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      1 /*
      2  * Copyright (C) 2015 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_LIBARTBASE_BASE_LENGTH_PREFIXED_ARRAY_H_
     18 #define ART_LIBARTBASE_BASE_LENGTH_PREFIXED_ARRAY_H_
     19 
     20 #include <stddef.h>  // for offsetof()
     21 #include <string.h>  // for memset()
     22 
     23 #include "base/bit_utils.h"
     24 #include "base/casts.h"
     25 #include "base/iteration_range.h"
     26 #include "base/stride_iterator.h"
     27 
     28 namespace art {
     29 
     30 template<typename T>
     31 class LengthPrefixedArray {
     32  public:
     33   explicit LengthPrefixedArray(size_t length)
     34       : size_(dchecked_integral_cast<uint32_t>(length)) {}
     35 
     36   T& At(size_t index, size_t element_size = sizeof(T), size_t alignment = alignof(T)) {
     37     DCHECK_LT(index, size_);
     38     return AtUnchecked(index, element_size, alignment);
     39   }
     40 
     41   const T& At(size_t index, size_t element_size = sizeof(T), size_t alignment = alignof(T)) const {
     42     DCHECK_LT(index, size_);
     43     return AtUnchecked(index, element_size, alignment);
     44   }
     45 
     46   StrideIterator<T> begin(size_t element_size = sizeof(T), size_t alignment = alignof(T)) {
     47     return StrideIterator<T>(&AtUnchecked(0, element_size, alignment), element_size);
     48   }
     49 
     50   StrideIterator<const T> begin(size_t element_size = sizeof(T),
     51                                 size_t alignment = alignof(T)) const {
     52     return StrideIterator<const T>(&AtUnchecked(0, element_size, alignment), element_size);
     53   }
     54 
     55   StrideIterator<T> end(size_t element_size = sizeof(T), size_t alignment = alignof(T)) {
     56     return StrideIterator<T>(&AtUnchecked(size_, element_size, alignment), element_size);
     57   }
     58 
     59   StrideIterator<const T> end(size_t element_size = sizeof(T),
     60                               size_t alignment = alignof(T)) const {
     61     return StrideIterator<const T>(&AtUnchecked(size_, element_size, alignment), element_size);
     62   }
     63 
     64   static size_t OffsetOfElement(size_t index,
     65                                 size_t element_size = sizeof(T),
     66                                 size_t alignment = alignof(T)) {
     67     DCHECK_ALIGNED_PARAM(element_size, alignment);
     68     return RoundUp(offsetof(LengthPrefixedArray<T>, data_), alignment) + index * element_size;
     69   }
     70 
     71   static size_t ComputeSize(size_t num_elements,
     72                             size_t element_size = sizeof(T),
     73                             size_t alignment = alignof(T)) {
     74     size_t result = OffsetOfElement(num_elements, element_size, alignment);
     75     DCHECK_ALIGNED_PARAM(result, alignment);
     76     return result;
     77   }
     78 
     79   size_t size() const {
     80     return size_;
     81   }
     82 
     83   // Update the length but does not reallocate storage.
     84   void SetSize(size_t length) {
     85     size_ = dchecked_integral_cast<uint32_t>(length);
     86   }
     87 
     88   // Clear the potentially uninitialized padding between the size_ and actual data.
     89   void ClearPadding(size_t element_size = sizeof(T), size_t alignment = alignof(T)) {
     90     size_t gap_offset = offsetof(LengthPrefixedArray<T>, data_);
     91     size_t gap_size = OffsetOfElement(0, element_size, alignment) - gap_offset;
     92     memset(reinterpret_cast<uint8_t*>(this) + gap_offset, 0, gap_size);
     93   }
     94 
     95  private:
     96   T& AtUnchecked(size_t index, size_t element_size, size_t alignment) {
     97     return *reinterpret_cast<T*>(
     98         reinterpret_cast<uintptr_t>(this) + OffsetOfElement(index, element_size, alignment));
     99   }
    100 
    101   const T& AtUnchecked(size_t index, size_t element_size, size_t alignment) const {
    102     return *reinterpret_cast<T*>(
    103         reinterpret_cast<uintptr_t>(this) + OffsetOfElement(index, element_size, alignment));
    104   }
    105 
    106   uint32_t size_;
    107   uint8_t data_[0];
    108 };
    109 
    110 // Returns empty iteration range if the array is null.
    111 template<typename T>
    112 IterationRange<StrideIterator<T>> MakeIterationRangeFromLengthPrefixedArray(
    113     LengthPrefixedArray<T>* arr, size_t element_size = sizeof(T), size_t alignment = alignof(T)) {
    114   return arr != nullptr ?
    115       MakeIterationRange(arr->begin(element_size, alignment), arr->end(element_size, alignment)) :
    116       MakeEmptyIterationRange(StrideIterator<T>(nullptr, 0));
    117 }
    118 
    119 }  // namespace art
    120 
    121 #endif  // ART_LIBARTBASE_BASE_LENGTH_PREFIXED_ARRAY_H_
    122