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Message-ID: <023b01cc5387$79b09dd0$6d11d970$@systemfabricworks.com> Date: Fri, 5 Aug 2011 10:51:00 -0500 From: "Bob Pearson" <rpearson@...temfabricworks.com> To: "'Joakim Tjernlund'" <joakim.tjernlund@...nsmode.se> Cc: "'Andrew Morton'" <akpm@...ux-foundation.org>, "'frank zago'" <fzago@...temfabricworks.com>, <linux-kernel@...r.kernel.org> Subject: RE: [PATCH] add slice by 8 algorithm to crc32.c > > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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