1 /*
2 * Copyright (C) 2017 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 #ifndef ART_RUNTIME_SUBTYPE_CHECK_BITS_AND_STATUS_H_
18 #define ART_RUNTIME_SUBTYPE_CHECK_BITS_AND_STATUS_H_
19
20 #include "base/bit_struct.h"
21 #include "base/bit_utils.h"
22 #include "base/casts.h"
23 #include "class_status.h"
24 #include "subtype_check_bits.h"
25
26 namespace art {
27
28 /*
29 * Enables a highly efficient O(1) subtype comparison by storing extra data
30 * in the unused padding bytes of ClassStatus.
31 */
32
33 // TODO: Update BitSizeOf with this version.
34 template <typename T>
NonNumericBitSizeOf()35 static constexpr size_t NonNumericBitSizeOf() {
36 return kBitsPerByte * sizeof(T);
37 }
38
39 /**
40 * MSB (most significant bit) LSB
41 * +---------------+---------------------------------------------------+
42 * | | |
43 * | ClassStatus | SubtypeCheckBits |
44 * | | |
45 * +---------------+---------------------------------------------------+
46 * <-- 4 bits --> <----- 28 bits ----->
47 *
48 * Invariants:
49 *
50 * AddressOf(ClassStatus) == AddressOf(SubtypeCheckBitsAndStatus)
51 * BitSizeOf(SubtypeCheckBitsAndStatus) == 32
52 *
53 * Note that with this representation the "Path To Root" in the MSB of this 32-bit word:
54 * This enables a highly efficient path comparison between any two labels:
55 *
56 * src <: target :=
57 * (src & mask) == (target & mask) where mask := (1u << len(path-to-root(target)) - 1u
58 *
59 * In the above example, the `len()` (and thus `mask`) is a function of the depth.
60 * Since the target is known at compile time, it becomes
61 * (src & #imm_mask) == #imm
62 * or
63 * ((src - #imm) << #imm_shift_to_remove_high_bits) == 0
64 * or a similar expression chosen for the best performance or code size.
65 *
66 * (This requires that path-to-root in `target` is not truncated, i.e. it is in the Assigned state).
67 */
68 static constexpr size_t kClassStatusBitSize = MinimumBitsToStore(enum_cast<>(ClassStatus::kLast));
69 static_assert(kClassStatusBitSize == 4u, "ClassStatus should need 4 bits.");
70 BITSTRUCT_DEFINE_START(SubtypeCheckBitsAndStatus, BitSizeOf<BitString::StorageType>())
71 BITSTRUCT_FIELD(SubtypeCheckBits,
72 /*lsb=*/ 0,
73 /*width=*/ SubtypeCheckBits::BitStructSizeOf()) subtype_check_info_;
74 BITSTRUCT_FIELD(ClassStatus,
75 /*lsb=*/ SubtypeCheckBits::BitStructSizeOf(),
76 /*width=*/ kClassStatusBitSize) status_;
77 BITSTRUCT_INT(/*lsb=*/ 0, /*width=*/ BitSizeOf<BitString::StorageType>()) int32_alias_;
78 BITSTRUCT_DEFINE_END(SubtypeCheckBitsAndStatus);
79
80 // Use the spare alignment from "ClassStatus" to store all the new SubtypeCheckInfo data.
81 static_assert(sizeof(SubtypeCheckBitsAndStatus) == sizeof(uint32_t),
82 "All of SubtypeCheckInfo+ClassStatus should fit into 4 bytes");
83 } // namespace art
84
85 #endif // ART_RUNTIME_SUBTYPE_CHECK_BITS_AND_STATUS_H_
86