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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_INDEX_BSS_MAPPING_H_
     18 #define ART_RUNTIME_INDEX_BSS_MAPPING_H_
     19 
     20 #include <android-base/logging.h>
     21 
     22 #include "base/bit_utils.h"
     23 
     24 namespace art {
     25 
     26 template<typename T> class LengthPrefixedArray;
     27 
     28 // IndexBssMappingEntry describes a mapping of one or more indexes to their offsets in the .bss.
     29 // A sorted array of IndexBssMappingEntry is used to describe the mapping of method indexes,
     30 // type indexes or string indexes to offsets of their assigned slots in the .bss.
     31 //
     32 // The highest index and a mask are stored in a single `uint32_t index_and_mask` and the split
     33 // between the index and the mask is provided externally. The "mask" bits specify whether some
     34 // of the previous indexes are mapped to immediately preceding slots. This is permissible only
     35 // if the slots are consecutive and in the same order as indexes.
     36 //
     37 // The .bss offset of the slot associated with the highest index is stored in plain form as
     38 // `bss_offset`. If the mask specifies any smaller indexes being mapped to immediately
     39 // preceding slots, their offsets are calculated using an externally supplied size of the slot.
     40 struct IndexBssMappingEntry {
     41   static size_t IndexBits(uint32_t number_of_indexes) {
     42     DCHECK_NE(number_of_indexes, 0u);
     43     return MinimumBitsToStore(number_of_indexes - 1u);
     44   }
     45 
     46   static uint32_t IndexMask(size_t index_bits) {
     47     DCHECK_LE(index_bits, 32u);
     48     constexpr uint32_t kAllOnes = static_cast<uint32_t>(-1);
     49     // Handle `index_bits == 32u` explicitly; shifting uint32_t left by 32 is undefined behavior.
     50     return (index_bits == 32u) ? kAllOnes : ~(kAllOnes << index_bits);
     51   }
     52 
     53   uint32_t GetIndex(size_t index_bits) const {
     54     return index_and_mask & IndexMask(index_bits);
     55   }
     56 
     57   uint32_t GetMask(size_t index_bits) const {
     58     DCHECK_LT(index_bits, 32u);  // GetMask() is valid only if there is at least 1 mask bit.
     59     return index_and_mask >> index_bits;
     60   }
     61 
     62   size_t GetBssOffset(size_t index_bits, uint32_t index, size_t slot_size) const;
     63 
     64   uint32_t index_and_mask;
     65   uint32_t bss_offset;
     66 };
     67 
     68 using IndexBssMapping = LengthPrefixedArray<IndexBssMappingEntry>;
     69 
     70 class IndexBssMappingLookup {
     71  public:
     72   static constexpr size_t npos = static_cast<size_t>(-1);
     73 
     74   static size_t GetBssOffset(const IndexBssMapping* mapping,
     75                              uint32_t index,
     76                              uint32_t number_of_indexes,
     77                              size_t slot_size);
     78 };
     79 
     80 }  // namespace art
     81 
     82 #endif  // ART_RUNTIME_INDEX_BSS_MAPPING_H_
     83