prefetch.h

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// This file's extension implies that it's C, but it's really -*- C++ -*-.
 
/*
  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
*/
 
// $Id$
#ifndef CXXUTILS_PREFETCH_H
#define CXXUTILS_PREFETCH_H
 
 
#include <cstdlib>
 
 
// Macro implementing prefetch on an address.
// Set to a no-op if we don't know how to implement it for this
// compiler / architecture.
// One can also predefine CXXUTILS_PREFETCH_ADDRESS for testing purposes.
#ifndef CXXUTILS_PREFETCH_ADDRESS
# ifdef __GNUC__
// gcc provides special builtin
#  define CXXUTILS_PREFETCH_ADDRESS(ADDR)  __builtin_prefetch(ADDR)
# else
#  define CXXUTILS_PREFETCH_ADDRESS(ADDR) do {} while(0) // no-op
# endif
#endif
 
 
namespace CxxUtils {
 
 
enum { CacheLineSize = 64 };
 
 
inline
void prefetchOne (const void* address)
{
  CXXUTILS_PREFETCH_ADDRESS(address);
}
 
 
template <size_t N>
inline
void prefetchN (const void* ptr)
{
  const char* pp = reinterpret_cast<const char*> (ptr);
 
  // The compiler will unroll this loop for small N.
  for (size_t i = 0; i < N; i += CacheLineSize)
    prefetchOne (pp + i);
 
  // Issue a prefetch for the last byte as well, in case it crosses
  // cache lines.
  prefetchOne (pp + N - 1);
}
 
 
template <typename T>
inline
void prefetchObj(const T* ptr)
{
  prefetchN<sizeof(T)> (ptr);
}
 
template <typename Iter>
inline
void prefetchNext (Iter iter, Iter endIter)
{
  if (++iter != endIter)
    prefetchObj(*iter);
}
 
 
template <typename Iter>
inline
void prefetchTwo (Iter iter, Iter endIter)
{
  if (iter != endIter) {
    prefetchObj(*iter);
    if (++iter != endIter) {
      prefetchObj(*iter);
    }
  }
}
 
 
} // namespace CxxUtils
 
 
#endif // not CXXUTILS_PREFETCH_H