Path: utzoo!attcan!uunet!husc6!cmcl2!nrl-cmf!ames!pasteur!ucbvax!decwrl!labrea!sri-unix!garth!smryan From: smryan@garth.UUCP (Steven Ryan) Newsgroups: comp.lang.c Subject: Re: Safe optimization Summary: On (in)formally undecidable assumptions. Message-ID: <798@garth.UUCP> Date: 24 Jun 88 23:16:14 GMT References: <16271@brl-adm.ARPA> Reply-To: smryan@garth.UUCP (Steven Ryan) Organization: INTERGRAPH (APD) -- Palo Alto, CA Lines: 43 Church: Our Lady of Reluctant Software. In article <16271@brl-adm.ARPA> dsill@nswc-oas.arpa (Dave Sill) writes: >>> You are implying that programmers can solve unsolvable problems but >>> that programs cannot, which is simply not true. >> >>Actually, you should keep your referents straight. I might be >>implying that humans can solve problems that are unsolvable by >>COMPUTERS. The truth of that is a matter of conjecture. > >You're absolutely right. I *have* been assuming that the >computational power of the human brain is subject to the Church-Turing >[hypo-]Thesis. I have no reason to believe that that is an incorrect >assumption. I have reason to believe that is an incorrect assumption, and I have no reason to believe it's a correct assumption. No proof of course. Dear friends, not everybody assumes this. So puleeeease, don't hold compilers up to this possible/impossible standard. If you want something everybody can accept, deal with the existing technology rather than building castles in the air. >Yes, we've been through all this before. Your example is .... Before another round of examples and counterexamples, are you prepared to write a total recursive predicate that can verify all practical cases of aliassing? (Not all possible cases--a sufficiently large set of practical cases will do.) Given the modern technology, can you guarentee a compiler could recognise the predicate? > assert (i != j); Actually on Vax or Cyber 180, words need not be on word boundaries, so you need assert(abs(i-j)>=word_separation) The real issue is not whether this can be detected, but what is the cost/risk tradeoff? Is it so risky we might as well use assembly? Is it so costly we have to use assembly? ----------------------------------- Neurotics build castles in the air. Psychotics live in them. Psychiatrists collect the rent.