1. 16 Dec, 2016 1 commit
    • Jim Bankoski's avatar
      vp8 : use threading mutex's for tsan only. · 318a1ff5
      Jim Bankoski authored
      To avoid decode performance hit of 2% when running on hyperthreaded
      cores.
      
      This patch only uses the mutex's when we are running tsan.
      
      This is safe because 32 bit operations like read and store are atomic
      on all the platforms we care about. Tsan warns about race situations,
      but in this case either situation ( read occurs before write or write
      before read) the worst case is that we go around one extra time in the
      loop.  So the ordering doesn't really matter.
      
      That said a few other things have been tried :
      
      for instance as per here:
      webrtc/base/atomicops.h#52
      
      In this patch they use:
      __atomic_load_n(i, __ATOMIC_ACQUIRE);
      __atomic_store_n(i, value, __ATOMIC_RELEASE);
      
      This code works on gcc, clang ( replacing protected write and read), and
      avoids tsan errors. Incurring no penalty in performance.  In C11 its
      replaced by straight atomic operands.
      
      However there is no equivalent in the visual studio's we support as
      int32 on all windows platforms is already atomic.  To avoid tsan like
      warnings on windows we'd need to use interlocked exchange and the
      end result doesn't gain us any thing.
      
      Change-Id: I2066e3c7f42641ebb23d53feb1f16f23f85bcf59
      318a1ff5
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