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Re^6: The 10**21 Problem (Part 3)

by eyepopslikeamosquito (Chancellor)
on May 17, 2014 at 23:13 UTC ( #1086471=note: print w/replies, xml ) Need Help??

in reply to Re^5: The 10**21 Problem (Part 3)
in thread The 10**21 Problem (Part 3)

There's no need to align the prefetch pointer.
Whoops, yes you are right. I made a silly mistake in my original test; with that blunder fixed, with your version, there is no difference in running speed (both run in 38 seconds).

Curiously, my version runs in 37 seconds with:

_mm_prefetch(&bytevecM[(unsigned int)m7 & 0xffffff80], _MM_HINT_T0); _mm_prefetch(&bytevecM[64+((unsigned int)m7 & 0xffffff80)], _MM_HINT_T +0);
versus 40 seconds with:
_mm_prefetch(&bytevecM[(unsigned int)m7], _MM_HINT_T0); _mm_prefetch(&bytevecM[(unsigned int)m7 ^ 64], _MM_HINT_T0);
I have no explanation for that, unless perhaps the prefetcher likes to prefetch in order (?).

Thanks for the other tips. Optimizing for the prefetcher seems to be something of a dark art -- if you know of any cool links on that, please let me know.

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