C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Why is the std::complex class so complex? Goto page 1, 2, 3  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Hanyou Chu Guest Posted: Fri Nov 12, 2004 3:07 pm    Post subject: Why is the std::complex class so complex? This question is probably for P.J. Plauger who supplied stl library for microsoft and gcc. The operator * for two complex numbers is written as (not literarily) complex operator*(const complex &a,const complex& b) { complex t=a; t *= b; return t; } It prevents Microsoft compilers(vc 6 and 7) from optimizing it in such a way that it slows the computations down by more than a factor of two(probably three). The Intel compiler is much better at optimizing this. My question is why isn't it written in the following way instead: complex operator*(const complex &a,const complex& b) { return complex(a.real()*b.real()-a.imag()*b.imag(), a.real()*b.imag()+a.imag()*b.real()); } People suggested transforming the above into 3 multiplications instead of 4, but it requires two temporaries, and as a result the code is slower. The other thing is related to the complex exponential function, which is again written in such a way that the calculation is slowed down by a factor of two. As calculational speed is critical to our product, this is unacceptible. I am attempted to write my own complex class, but that would defeat the purpose of having a stl library. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hanyou Chu Guest Posted: Sat Nov 13, 2004 2:48 am    Post subject: Re: Why is the std::complex class so complex? I forgot to add that the code really looks too complicated. Inline functions call identical inline functions with another name. We finally decided to look at the assembly code, it turns out that the MS compiler is not able to inline the multiplications due to multilayer function calls. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Frank Birbacher Guest Posted: Sun Nov 14, 2004 5:01 am    Post subject: Re: Why is the std::complex class so complex? Hi! Hanyou Chu wrote: Quote: The operator * for two complex numbers is written as (not literarily) complex operator*(const complex &a,const complex& b) { complex t=a; t *= b; return t; } That's a reasonable implementation. Quote: It prevents Microsoft compilers(vc 6 and 7) from optimizing it in such a way that it slows the computations down by more than a factor of two(probably three). The Intel compiler is much better at optimizing this. Back luck for MS here. Quote: My question is why isn't it written in the following way instead: complex operator*(const complex &a,const complex& b) { return complex(a.real()*b.real()-a.imag()*b.imag(), a.real()*b.imag()+a.imag()*b.real()); } It stems from a common design principle: Avoid redundant code. Because if you did something wrong when implementing the multiplication, then you probably did it wrong in two places (operator * and *=). By calling one from the other, you avoid this. See, operator * is just a convinience function. Quote: As calculational speed is critical to our product, this is unacceptible. I am attempted to write my own complex class, but that would defeat the purpose of having a stl library. If speed is critical, then change *your* code to use operator *= instead. This will save you some temporaries. Frank [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top M Jared Finder Guest Posted: Sun Nov 14, 2004 10:32 am    Post subject: Re: Why is the std::complex class so complex? Hanyou Chu wrote: Quote: The other thing is related to the complex exponential function, which is again written in such a way that the calculation is slowed down by a factor of two. As calculational speed is critical to our product, this is unacceptible. I am attempted to write my own complex class, but that would defeat the purpose of having a stl library. Complex is a very simple class to write, so if you really need the speed improvements, why not write a custom <complex>? There is a lot of functionality, but it should be relatively straightforward to implement; no more than a few days work. If your new implementation complies with the standard, then other programmers still do not need to learn Yet Another API. -- MJF [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Peter Koch Larsen Guest Posted: Sun Nov 14, 2004 4:55 pm    Post subject: [OT] Re: Why is the std::complex class so complex? "Hanyou Chu" <hchu (AT) thermawave (DOT) com> skrev i en meddelelse news:ceee3b79.0411121201.5dfd65e8 (AT) posting (DOT) google.com... Quote: I forgot to add that the code really looks too complicated. Inline functions call identical inline functions with another name. We finally decided to look at the assembly code, it turns out that the MS compiler is not able to inline the multiplications due to multilayer function calls. This has also been my experience with VC++ 6.0: deeply nested inline functions are not inlined however simple they are. I believe that a switch to 7.1 would fix that. You could also declare the functions __forceinline (not sure this is exact spelling, so you have to look it up). This is of course inconvenient if you have to modify system-headers, but perhaps Dinkumware has a patch? Kind regards Peter Koch Larsen [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alberto Barbati Guest Posted: Sun Nov 14, 2004 5:00 pm    Post subject: Re: Why is the std::complex class so complex? Hanyou Chu wrote: Quote: I forgot to add that the code really looks too complicated. Inline functions call identical inline functions with another name. We finally decided to look at the assembly code, it turns out that the MS compiler is not able to inline the multiplications due to multilayer function calls. You question is MS specific, this is the wrong place for that. You should post your question to microsoft.public.vc.stl. Alberto [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Glen Low Guest Posted: Sun Nov 14, 2004 8:38 pm    Post subject: Re: Why is the std::complex class so complex? Quote: My question is why isn't it written in the following way instead: complex operator*(const complex &a,const complex& b) { return complex(a.real()*b.real()-a.imag()*b.imag(), a.real()*b.imag()+a.imag()*b.real()); } Most people recommended implementing operator* in terms of operator*=, but even on a modern compiler like gcc 3.3, it pessimizes temporary-laden code as a result e.g. complex a = b * c + d * e; has at least 2 temporaries that are slowed down by having the operator* defined in terms of operator*=. Do it the other way around and it becomes much faster, likely because the compiler knows it doesn't need to push the values back to memory and simply keeps them in registers ("enregistration"). Quote: People suggested transforming the above into 3 multiplications instead of 4, but it requires two temporaries, and as a result the code is slower. Not necessarily. If the temporaries are enregistered you don't really pay a thing. I know gcc 3.3 properly enregisters structs with single members and inbuilt types. Quote: The other thing is related to the complex exponential function, which is again written in such a way that the calculation is slowed down by a factor of two. As calculational speed is critical to our product, this is unacceptible. I am attempted to write my own complex class, but that would defeat the purpose of having a stl library. Hmm... I'm in the process of writing an Altivec-enhanced version of the valarray and complex implementations in my macstl library. I haven't gotten round to doing the intended optimization in SSE2 yet, but the Altivec-enhanced multiply is already 1.75x faster than scalar multiply on complex<float>. Cheers, Glen Low, Pixelglow Software www.pixelglow.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Mickey Moore Guest Posted: Mon Nov 15, 2004 11:41 am    Post subject: Re: Why is the std::complex class so complex? It seems to me that this is a case of just using the canonical idiom ("Implement operator@ in terms of operator@=") instead of bothering to actually *think* about the best implementation or understand the requirements of the typical client of the class. The canonical idiom is almost always The Right Thing To Do, but when it's not, then it shouldn't be done! Your question highlights an implementation issue, but the standard for complex<T> itself contains an instance of doing the Canonical Thing instead of the Right Thing. After working on a recent project, it's become very clear to me that complex<T> should have been implemented with public data members (named real and imag). It's too late to do that now of course. ("Get your interfaces right first. If you get the interface wrong you may never be allowed to fix it." -- Sutter) At the very least, complex<T> needs a way to handle the real and imaginary parts separately. Instead, the class remains broken, two years after the defect was formally publicized! (See library issue #387.) (Fortunately, the g++ library contains a non- standard extension which implements suggested fix #2 in the standard's document n1589, which I've come to believe is the only acceptable solution to the second subissue of issue #387.) Quote: As calculational speed is critical to our product, this is unacceptible. Calculation speed is almost always critical when dealing with mathematical primitives in numerical work. --------------------------------------------- Mitchell G. (Mickey) Moore, Ph.D. (Physics) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top chris Guest Posted: Mon Nov 15, 2004 11:20 pm    Post subject: [OT] Re: Why is the std::complex class so complex? Glen Low wrote: Quote: My question is why isn't it written in the following way instead: complex operator*(const complex &a,const complex& b) { return complex(a.real()*b.real()-a.imag()*b.imag(), a.real()*b.imag()+a.imag()*b.real()); } Most people recommended implementing operator* in terms of operator*=, but even on a modern compiler like gcc 3.3, it pessimizes temporary-laden code as a result e.g. complex a = b * c + d * e; has at least 2 temporaries that are slowed down by having the operator* defined in terms of operator*=. Do it the other way around and it becomes much faster, likely because the compiler knows it doesn't need to push the values back to memory and simply keeps them in registers ("enregistration"). Sorry for the slightly off-topic post, but for those who are tempted to start fiddling with their code due to this lack of optimisation, it's worth noting that gcc's current CVS version, and therefore the upcoming gcc 4 will optimise all the temporaries away in this statement. Chris [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hanyou Chu Guest Posted: Wed Nov 17, 2004 11:58 pm    Post subject: Re: Why is the std::complex class so complex? M Jared Finder <jared (AT) hpalace (DOT) com> wrote Quote: Hanyou Chu wrote: The other thing is related to the complex exponential function, which is again written in such a way that the calculation is slowed down by a factor of two. As calculational speed is critical to our product, this is unacceptible. I am attempted to write my own complex class, but that would defeat the purpose of having a stl library. Complex is a very simple class to write, so if you really need the speed improvements, why not write a custom <complex>? There is a lot of functionality, but it should be relatively straightforward to implement; no more than a few days work. If your new implementation complies with the standard, then other programmers still do not need to learn Yet Another API. I am aware of that, we already did write our own class. I just assumed that the stl implementation was high quality. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hanyou Chu Guest Posted: Wed Nov 17, 2004 11:58 pm    Post subject: Re: Why is the std::complex class so complex? Quote: Most people recommended implementing operator* in terms of operator*=, but even on a modern compiler like gcc 3.3, it pessimizes temporary-laden code as a result e.g. complex a = b * c + d * e; has at least 2 temporaries that are slowed down by having the operator* defined in terms of operator*=. Do it the other way around and it becomes much faster, likely because the compiler knows it doesn't need to push the values back to memory and simply keeps them in registers ("enregistration"). gcc is not particularly good at optimizing code. On a 64 bit opteron the 64bit code generated by gcc is about twice as slow compared to the same code generated on windows running 32bit operating system. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hanyou Chu Guest Posted: Wed Nov 17, 2004 11:59 pm    Post subject: Re: Why is the std::complex class so complex? Quote: It stems from a common design principle: Avoid redundant code. Because if you did something wrong when implementing the multiplication, then you probably did it wrong in two places (operator * and *=). By calling one from the other, you avoid this. See, operator * is just a convinience function. It is so simple that there is no reason not to. Quote: If speed is critical, then change *your* code to use operator *= instead. This will save you some temporaries. It would become unreadable. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hanyou Chu Guest Posted: Thu Nov 18, 2004 12:07 am    Post subject: Re: Why is the std::complex class so complex? Alberto Barbati <AlbertoBarbati (AT) libero (DOT) it> wrote Quote: You question is MS specific, this is the wrong place for that. You should post your question to microsoft.public.vc.stl. gcc uses the same stl implementation. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top P.J. Plauger Guest Posted: Thu Nov 18, 2004 12:15 pm    Post subject: Re: Why is the std::complex class so complex? "Hanyou Chu" <hchu (AT) thermawave (DOT) com> wrote Quote: Alberto Barbati <AlbertoBarbati (AT) libero (DOT) it> wrote in message news: You question is MS specific, this is the wrong place for that. You should post your question to microsoft.public.vc.stl. gcc uses the same stl implementation. No, it doesn't. P.J. Plauger Dinkumware, Ltd. http://www.dinkumware.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Mickey Moore Guest Posted: Fri Nov 19, 2004 3:29 pm    Post subject: Re: Why is the std::complex class so complex? Quote: gcc uses the same stl implementation [as MS]. No, it doesn't. P.J. Plauger PJP is of course correct; in fact, I'm rather amazed at how differently the complex classes are implemented! However, the OP's complaint, that operator* is implemented in terms of operator*=, is true of both the MS and gcc3 versions. I came across this just recently in Sutter and Alexandrescu's new book 'C++ Coding Standards': Item 27 is 'Prefer the canonical forms of arithmetic and assignment operators', which among other things means 'implement operator@ in terms of operator@='. But their exceptions clause reads: "In some cases (e.g., operator*= on complex numbers), an operator might mutate its left-hand side so significantly that it can be more advantageous to implement operator*= in terms of operator* rather than the reverse." In mild defense of gcc3's complex, they do admit in the header file several times that "This is a grammar school implementation.". They also state that it is not conforming, though it's not obvious how. (Does adding an extension make something formally non-conforming even if all the standard defined behavior is correct?) --------------------------------------------- Mitchell G. (Mickey) Moore, Ph.D. (Physics) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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