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 {
IndexBitsIndexBssMappingEntry41   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 
IndexMaskIndexBssMappingEntry46   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 
GetIndexIndexBssMappingEntry53   uint32_t GetIndex(size_t index_bits) const {
54     return index_and_mask & IndexMask(index_bits);
55   }
56 
GetMaskIndexBssMappingEntry57   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