C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  For loop compilation optimization          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message klaus Guest Posted: Mon Aug 25, 2003 10:25 pm    Post subject: For loop compilation optimization Hello Consider the following code: for(int i=0; i < something.GetNoOfSomething(); i++) { something.DoSomething(); } I am wondering how the compiler optimizes or doesn't optimize the above code. Is the something.GetNoOfSomething() function executed every time the for loop takes a round? I suppose the compiler would have to consider if the something.DoSomething() would affect the result of something.GetNoOfSomething(). Thanks Klaus [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jeff Greif Guest Posted: Tue Aug 26, 2003 8:26 am    Post subject: Re: For loop compilation optimization Besides the reason you mention, most compilers cannot take a chance on optimizing this. -- Unless getNoOfSomething looks like int GetNoOfSomething() const the compiler cannot tell that it doesn't modify something. -- Without analyzing the body of GetNoOfSomething, it cannot tell whether it returns the same thing on successive calls if the object 'something' has not changed. For example, it could return the number of nanoseconds since the Big Bang. -- The 'something' container could be modified in another thread, so the compiler cannot optimize unless the compiler can know that no other threads will run (owing to some compiler setting whose semantics it understands and which will be enforced at runtime). There a probably some other possible complications also. Jeff "klaus" <kh (AT) slet (DOT) ingenioer.dk> wrote Quote: Hello Consider the following code: for(int i=0; i < something.GetNoOfSomething(); i++) { something.DoSomething(); } I am wondering how the compiler optimizes or doesn't optimize the above code. Is the something.GetNoOfSomething() function executed every time the for loop takes a round? I suppose the compiler would have to consider if the something.DoSomething() would affect the result of something.GetNoOfSomething(). [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top llewelly Guest Posted: Tue Aug 26, 2003 7:08 pm    Post subject: Re: For loop compilation optimization "klaus" <kh (AT) slet (DOT) ingenioer.dk> writes: Quote: Hello Consider the following code: for(int i=0; i < something.GetNoOfSomething(); i++) { something.DoSomething(); } I am wondering how the compiler optimizes or doesn't optimize the above code. Is the something.GetNoOfSomething() function executed every time the for loop takes a round? [snip] In general, if GetNoOfSomething() gets inlined, the inlined code may get hoisted out of the loop. If it doesn't get inlined, it almost certainly won't; I don't know of any compilers which attempt to analyze non-inlined functions for side-effects. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Leimbach Guest Posted: Wed Aug 27, 2003 8:58 am    Post subject: Re: For loop compilation optimization [email]ryan.winter (AT) optusnet (DOT) com.au[/email] (Ryan Winter) wrote in message news:<5dadf782.0308251700.5079c233 (AT) posting (DOT) google.com>... Quote: "klaus" <kh (AT) slet (DOT) ingenioer.dk> wrote Hello Consider the following code: for(int i=0; i < something.GetNoOfSomething(); i++) { something.DoSomething(); } I am wondering how the compiler optimizes or doesn't optimize the above code. Is the something.GetNoOfSomething() function executed every time the for loop takes a round? I suppose the compiler would have to consider if the something.DoSomething() would affect the result of something.GetNoOfSomething(). You are correct. If you defined the something.DoSomething() as const, then the compiler would know that it could easily optimise the getNoOfSomething to be called only once. Otherwise it has no way to tell. Actually const member fuctions can still modify "mutable" member data can't they? In this case you cannot annotate a member function such that the state of even a const object won't be modified. #include class A { mutable int x; public: A (A const & a) : x(a.x) {} A () : x() {} void blah () const { ++x; } int get () const { return x; } }; int main () { const A a; for (int i = 0; i < 101; ++i) a.blah(); std::cout << a.get() << std::endl; } Quote: You would be much better of calling GetNoOfSomething() before the loop. Yes . But one must be careful with this rule: for (m_iterator = m.begin(); m_iterator != m.end(); ++m_itereator) { //some stuff that might make the data structure grow //or invalidate your iterators BE CAREFUL! } Sometimes you actually need to constantly query the object for the state of where a pointer/iterator may lead. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jeff Greif Guest Posted: Wed Aug 27, 2003 9:50 am    Post subject: Re: For loop compilation optimization "Ben Hutchings" <do-not-spam-benh (AT) bwsint (DOT) com> wrote Quote: In article <e2y2b.97512$2x.28629 (AT) rwcrnsc52 (DOT) ops.asp.att.net>, Jeff Greif wrote: ..... -- The 'something' container could be modified in another thread, so the compiler cannot optimize unless the compiler can know that no other threads will run (owing to some compiler setting whose semantics it understands and which will be enforced at runtime). Wrong. The compiler is not required to synchronise access by different threads; that's the responsibility of the programmer. I was not referring to synchronization issues that are the responsibility of the programmer. What I meant was that if the compiler were invoked with options like --singleThreaded --noSharedMemory --singleThreadedLibraries and these options guaranteed that there could be no other access to 'something' for the duration of the loop, the compiler would be allowed to take advantage of this information to optimize the loop. Jeff [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Wed Aug 27, 2003 3:47 pm    Post subject: Re: For loop compilation optimization Ben Hutchings <do-not-spam-benh (AT) bwsint (DOT) com> wrote Quote: In article <e2y2b.97512$2x.28629 (AT) rwcrnsc52 (DOT) ops.asp.att.net>, Jeff Greif wrote: Besides the reason you mention, most compilers cannot take a chance on optimizing this. -- Unless getNoOfSomething looks like int GetNoOfSomething() const the compiler cannot tell that it doesn't modify something. A const member function can still modify static data and data reachable through static pointers. A const member function could also modify mutable data. For that matter, a const member function can use const_cast to modify anything it pleases, as long as the original object is not const. Basically, const means nothing to the optimizer. Also, of course, the GetNoOfSomething() function may have observable side effects, writing to a file, for example, and these must occur. With regards to the original question, the answer is simple: the compiler can do anything it wishes, as long as the observable behavior (output, access to volatile variables) is the same as if it had called the function each time through the loop. In practice, of course, unless the function is inline and trivial, there are very, very few optimizers which will be capable of suppressing the call each time through the loop. -- James Kanze GABI Software mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ http://www.gabi-soft.fr Beratung in objektorientierter Datenverarbeitung 11 rue de Rambouillet, 78460 Chevreuse, France, +33 (0)1 30 23 45 16 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Kevin Cline Guest Posted: Sat Aug 30, 2003 1:17 pm    Post subject: Re: For loop compilation optimization [email]ryan.winter (AT) optusnet (DOT) com.au[/email] (Ryan Winter) wrote in message news:<5dadf782.0308251700.5079c233 (AT) posting (DOT) google.com>... Quote: "klaus" <kh (AT) slet (DOT) ingenioer.dk> wrote Hello Consider the following code: for(int i=0; i < something.GetNoOfSomething(); i++) { something.DoSomething(); } I am wondering how the compiler optimizes or doesn't optimize the above code. Is the something.GetNoOfSomething() function executed every time the for loop takes a round? I suppose the compiler would have to consider if the something.DoSomething() would affect the result of something.GetNoOfSomething(). You are correct. If you defined the something.DoSomething() as const, then the compiler would know that it could easily optimise the getNoOfSomething to be called only once. Otherwise it has no way to tell. You would be much better of calling GetNoOfSomething() before the loop. Maybe not. Doing that makes the code just a bit harder to read. It might be better to make GetNoOfSomething() an inline function. Or to redesign the interface so that you don't have to write all those infernal loops: something.ForEachSomething(myFunction); [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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