83 lines
3.0 KiB
C++
83 lines
3.0 KiB
C++
/*
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* Copyright (C) 2017 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#ifndef ART_RUNTIME_INDEX_BSS_MAPPING_H_
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#define ART_RUNTIME_INDEX_BSS_MAPPING_H_
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#include <android-base/logging.h>
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#include "base/bit_utils.h"
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namespace art {
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template<typename T> class LengthPrefixedArray;
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// IndexBssMappingEntry describes a mapping of one or more indexes to their offsets in the .bss.
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// A sorted array of IndexBssMappingEntry is used to describe the mapping of method indexes,
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// type indexes or string indexes to offsets of their assigned slots in the .bss.
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//
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// The highest index and a mask are stored in a single `uint32_t index_and_mask` and the split
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// between the index and the mask is provided externally. The "mask" bits specify whether some
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// of the previous indexes are mapped to immediately preceding slots. This is permissible only
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// if the slots are consecutive and in the same order as indexes.
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//
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// The .bss offset of the slot associated with the highest index is stored in plain form as
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// `bss_offset`. If the mask specifies any smaller indexes being mapped to immediately
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// preceding slots, their offsets are calculated using an externally supplied size of the slot.
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struct IndexBssMappingEntry {
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static size_t IndexBits(uint32_t number_of_indexes) {
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DCHECK_NE(number_of_indexes, 0u);
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return MinimumBitsToStore(number_of_indexes - 1u);
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}
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static uint32_t IndexMask(size_t index_bits) {
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DCHECK_LE(index_bits, 32u);
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constexpr uint32_t kAllOnes = static_cast<uint32_t>(-1);
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// Handle `index_bits == 32u` explicitly; shifting uint32_t left by 32 is undefined behavior.
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return (index_bits == 32u) ? kAllOnes : ~(kAllOnes << index_bits);
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}
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uint32_t GetIndex(size_t index_bits) const {
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return index_and_mask & IndexMask(index_bits);
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}
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uint32_t GetMask(size_t index_bits) const {
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DCHECK_LT(index_bits, 32u); // GetMask() is valid only if there is at least 1 mask bit.
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return index_and_mask >> index_bits;
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}
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size_t GetBssOffset(size_t index_bits, uint32_t index, size_t slot_size) const;
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uint32_t index_and_mask;
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uint32_t bss_offset;
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};
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using IndexBssMapping = LengthPrefixedArray<IndexBssMappingEntry>;
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class IndexBssMappingLookup {
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public:
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static constexpr size_t npos = static_cast<size_t>(-1);
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static size_t GetBssOffset(const IndexBssMapping* mapping,
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uint32_t index,
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uint32_t number_of_indexes,
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size_t slot_size);
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};
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} // namespace art
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#endif // ART_RUNTIME_INDEX_BSS_MAPPING_H_
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