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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_H_
     18 #define ART_RUNTIME_SUBTYPE_CHECK_BITS_H_
     19 
     20 #include "base/bit_string.h"
     21 #include "base/bit_struct.h"
     22 #include "base/bit_utils.h"
     23 
     24 namespace art {
     25 
     26 /**
     27  * The SubtypeCheckBits memory layout (in bits):
     28  *
     29  *   1 bit       Variable
     30  *     |             |
     31  *     v             v     <---- up to 23 bits ---->
     32  *
     33  *  +----+-----------+--------+-------------------------+
     34  *  |    |                  Bitstring                   |
     35  *  +    +-----------+--------+-------------------------+
     36  *  | OF | (unused)  |  Next  |      Path To Root       |
     37  *  +    |           |--------+----+----------+----+----+
     38  *  |    | (0....0)  |        |    |   ...    |    |    |
     39  *  +----+-----------+--------+----+----------+----+----+
     40  * MSB (most significant bit)                          LSB
     41  *
     42  * The bitstring takes up to 23 bits; anything exceeding that is truncated:
     43  * - Path To Root is a list of chars, encoded as a BitString:
     44  *     starting at the root (in LSB), each character is a sibling index unique to the parent,
     45  *   Paths longer than BitString::kCapacity are truncated to fit within the BitString.
     46  * - Next is a single BitStringChar (immediatelly following Path To Root)
     47  *     When new children are assigned paths, they get allocated the parent's Next value.
     48  *     The next value is subsequently incremented.
     49  *
     50  * The exact bit position of (unused) is variable-length:
     51  * In the cases that the "Path To Root" + "Next" does not fill up the entire
     52  * BitString capacity, the remaining bits are (unused) and left as 0s.
     53  *
     54  * There is also an additional "OF" (overflow) field to indicate that the
     55  * PathToRoot has been truncated.
     56  *
     57  * See subtype_check.h and subtype_check_info.h for more details.
     58  */
     59 BITSTRUCT_DEFINE_START(SubtypeCheckBits, /*size*/ BitString::BitStructSizeOf() + 1u)
     60   BitStructField<BitString, /*lsb*/ 0> bitstring_;
     61   BitStructUint</*lsb*/ BitString::BitStructSizeOf(), /*width*/ 1> overflow_;
     62 BITSTRUCT_DEFINE_END(SubtypeCheckBits);
     63 
     64 }  // namespace art
     65 
     66 #endif  // ART_RUNTIME_SUBTYPE_CHECK_BITS_H_
     67