Article View: comp.arch
Article #11352Re: DEC Alpha architecture issues
From: Eliot Moss
Date: Fri, 20 Nov 1992 15:17
Date: Fri, 20 Nov 1992 15:17
20 lines
1899 bytes
1899 bytes
Regarding conditional moves, I think that branch prediction is not the only issue. Another issue is code density. A comparison followed by a conditional move is one instruction shorter than a comparison followed by a branch followed by an unconditional move. The number of instructions moved through the pipe must be at least two (unless you are lucky to have a situation where a compare and branch can fold into one instruction) and the conditional move approach gives you that, but with no pipeline "bubble". In fact, conditional instructions are easy to handle in a regular pipeline -- you just suppress the store in the last step, and you certainly know the outcome of the comparison by then. In a superscalar, where things get reordered, it is admittedly not quite so simple. -- J. Eliot B. Moss, Associate Professor Visiting Associate Professor Department of Computer Science School of Computer Science Lederle Graduate Research Center Carnegie Mellon University University of Massachusetts 5000 Forbes Avenue Amherst, MA 01003 Pittsburgh, PA 15213-3891 (413) 545-4206, 545-1249 (fax) (412) 268-6767, 681-5739 (fax) Moss@cs.umass.edu Moss@cs.cmu.edu
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Newsgroups: comp.arch Path: arc1-ca.pugleaf.net!sparky!uunet!zaphod.mps.ohio-state.edu!wupost!udel!rochester!cantaloupe.srv.cs.cmu.edu!netnews-2.srv.cs.cmu.edu!moss From: moss@cs.cmu.edu (Eliot Moss) Subject: Re: DEC Alpha architecture issues In-Reply-To: tremblay@flayout.Eng.Sun.COM's message of 19 Nov 92 18:52:35 GMT Message-ID: <MOSS.92Nov20101727@CRAFTY.cs.cmu.edu> Sender: news@cs.cmu.edu (Usenet News System) Nntp-Posting-Host: crafty.fox.cs.cmu.edu Reply-To: moss@cs.cmu.edu Organization: Dept of Comp and Info Sci, Univ of Mass (Amherst) References: <1992Nov18.112407.2518@doug.cae.wisc.edu> <1992Nov18.191730.1044@meiko.com> <lgnojjINN627@exodus.Eng.Sun.COM> Date: Fri, 20 Nov 1992 15:17:27 GMT Lines: 20