1 /*
 2  * Copyright (c) 2023, 2025, Red Hat, Inc. All rights reserved.
 3  * Copyright (c) 2023, 2025, Oracle and/or its affiliates. All rights reserved.
 4  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 5  *
 6  * This code is free software; you can redistribute it and/or modify it
 7  * under the terms of the GNU General Public License version 2 only, as
 8  * published by the Free Software Foundation.
 9  *
10  * This code is distributed in the hope that it will be useful, but WITHOUT
11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
13  * version 2 for more details (a copy is included in the LICENSE file that
14  * accompanied this code).
15  *
16  * You should have received a copy of the GNU General Public License version
17  * 2 along with this work; if not, write to the Free Software Foundation,
18  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19  *
20  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21  * or visit www.oracle.com if you need additional information or have any
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24  */
25 
26 #ifdef _LP64
27 
28 #include "memory/metaspace.hpp"
29 #include "oops/compressedKlass.hpp"
30 #include "utilities/globalDefinitions.hpp"
31 
32 char* CompressedKlassPointers::reserve_address_space_for_compressed_classes(size_t size, bool aslr, bool optimize_for_zero_base) {
33 
34   char* result = nullptr;
35 
36   assert(CompressedKlassPointers::narrow_klass_pointer_bits() == 32 ||
37          CompressedKlassPointers::narrow_klass_pointer_bits() == 22, "Rethink if we ever use different nKlass bit sizes");
38 
39   // Unconditionally attempting to reserve in lower 4G first makes always sense:
40   // -CDS -COH: Try to get unscaled mode (zero base, zero shift)
41   // +CDS -COH: No zero base possible (CDS prevents it); but we still benefit from small base pointers (imm32 movabs)
42   // -CDS +COH: No zero base possible (22bit nKlass + zero base zero shift = 4MB encoding range, way too small);
43   //            but we still benefit from small base pointers (imm32 movabs)
44   // +CDS +COH: No zero base possible for multiple reasons (CDS prevents it and encoding range too small);
45   //            but we still benefit from small base pointers (imm32 movabs)
46 
47   result = reserve_address_space_below_4G(size, aslr);
48 
49   if (result == nullptr && optimize_for_zero_base) {
50     // Failing that, if we are running without CDS, attempt to allocate below 32G.
51     // This allows us to use zero-based encoding with a non-zero shift.
52     result = reserve_address_space_for_zerobased_encoding(size, aslr);
53   }
54 
55   return result;
56 }
57 
58 #endif // _LP64