Date: Sun, 8 Jul 2012 10:28:21 +0400 From: Solar Designer <solar@...nwall.com> To: john-dev@...ts.openwall.com Subject: Re: Rotate and bitselect investigation Sayantan, On Sun, Jul 08, 2012 at 10:51:54AM +0530, Sayantan Datta wrote: > I have investigated the rotate and bitselct issue on 7970. Thank you! > Both type of rotate(manual and inbuilt opencl function) use bitalign > instruction. I investigated using rotate(x,(uint)30) and ((x << 30) | ((x > ) >> 2)). Also the values loaded in the bitalign instructions are exactly > same except they operate on different registers. So you won't see any > performance increase in this case. Sounds good. > However with bitselct the situation is different. The inbuilt function uses > an alien bfi instruction This is precisely what we expected. :-) > which I couldn't find anywhere in the docs. The > manual version uses ixor and iand. So, any explanation why there's no measurable speedup (at least in my tests) from using bitselect() in SHA-1's F in MSCash2? Is there some kind of stall, so that the reduction in instruction count doesn't help? Or is there somehow no such reduction (e.g., an extra move is added)? Alexander
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