
 #else
     uint32_t raw_addr = V8::RandomPrivate(isolate);
+
+    raw_addr &= 0x3ffff000;
+
+# ifdef __sun
+    // For our Solaris/illumos mmap hint, we pick a random address in the bottom
+    // half of the top half of the address space (that is, the third quarter).
+    // Because we do not MAP_FIXED, this will be treated only as a hint -- the
+    // system will not fail to mmap() because something else happens to already
+    // be mapped at our random address. We deliberately set the hint high enough
+    // to get well above the system's break (that is, the heap); Solaris and
+    // illumos will try the hint and if that fails allocate as if there were
+    // no hint at all. The high hint prevents the break from getting hemmed in
+    // at low values, ceding half of the address space to the system heap.
+    raw_addr += 0x80000000;
+# else
     // The range 0x20000000 - 0x60000000 is relatively unpopulated across a
     // variety of ASLR modes (PAE kernel, NX compat mode, etc) and on macos
     // 10.6 and 10.7.
-    raw_addr &= 0x3ffff000;
     raw_addr += 0x20000000;
+# endif
 #endif
     return reinterpret_cast<void*>(raw_addr);
   }