common/ringbuffer.h

   1 #ifndef RINGBUFFER_H
   2 #define RINGBUFFER_H
   3
   4 #include <atomic>
   5 #include <cassert>
   6 #include <cstddef>
   7 #include <memory>
   8 #include <new>
   9 #include <utility>
  10
  11 #include "almalloc.h"
  12 #include "flexarray.h"
  13
  14
  15 /* NOTE: This lockless ringbuffer implementation is copied from JACK, extended
  16  * to include an element size. Consequently, parameters and return values for a
  17  * size or count are in 'elements', not bytes. Additionally, it only supports
  18  * single-consumer/single-provider operation.
  19  */
  20
  21 struct RingBuffer {
  22 private:
  23 #if defined(__cpp_lib_hardware_interference_size) && !defined(_LIBCPP_VERSION)
  24     static constexpr std::size_t sCacheAlignment{std::hardware_destructive_interference_size};
  25 #else
  26     /* Assume a 64-byte cache line, the most common/likely value. */
  27     static constexpr std::size_t sCacheAlignment{64};
  28 #endif
  29     alignas(sCacheAlignment) std::atomic<std::size_t> mWriteCount{0u};
  30     alignas(sCacheAlignment) std::atomic<std::size_t> mReadCount{0u};
  31
  32     alignas(sCacheAlignment) const std::size_t mWriteSize;
  33     const std::size_t mSizeMask;
  34     const std::size_t mElemSize;
  35
  36     al::FlexArray<std::byte, 16> mBuffer;
  37
  38 public:
  39     struct Data {
  40         std::byte *buf;
  41         std::size_t len;
  42     };
  43     using DataPair = std::pair<Data,Data>;
  44
  45     RingBuffer(const std::size_t writesize, const std::size_t mask, const std::size_t elemsize,
  46         const std::size_t numbytes)
  47         : mWriteSize{writesize}, mSizeMask{mask}, mElemSize{elemsize}, mBuffer{numbytes}
  48     { }
  49
  50     /** Reset the read and write pointers to zero. This is not thread safe. */
  51     auto reset() noexcept -> void;
  52
  53     /**
  54      * Return the number of elements available for reading. This is the number
  55      * of elements in front of the read pointer and behind the write pointer.
  56      */
  57     [[nodiscard]] auto readSpace() const noexcept -> std::size_t
  58     {
  59         const std::size_t w{mWriteCount.load(std::memory_order_acquire)};
  60         const std::size_t r{mReadCount.load(std::memory_order_acquire)};
  61         /* mWriteCount is never more than mWriteSize greater than mReadCount. */
  62         return w - r;
  63     }
  64
  65     /**
  66      * The copying data reader. Copy at most `count' elements into `dest'.
  67      * Returns the actual number of elements copied.
  68      */
  69     [[nodiscard]] auto read(void *dest, std::size_t count) noexcept -> std::size_t;
  70     /**
  71      * The copying data reader w/o read pointer advance. Copy at most `count'
  72      * elements into `dest'. Returns the actual number of elements copied.
  73      */
  74     [[nodiscard]] auto peek(void *dest, std::size_t count) const noexcept -> std::size_t;
  75
  76     /**
  77      * The non-copying data reader. Returns two ringbuffer data pointers that
  78      * hold the current readable data. If the readable data is in one segment
  79      * the second segment has zero length.
  80      */
  81     [[nodiscard]] auto getReadVector() noexcept -> DataPair;
  82     /** Advance the read pointer `count' places. */
  83     auto readAdvance(std::size_t count) noexcept -> void
  84     {
  85         const std::size_t w{mWriteCount.load(std::memory_order_acquire)};
  86         const std::size_t r{mReadCount.load(std::memory_order_relaxed)};
  87         [[maybe_unused]] const std::size_t readable{w - r};
  88         assert(readable >= count);
  89         mReadCount.store(r+count, std::memory_order_release);
  90     }
  91
  92
  93     /**
  94      * Return the number of elements available for writing. This is the total
  95      * number of writable elements excluding what's readable (already written).
  96      */
  97     [[nodiscard]] auto writeSpace() const noexcept -> std::size_t
  98     { return mWriteSize - readSpace(); }
  99
 100     /**
 101      * The copying data writer. Copy at most `count' elements from `src'. Returns
 102      * the actual number of elements copied.
 103      */
 104     [[nodiscard]] auto write(const void *src, std::size_t count) noexcept -> std::size_t;
 105
 106     /**
 107      * The non-copying data writer. Returns two ringbuffer data pointers that
 108      * hold the current writeable data. If the writeable data is in one segment
 109      * the second segment has zero length.
 110      */
 111     [[nodiscard]] auto getWriteVector() noexcept -> DataPair;
 112     /** Advance the write pointer `count' places. */
 113     auto writeAdvance(std::size_t count) noexcept -> void
 114     {
 115         const std::size_t w{mWriteCount.load(std::memory_order_relaxed)};
 116         const std::size_t r{mReadCount.load(std::memory_order_acquire)};
 117         [[maybe_unused]] const std::size_t writable{mWriteSize - (w - r)};
 118         assert(writable >= count);
 119         mWriteCount.store(w+count, std::memory_order_release);
 120     }
 121
 122     [[nodiscard]] auto getElemSize() const noexcept -> std::size_t { return mElemSize; }
 123
 124     /**
 125      * Create a new ringbuffer to hold at least `sz' elements of `elem_sz'
 126      * bytes. The number of elements is rounded up to a power of two. If
 127      * `limit_writes' is true, the writable space will be limited to `sz'
 128      * elements regardless of the rounded size.
 129      */
 130     [[nodiscard]] static
 131     auto Create(std::size_t sz, std::size_t elem_sz, bool limit_writes) -> std::unique_ptr<RingBuffer>;
 132
 133     DEF_FAM_NEWDEL(RingBuffer, mBuffer)
 134 };
 135 using RingBufferPtr = std::unique_ptr<RingBuffer>;
 136
 137 #endif /* RINGBUFFER_H */