Messages in this thread | | | From | "Bob Pearson" <> | Subject | RE: [PATCH] add slice by 8 algorithm to crc32.c | Date | Fri, 5 Aug 2011 10:51:00 -0500 |
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> > version. While I haven't done the experiment you suggest there is > something > > to the point that the second q computation in the new version can be > moved > > ahead of the table lookups from the first q computation . My guess is that > > the unrolled version will be significantly slower. > > Ah, didn't see that. Don't understand how this works though. > Why do you do two 32 bit loads instead of one 64 bit load? > > >
The two expression trees can be computed in parallel and combined with the final xor. If the compiler/instruction scheduler are smart enough and can process enough instructions per cycle they overlap well and you get some speedup. I did try a 64 bit load on Nehalem but got about 2 cycles per byte which is a little worse than doing two loads and better than the 32 bit version. I'm not really sure why.
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