mpmc-queue/mpmc-queue.h

   1 #include <stdatomic.h>
   2 #include <unrelacy.h>
   3
   4 template <typename t_element, size_t t_size>
   5 struct mpmc_boundq_1_alt
   6 {
   7 private:
   8
   9         // elements should generally be cache-line-size padded :
  10         t_element               m_array[t_size];
  11
  12         // rdwr counts the reads & writes that have started
  13         atomic<unsigned int>    m_rdwr;
  14         // "read" and "written" count the number completed
  15         atomic<unsigned int>    m_read;
  16         atomic<unsigned int>    m_written;
  17
  18 public:
  19
  20         mpmc_boundq_1_alt()
  21         {
  22                 m_rdwr = 0;
  23                 m_read = 0;
  24                 m_written = 0;
  25         }
  26
  27         //-----------------------------------------------------
  28
  29         t_element * read_fetch() {
  30                 unsigned int rdwr = m_rdwr.load(mo_acquire);
  31                 unsigned int rd,wr;
  32                 for(;;) {
  33                         rd = (rdwr>>16) & 0xFFFF;
  34                         wr = rdwr & 0xFFFF;
  35
  36                         if ( wr == rd ) // empty
  37                                 return false;
  38
  39                         if ( m_rdwr.compare_exchange_weak(rdwr,rdwr+(1<<16),mo_acq_rel) )
  40                                 break;
  41                 }
  42
  43                 // (*1)
  44                 rl::backoff bo;
  45                 while ( (m_written.load(mo_acquire) & 0xFFFF) != wr ) {
  46                         bo.yield();
  47                 }
  48
  49                 t_element * p = & ( m_array[ rd % t_size ] );
  50
  51                 return p;
  52         }
  53
  54         void read_consume() {
  55                 m_read.fetch_add(1,mo_release);
  56         }
  57
  58         //-----------------------------------------------------
  59
  60         t_element * write_prepare() {
  61                 unsigned int rdwr = m_rdwr.load(mo_acquire);
  62                 unsigned int rd,wr;
  63                 for(;;) {
  64                         rd = (rdwr>>16) & 0xFFFF;
  65                         wr = rdwr & 0xFFFF;
  66
  67                         if ( wr == ((rd + t_size)&0xFFFF) ) // full
  68                                 return NULL;
  69
  70                         if ( m_rdwr.compare_exchange_weak(rdwr,(rd<<16) | ((wr+1)&0xFFFF),mo_acq_rel) )
  71                                 break;
  72                 }
  73
  74                 // (*1)
  75                 rl::backoff bo;
  76                 while ( (m_read.load(mo_acquire) & 0xFFFF) != rd ) {
  77                         bo.yield();
  78                 }
  79
  80                 t_element * p = & ( m_array[ wr % t_size ] );
  81
  82                 return p;
  83         }
  84
  85         void write_publish()
  86         {
  87                 m_written.fetch_add(1,mo_release);
  88         }
  89
  90         //-----------------------------------------------------
  91
  92
  93 };