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25 
26 #include "memory/metaspace.hpp"
27 #include "oops/compressedKlass.hpp"
28 #include "utilities/globalDefinitions.hpp"
29 
30 char* CompressedKlassPointers::reserve_address_space_for_compressed_classes(size_t size, bool aslr, bool optimize_for_zero_base) {
31 
32   char* result = nullptr;
33 
34   assert(CompressedKlassPointers::narrow_klass_pointer_bits() == 32 ||
35          CompressedKlassPointers::narrow_klass_pointer_bits() == 19, "Rethink if we ever use different nKlass bit sizes");
36 
37   // Unconditionally attempting to reserve in lower 4G first makes always sense:
38   // -CDS -COH: Try to get unscaled mode (zero base, zero shift)
39   // +CDS -COH: No zero base possible (CDS prevents it); but we still benefit from small base pointers (imm32 movabs)
40   // -CDS +COH: No zero base possible (22bit nKlass + zero base zero shift = 4MB encoding range, way too small);
41   //            but we still benefit from small base pointers (imm32 movabs)
42   // +CDS +COH: No zero base possible for multiple reasons (CDS prevents it and encoding range too small);
43   //            but we still benefit from small base pointers (imm32 movabs)
44 
45   result = reserve_address_space_below_4G(size, aslr);
46 
47   if (result == nullptr && optimize_for_zero_base) {
48     // Failing that, if we are running without CDS, attempt to allocate below 32G.
49     // This allows us to use zero-based encoding with a non-zero shift.
50     result = reserve_address_space_for_zerobased_encoding(size, aslr);
51   }
52 
53   return result;
54 }