C++ How to force prefetch data to cache? (array loop)

c++ cpu-cache prefetch
lizaczek picture lizaczek · Jan 9, 2013 · Viewed 18.3k times · Source

I have loop like this

start = __rdtsc();
unsigned long long count = 0;
for(int i = 0; i < N; i++)
    for(int j = 0; j < M; j++)
        count += tab[i][j];
stop = __rdtsc();
time = (stop - start) * 1/3;

Need to check how prefetch data influences on efficiency. How to force prefetch some values from memory into cache before they will be counted?

Answer

Leonid Volnitsky picture Leonid Volnitsky · Jan 9, 2013

For GCC only:

__builtin_prefetch((const void*)(prefetch_address),0,0);

prefetch_address can be invalid, there will be no segfault. If there too small difference between prefetch_address and current location, there might be no effect or even slowdown. Try to set it at least 1k ahead.

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