From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-2.9 required=5.0 tests=BAYES_00,MAILING_LIST_MULTI autolearn=unavailable autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 103376,a00006d3c4735d70 X-Google-Attributes: gid103376,public X-Google-ArrivalTime: 2004-02-26 21:01:01 PST Path: archiver1.google.com!news2.google.com!news.maxwell.syr.edu!newsfeed.icl.net!newsfeed.fjserv.net!nnx.oleane.net!oleane!freenix!enst.fr!melchior!cuivre.fr.eu.org!melchior.frmug.org!not-for-mail From: Stephen Leake Newsgroups: comp.lang.ada Subject: Re: left-to-right (was In-Out Parameters for functions) Date: 26 Feb 2004 23:58:22 -0500 Organization: Cuivre, Argent, Or Message-ID: References: <1075851506.238480@master.nyc.kbcfp.com> <4020C947.81A6D703@0.0> <1075907239.138068@master.nyc.kbcfp.com> <402232E9.3EE15B4B@0.0> <1075987360.225622@master.nyc.kbcfp.com> <40236C0B.E988E003@0.0> <1077634311.254581@master.nyc.kbcfp.com> NNTP-Posting-Host: lovelace.ada-france.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: melchior.cuivre.fr.eu.org 1077857934 33645 212.85.156.195 (27 Feb 2004 04:58:54 GMT) X-Complaints-To: usenet@melchior.cuivre.fr.eu.org NNTP-Posting-Date: Fri, 27 Feb 2004 04:58:54 +0000 (UTC) To: comp.lang.ada@ada-france.org Return-Path: In-Reply-To: User-Agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.3 X-Virus-Scanned: by amavisd-new-20030616-p7 (Debian) at ada-france.org X-BeenThere: comp.lang.ada@ada-france.org X-Mailman-Version: 2.1.4 Precedence: list List-Id: "Gateway to the comp.lang.ada Usenet newsgroup" List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Xref: archiver1.google.com comp.lang.ada:5878 Date: 2004-02-26T23:58:22-05:00 Dmitry A.Kazakov writes: > On 24 Feb 2004 19:44:12 -0500, Stephen Leake > wrote: > > >I think Hyman has two valid points: > > > >1) if the language specified left-to-right order, there would never be > > a need to "kill the original smart-ass programmer" > > But also, if the language disallowed side-effects when there could be > more than one order. I prefer the second, provided that the term > "side-effect" should be formally defined by the language and visible > from the specifications. That's never going to happen! > >2) Does anyone have a real example of a compiler taking advantage > >of the evaluation order freedom to speed up a program? > > The question is what gets lost if an evaluation order is fixed. I gave > an example earlier: > > for I in A'Range loop > Sum := Sum + A (I) + B (B'First + I - A'First); > ... > > Here with left-to-right order the compiler cannot optimize neither the > array index of B, What do you mean by "optimize" in this sentence? the array index of B is B'First + I - A'First. It must be computed before B(). In what way would this be done differently by a "real Ada" compiler vs a "left-to-right" Ada compiler? > nor can it increment Sum by adding A()+B(). True; it must compute temp := Sum + A(), then Sum := temp + B(). What difference does that make, in actual practice? How many compilers do anything else? Hmm. For scalars, if A and B happen to be in registers, and Sum in ram, I can see compilers messing with the order. Can anyone confirm that actually happens in real compilers? It's been a while since I messed with gcc code generation. > Well, these optimizations could be made possible if the compiler > might prove that the result does not depend on the order. That could > be quite difficult to formally define, especially regarding numeric > exceptions and floating-point accuracy stuff. Even more difficult it > would be check. That is _precisely_ the point. Since it is difficult to formally define and check the meaning of "does not depend on order", why do we expect people to be able to do it? On the other hand, that's also why it's not likely such restrictions will be added to the definition of "pure" functions in Ada. > But finally, if all that could be achived, then the compiler will be > capable to simply detect that different orders give different > results, and threat it as an error. Much better than fixing an > arbitrary order! Why is that better? The goal is to have code that works (meaning produces correct results :), on all compilers/platforms. If I only have to prove that one evaluation order is correct, that is easier than if I have to think about many evaluation orders. > So the bottom line, IMO, 1 makes no sense. But you haven't answered my question. Is there, in actual fact, any compiler that actually does these optimizations? > >Ada is supposed to be about clear, unsurprising code. Subtle order > >issues are just that - "subtle". If the language _could_ make them a > >non-issue, at very little cost, I think it _should_. > > Right, the present situation is not good. But IMO the solution lies in > introducing formally pure functions and imposing limitations on use of > impure functions (and procedures with results) in expressions. That's way more work than simply requiring left-to-right, and therefore far less likely to happen. -- -- Stephe