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