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      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>
     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   BitStructField<SubtypeCheckBits, /*lsb*/ 0> subtype_check_info_;
     72   BitStructField<ClassStatus,
     73                  /*lsb*/ SubtypeCheckBits::BitStructSizeOf(),
     74                  /*width*/ kClassStatusBitSize> status_;
     75   BitStructInt</*lsb*/ 0, /*width*/ BitSizeOf<BitString::StorageType>()> int32_alias_;
     76 BITSTRUCT_DEFINE_END(SubtypeCheckBitsAndStatus);
     77 
     78 // Use the spare alignment from "ClassStatus" to store all the new SubtypeCheckInfo data.
     79 static_assert(sizeof(SubtypeCheckBitsAndStatus) == sizeof(uint32_t),
     80               "All of SubtypeCheckInfo+ClassStatus should fit into 4 bytes");
     81 }  // namespace art
     82 
     83 #endif  // ART_RUNTIME_SUBTYPE_CHECK_BITS_AND_STATUS_H_
     84