C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Why are calls to virtual functions not linked statically if          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Gino van den Bergen Guest Posted: Mon Sep 13, 2004 8:22 pm    Post subject: Why are calls to virtual functions not linked statically if I have the following situation: An abstract class for shapes: class Shape() { public: virtual BBox bbox() const = 0; }; A number of primitive types that implement this interface, and a class template template <typename Shape1, typename Shape2> class MinkowskiSum : public Shape { public: BBox bbox() const { return m_first.bbox() + m_second.bbox(); } private: Shape1 m_first; Shape2 m_second; }; (Constructors and accessors have been left out for the sake of brevity.) As you can see, the run-time types of the attributes of, for instance, a MinkowskiSum<Point, Sphere> are known at compile-time so you would expect a decent compiler to optimizie out the dereferencing of virtual functions and to simply link the calls to Point::bbox() and Sphere::bbox() statically into MinkowskiSum<Point, Sphere>::bbox(). I do not see this happening in Visual C++ 7.1. There is no optimization flag that will result in the virtual calls being removed. Now my question: Is this a flaw in the compiler or are there other reasons why the virtual calls cannot be removed here? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Balog Pal Guest Posted: Wed Sep 15, 2004 6:40 pm    Post subject: Re: Why are calls to virtual functions not linked statically "Gino van den Bergen" <gino (AT) dtecta (DOT) COMEAGAIN> wrote Quote: BBox bbox() const { return m_first.bbox() + m_second.bbox(); } As you can see, the run-time types of the attributes of, for instance, a MinkowskiSum<Point, Sphere are known at compile-time so you would expect a decent compiler to optimizie out the dereferencing of virtual functions and to simply link the calls to Point::bbox() and Sphere::bbox() statically into MinkowskiSum Sphere>::bbox(). I do not see this happening in Visual C++ 7.1. Strange, but that's exattly what i see happening. Same thing without templates. If you create a local Shape1 the compiler calls staticly, but it seems for *member* it sticks to virtual call. Until you find some better solution you can force a static call by return m_first.Shape1::bbox() + m_second.Shape2::bbox(); ugly and redundancy-stuffed, but does the job. Quote: Is this a flaw in the compiler or are there other reasons why the virtual calls cannot be removed here? Good question. Paul [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Michael Karcher Guest Posted: Fri Sep 17, 2004 2:42 am    Post subject: Re: Why are calls to virtual functions not linked statically Balog Pal <pasa (AT) lib (DOT) hu> wrote: Quote: are known at compile-time so you would expect a decent compiler to optimizie out the dereferencing of virtual functions and to simply link the calls to Point::bbox() and Sphere::bbox() statically into MinkowskiSum<Point, Sphere>::bbox(). I do not see this happening in Visual C++ 7.1. Strange, but that's exattly what i see happening. Same thing without templates. If you create a local Shape1 the compiler calls staticly, but it seems for *member* it sticks to virtual call. Using g++ 3.3.4 (Debian 1:3.3.4-6sarge1) and g++ 3.4.1 (Debian 3.4.1-4sarge1) the calls get compiled statically, even without any optimization enabled. The test source used is following the text. Using optimization, the compiler is also able to inline the two bbox calls. (Yes, I know the example source does not do anything useful or even remotely related to calculating bounding boxes) Michael Karcher typedef int BBox; class Shape { public: virtual BBox bbox() const = 0; }; class Sphere : public Shape { int radius; public: BBox bbox() const { return 3*radius; } }; class Point : public Shape { public: BBox bbox() const { return 1; } }; template <typename Shape1, typename Shape2> class MinkowskiSum : public Shape { public: BBox bbox() const { return m_first.bbox() + m_second.bbox(); } private: Shape1 m_first; Shape2 m_second; }; template class MinkowskiSum<Point,Sphere>; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gino van den Bergen Guest Posted: Fri Sep 17, 2004 2:44 am    Post subject: Re: Why are calls to virtual functions not linked statically I guess with templates it's a little harder to decide to link calls to virtual members statically, since the substituted type could very well be a reference. However, this still leaves the issue of why virtual calls in non-templated classes are not linked statically. "Balog Pal" <pasa (AT) lib (DOT) hu> wrote Quote: "Gino van den Bergen" <gino (AT) dtecta (DOT) COMEAGAIN> wrote in message news:b211$41459a7b$d468cfd2$23430 (AT) nf1 (DOT) news-service-com... BBox bbox() const { return m_first.bbox() + m_second.bbox(); } As you can see, the run-time types of the attributes of, for instance, a MinkowskiSum<Point, Sphere are known at compile-time so you would expect a decent compiler to optimizie out the dereferencing of virtual functions and to simply link the calls to Point::bbox() and Sphere::bbox() statically into MinkowskiSum Sphere>::bbox(). I do not see this happening in Visual C++ 7.1. Strange, but that's exattly what i see happening. Same thing without templates. If you create a local Shape1 the compiler calls staticly, but it seems for *member* it sticks to virtual call. Until you find some better solution you can force a static call by return m_first.Shape1::bbox() + m_second.Shape2::bbox(); ugly and redundancy-stuffed, but does the job. Is this a flaw in the compiler or are there other reasons why the virtual calls cannot be removed here? Good question. Paul [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel Krügler (ne Spange Guest Posted: Fri Sep 17, 2004 5:40 pm    Post subject: Re: Why are calls to virtual functions not linked statically Hello Gino van den Bergen, Gino van den Bergen schrieb: Quote: I guess with templates it's a little harder to decide to link calls to virtual members statically, since the substituted type could very well be a reference. Actually I don't think that this is a template-related problem, because the compiler knows the type of the (data) members of a given class template if it is going to instantiate member functions of that class. What do you think? Greetings from Bremen, Daniel Krügler Quote: [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dave Harris Guest Posted: Sat Sep 18, 2004 2:34 pm    Post subject: Re: Why are calls to virtual functions not linked statically [email]gino (AT) dtecta (DOT) COME[/email]AGAIN (Gino van den Bergen) wrote (abridged): Quote: As you can see, the run-time types of the attributes of, for instance, a MinkowskiSum<Point, Sphere are known at compile-time so you would expect a decent compiler to optimizie out the dereferencing of virtual functions and to simply link the calls to Point::bbox() and Sphere::bbox() statically into MinkowskiSum A /very/ decent compiler might be trying to avoid code bloat. The machine code for a vtable call to Point::bbox() is probably identical to one for Sphere::bbox(), so the implementation of MinkowskiSum<Point, Sphere>::bbox can probably be shared if it uses dynamic dispatch. I doubt that's the reason here, but it could be a valid concern in some cases. -- Dave Harris, Nottingham, UK [ 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: Mon Sep 20, 2004 6:28 pm    Post subject: Re: Why are calls to virtual functions not linked statically "Gino van den Bergen" <gino (AT) dtecta (DOT) COMEAGAIN> wrote Quote: I guess with templates it's a little harder to decide to link calls to virtual members statically, since the substituted type could very well be a reference. However, this still leaves the issue of why virtual calls in non-templated classes are not linked statically. I can't see templates playing a role here. By the time the compiler is generating code, the temlates have been fully instantiated, and it generates the code exactly as if it were not a template. -- James Kanze GABI Software http://www.gabi-soft.fr Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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