Path: utzoo!utgpu!watmath!clyde!att!pacbell!ames!mailrus!tut.cis.ohio-state.edu!ukma!husc6!yale!mfci!colwell From: colwell@mfci.UUCP (Robert Colwell) Newsgroups: comp.arch Subject: Re: Assembly or .... Message-ID: <569@m3.mfci.UUCP> Date: 29 Nov 88 14:23:03 GMT References: <1388@aucs.UUCP> <729@convex.UUCP> <1961@crete.cs.glasgow.ac.uk> <6529@june.cs.washington.edu> <1031@l.cc.purdue.edu> <1988Nov28.205129.2216@utzoo.uucp> Sender: colwell@mfci.UUCP Reply-To: colwell@mfci.UUCP (Robert Colwell) Organization: Multiflow Computer Inc., Branford Ct. 06405 Lines: 31 In article <1988Nov28.205129.2216@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes: ]In article <1031@l.cc.purdue.edu] cik@l.cc.purdue.edu (Herman Rubin) writes: ]]... But one ]]of the most amazing things that I have seen in the workings of the ]]designers is the assumption that the compiler has all the information ]]necessary to produce optimized code! There is no provision for input ]]as to frequency of branches... ] ]Not true; some C implementations will take advice from the profiler on ]this. It's practically a necessity for the VLIW folks. (Oh, you meant ]advice from the *programmer*? :-) The guy who's wrong 3/4 of the time ]about such things? Silly idea.) (No, I'm not kidding -- programmer ]intuition is vastly inferior to measured data in such matters. This ]has been known for many years.) Programmers ARE often wrong. We've even seen cases where the programmer had put in assertions for his Cray code that were provably incorrect for certain interesting sets of input data. But for branching, I wouldn't say it's a necessity for us to have the ability for the profiler to tell the compiler where branches are going. We have that mechanism, but it isn't often used. The heuristics built in to the compiler for predicting directions work awfully well. And if the programmer provides branch assertions, and they're wrong, and the performance is adversely affected, presumably he or she will discover that and fix it. Bob Colwell ..!uunet!mfci!colwell Multiflow Computer or colwell@multiflow.com 175 N. Main St. Branford, CT 06405 203-488-6090