Author: Matt Taylor
Date: 21:56:24 12/27/02
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On December 27, 2002 at 17:47:19, Walter Faxon wrote: >On December 26, 2002 at 22:48:35, Matt Taylor wrote: > ><snip lotso neat stuff> > >>The "cmove" instruction is "conditional move if equal/zero." If the previous >>comparison (val == some case) is true, it will replace the contents of result >>with the second operand. >> >>This strategy also works particularly well on IA-64 where the results of a >>comparison go into any one of 64 special 1-bit registers. In the example you >>gave, the particular symmetry would allow the processor to do the correct >>computation quickly without branching at all. >> >>In other cases where one is not so fortunate to have nice facilities, you can >>play games with masks. I won't get into all the many tricks & techniques for >>doing this because it would make this post even longer than it already is. In >>the case of MMX/SSE vector extensions to IA-32, comparisons will give you a >>resulting mask of -1 or 0 that you can manipulate. > ><snip more great stuff -- this time for SSE> > >>-Matt > > >Wow. > >Other than my feeling a little retarded regarding cmove, etc., all I want to >know now is: do you know of any "mother lode(s)" for coding tricks? (I already >have a copy of the C-centric "Hacker's Delight".) > >Either way, thanks, Matt. You've really opened my eyes. > >-- Walter It is alright as I would not expect anyone to be familiar with all the tricks in x86, and some people are still better at it than I am. It is also noteworthy that one can sometimes avoid the virtual function call with "virtual data" -- data that gets set based on the specific instantiation of the class. It's all a matter of knowing how well the compiler can optimize different constructs. I wish I knew of a repository for tips & tricks. I glean a lot of my knowledge of such topics from stuff I read on the web. I start out searching for something specific, and 3 hours later I've read several completely unrelated articles. -Matt
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