C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Why C++ is so slow to compile ? Goto page 1, 2, 3, 4, 5, 6  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Yves Dufournaud Guest Posted: Sun Apr 24, 2005 3:46 pm    Post subject: Why C++ is so slow to compile ? Hello All, This is a general question : from my experience C/C++ are very slow to compile compared to Java or C#. I have for example never see a C compiler that match Java compile time speed for example (even if it's faster to compile than C++). Used of precompiled header can boost things a bit, but figures that come to my mind and that are comming from real projects are for example : 3000 java file in order of 1 min (this is overpriced in fact) and on the same computer for about the same order of number of cpp files in many hours. I have got no personnals stats on C#, but I suspect that they match Java. I'am wondering what cause this slowliness : does it comes from the language itself that impose some different architecture on the compiler ? If so, which parts ? Yves [ 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 Apr 24, 2005 8:24 pm    Post subject: Re: Why C++ is so slow to compile ? Yves Dufournaud wrote: Quote: Hello All, This is a general question : from my experience C/C++ are very slow to compile compared to Java or C#. I have for example never see a C compiler that match Java compile time speed for example (even if it's faster to compile than C++). Used of precompiled header can boost things a bit, but figures that come to my mind and that are comming from real projects are for example : 3000 java file in order of 1 min (this is overpriced in fact) and on the same computer for about the same order of number of cpp files in many hours. I have got no personnals stats on C#, but I suspect that they match Java. I'am wondering what cause this slowliness : does it comes from the language itself that impose some different architecture on the compiler ? If so, which parts ? There are a two major things that make C++ slower to compile than Java or C#. 1. C++ is a much more complicated language than C# or Java. This makes it take much longer to parse. 2. Preprocessing file inclusion combined with pre-processing's conditional compilation means that each #include of a header file may generate differnt code. This means that to be correct, each header file must be reprocessed for each time it is #include'd, for each source file. Combining this with the more complicated parser makes things even worse. -- MJF [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Kanze Guest Posted: Sun Apr 24, 2005 8:57 pm    Post subject: Re: Why C++ is so slow to compile ? Yves Dufournaud wrote: Quote: This is a general question : from my experience C/C++ are very slow to compile compared to Java or C#. I have for example never see a C compiler that match Java compile time speed for example (even if it's faster to compile than C++). Used of precompiled header can boost things a bit, but figures that come to my mind and that are comming from real projects are for example : 3000 java file in order of 1 min (this is overpriced in fact) and on the same computer for about the same order of number of cpp files in many hours. I have got no personnals stats on C#, but I suspect that they match Java. I'am wondering what cause this slowliness : does it comes from the language itself that impose some different architecture on the compiler ? If so, which parts ? I'm not 100% sure of what you're really comparing, but certainly much of the difference could be explained by the fact that a C++ compiler usually compiles directly to machine code (including some degree of optimization), whereas as Java compilers normally compile to a relatively simple intermediate representation, and only really compile to machine code at run-time. Note the start-up times for Java programs, compared to C++, for example. Of course, I'm supposing that you are speaking of complete builds. I find that often, when I'm just trying to do a syntax check on a single module, C++ actually compiles faster. The Java compiler will automatically compile all of the imported packages, if the .class files for them aren't available, whereas the C++ compiler is quite happy with just a header. -- James Kanze mailto: [email]james.kanze (AT) free (DOT) fr[/email] Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 pl. Pierre 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. First time posters: Do this! ] Back to top Walter Guest Posted: Sun Apr 24, 2005 8:58 pm    Post subject: Re: Why C++ is so slow to compile ? "Yves Dufournaud" <yves.dufournaud (AT) free (DOT) fr> wrote Quote: I'am wondering what cause this slowliness : does it comes from the language itself that impose some different architecture on the compiler ? If so, which parts ? Some of it is due to inefficient compiler implementations, see http://biolpc22.york.ac.uk/wx/wxhatch/wxMSW_Compiler_choice.html A factor of 2 or 3 slowdown comes from needing to support the preprocessor and the multiple phases of translation. But the biggest culprit is the #include system. More and more of the semantics of a C++ program are being shoved into the .h files (macros, constants, templates, inlines), and the natural tendency of .cpp files is to #include every .h file in the project. Those .h files wind up #include'ing most of the system .h files. The end result is that to compile each .cpp source file, the compiler has to process an incredible amount of source text. Multiply that by the number of source files in the project, and a build is going to take a long time. Languages with true modules, such as the D programming language, Java, C#, etc., don't have this problem because each module is compiled only once, and then symbolically referenced (rather than textually #include'd). -Walter www.digitalmars.com C, C++, D compilers "code of the nerds" [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Graeme Prentice Guest Posted: Mon Apr 25, 2005 1:12 pm    Post subject: Re: Why C++ is so slow to compile ? On 24 Apr 2005 16:58:41 -0400, Walter wrote: Quote: "Yves Dufournaud" <yves.dufournaud (AT) free (DOT) fr> wrote in message news:426b858e$0$29863$636a15ce (AT) news (DOT) free.fr... I'am wondering what cause this slowliness : does it comes from the language itself that impose some different architecture on the compiler ? If so, which parts ? Some of it is due to inefficient compiler implementations, see http://biolpc22.york.ac.uk/wx/wxhatch/wxMSW_Compiler_choice.html A factor of 2 or 3 slowdown comes from needing to support the preprocessor and the multiple phases of translation. But the biggest culprit is the #include system. More and more of the semantics of a C++ program are being shoved into the .h files (macros, constants, templates, inlines), and the natural tendency of .cpp files is to #include every .h file in the project. Those .h files wind up #include'ing most of the system .h files. When you say "natural tendency" do you mean because of header files #including other header files or do you mean because the #include list in a .cpp includes more .h files than it needs to. If the latter, is it considered a bad thing to have more files included in the .cpp file than needed, or even to include all .h files, if compile times aren't an issue? Quote: The end result is that to compile each .cpp source file, the compiler has to process an incredible amount of source text. Multiply that by the number of source files in the project, and a build is going to take a long time. Languages with true modules, such as the D programming language, Java, C#, etc., don't have this problem because each module is compiled only once, and then symbolically referenced (rather than textually #include'd). How do you mean symbolically referenced? Are Java, C#, Delphi or D intermediate files "re-parsed" when they're referenced from another module or do the intermediate files contain an "index" that allows fast lookup on symbols? Graeme [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dmitry Mityugov Guest Posted: Mon Apr 25, 2005 1:21 pm    Post subject: Re: Why C++ is so slow to compile ? Yves Dufournaud wrote: Quote: Hello All, This is a general question : from my experience C/C++ are very slow to compile compared to Java or C#. I have for example never see a C compiler that match Java compile time speed for example (even if it's faster to compile than C++). Used of precompiled header can boost things a bit, but figures that come to my mind and that are comming from real projects are for example : 3000 java file in order of 1 min (this is overpriced in fact) and on the same computer for about the same order of number of cpp files in many hours. I have got no personnals stats on C#, but I suspect that they match Java. I'am wondering what cause this slowliness : does it comes from the language itself that impose some different architecture on the compiler ? If so, which parts ? I believe this is because meanings of many C++ keywords heavily depend on the context in which they're using. For example, keyword 'static' may denote at least 5 (*five*) different things, depending on the context: - a static variable defined outside of a function - a static variable defined inside of a function - a static function - a static method of a class - a static data member of a class Dmitry [ 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: Mon Apr 25, 2005 1:21 pm    Post subject: Re: Why C++ is so slow to compile ? [email]walter (AT) digitalmars (DOT) nospamm.com[/email] (Walter) wrote (abridged): Quote: But the biggest culprit is the #include system. More and more of the semantics of a C++ program are being shoved into the .h files (macros, constants, templates, inlines), and the natural tendency of .cpp files is to #include every .h file in the project. Those .h files wind up #include'ing most of the system .h files. [...] Languages with true modules, such as the D programming language, Java, C#, etc., don't have this problem because each module is compiled only once, and then symbolically referenced (rather than textually #include'd). Agreed. Also, I don't know about D and C#, but in Java user-defined types are always used by reference. This means they can avoid some transitive dependencies. In C++ we have the choice. If you write, for example: class Rect { Point topLeft; Point bottomRight; }; then you will need to #include "Point.h" because the compiler needs to know the size of Point. In Java you can only write the equivalent of: class Rect { Point *topLeft; Point *bottomRight; }; with indirection, and for this a forward-declaration is sufficient even in C++. -- 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 Apr 25, 2005 1:27 pm    Post subject: Re: Why C++ is so slow to compile ? Walter wrote: Quote: "Yves Dufournaud" <yves.dufournaud (AT) free (DOT) fr> wrote in message news:426b858e$0$29863$636a15ce (AT) news (DOT) free.fr... I'am wondering what cause this slowliness : does it comes from the language itself that impose some different architecture on the compiler ? If so, which parts ? Some of it is due to inefficient compiler implementations, see http://biolpc22.york.ac.uk/wx/wxhatch/wxMSW_Compiler_choice.html A factor of 2 or 3 slowdown comes from needing to support the preprocessor and the multiple phases of translation. But the biggest culprit is the #include system. More and more of the semantics of a C++ program are being shoved into the .h files (macros, constants, templates, inlines), and the natural tendency of .cpp files is to #include every .h file in the project. Those .h files wind up #include'ing most of the system .h files. If you have "a natural tendency of .cpp to #include every .h file in the project", I'd say that you have a serious design problem. In practice, the expansion due to include files isn't negligible, but it is far from being the problem you seem to suggest (unless someone is making exagerated use of templates). Quote: The end result is that to compile each .cpp source file, the compiler has to process an incredible amount of source text. Multiply that by the number of source files in the project, and a build is going to take a long time. For the system headers, I think most compilers today support some form of precompiled headers. For the project specific headers, you really do need some sort of dependancy management. But not just because of compile times. Quote: Languages with true modules, such as the D programming language, Java, C#, etc., don't have this problem because each module is compiled only once, and then symbolically referenced (rather than textually #include'd). It's a two edged sword. My only experience is with Java, and I found it a pain that 1) the compiler had to compile all of the modules I "included", including the implementation of the module (which meant I couldn't compile until there were implementations for everything I used), and 2) changing a single character in the implementation of a member function triggered the recompilation of all code which used the module. All in all, the C++ situation is pretty bad, but Java managed too do worse. (What is really needed is something along the lines of Ada.) -- James Kanze GABI Software 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. First time posters: Do this! ] Back to top E. Rosten Guest Posted: Mon Apr 25, 2005 11:05 pm    Post subject: Re: Why C++ is so slow to compile ? On Sun, 24 Apr 2005 11:46:45 -0400, Yves Dufournaud wrote: Quote: Hello All, This is a general question : from my experience C/C++ are very slow to compile compared to Java or C#. I have for example never see a C compiler that match Java compile time speed for example Since you're talking about C compilers as well here, have you tried TCC? It is a very fast compiler (~ 10x faster than gcc -O0), but it doesn't do much optimization. The benchmrks say that on a P4 2.4GHz, it does about 900,000 lines of code per second. You would have a hard job making a really fast C++ compiler, since template meta-programming is turing complete, so you could always write a program which takes for ever to compile. -Ed -- (You can't go wrong with psycho-rats.) (er258)(@)(eng.cam)(.ac.uk) /d{def}def/f{/Times findfont s scalefont setfont}d/s{10}d/r{roll}d f 5/m {moveto}d -1 r 230 350 m 0 1 179{1 index show 88 rotate 4 mul 0 rmoveto} for /s 15 d f pop 240 420 m 0 1 3 { 4 2 1 r sub -1 r show } for showpage [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Rob Guest Posted: Mon Apr 25, 2005 11:10 pm    Post subject: Re: Why C++ is so slow to compile ? Yves Dufournaud wrote: Quote: Hello All, This is a general question : from my experience C/C++ are very slow to compile compared to Java or C#. I have for example never see a C compiler that match Java compile time speed for example (even if it's faster to compile than C++). Used of precompiled header can boost things a bit, but figures that come to my mind and that are comming from real projects are for example : 3000 java file in order of 1 min (this is overpriced in fact) and on the same computer for about the same order of number of cpp files in many hours. I have got no personnals stats on C#, but I suspect that they match Java. I'am wondering what cause this slowliness : does it comes from the language itself that impose some different architecture on the compiler ? If so, which parts ? There are a few contributors I can think of. 1) C (and even more so, C++) is a complex language, so takes more time to parse. 2) The way Java works also means that part of the compilation can happen when the application is run, meaning there is a trade-off between compile time and run time performance when comparing with C/C++. 3) Good quality optimisers with C and C++ are typically more aggressive than the Java compiler, which also gives a trade-off between compile speeds and runtime performance. 4) C and C++ work with header files to facilitate use of libraries (which requires additional parsing) whereas the equivalent mechanism in Java either uses precompiled mechanisms or (in some cases) does not require the compiler to load relevant information. 5) A lot of things in Java are in the form of precompiled libraries, but equivalent facilities in C/C++ are in the form of code that needs to be recompiled (eg macros in C, templates in C++. Expansion of macros and instantiation of templates is a compile time overhead (although it is possible to reduce those overheads with selective programming techniques). [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Shannon Barber Guest Posted: Tue Apr 26, 2005 9:01 am    Post subject: Re: Why C++ is so slow to compile ? Try a Borland compiler. Some slowness is from the more complex parsing, much of the slowness is from the back-end optimizations, which is deferred until later with ..Net and either later or not at all with Java. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Antoun Kanawati Guest Posted: Tue Apr 26, 2005 9:20 am    Post subject: Re: Why C++ is so slow to compile ? Graeme Prentice wrote: Quote: On 24 Apr 2005 16:58:41 -0400, Walter wrote: Languages with true modules, such as the D programming language, Java, C#, etc., don't have this problem because each module is compiled only once, and then symbolically referenced (rather than textually #include'd). How do you mean symbolically referenced? Are Java, C#, Delphi or D intermediate files "re-parsed" when they're referenced from another module or do the intermediate files contain an "index" that allows fast lookup on symbols? In Java, a reference to Foo.bar() does not require that you have seen Foo.java. All it requires is the presence of Foo.class on the class path, and accessing the contents of Foo.class is generally a lot faster than parsing the source and reconstructing the class/interface declaration in every source file that refers to Foo. In preprocessor based compilers, like C/C++, even with include guards, you still have to go through the entrire text every time a file is #included. -- A. Kanawati [email]NO.antounk.SPAM (AT) comcast (DOT) net[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top clemd@acm.org Guest Posted: Tue Apr 26, 2005 9:30 am    Post subject: Re: Why C++ is so slow to compile ? [email]kanze (AT) gabi-soft (DOT) fr[/email] wrote: Quote: My only experience is with Java, and I found it a pain that 1) the compiler had to compile all of the modules I "included", including the implementation of the module ... All in all, the C++ situation is pretty bad, but Java managed to do worse. That is not how Java is supposed to be used. Use an "interface" to separate a caller from an implementation. You can write Java without interfaces, but the consequences are, well, just as you wrote. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Walter Guest Posted: Tue Apr 26, 2005 9:32 am    Post subject: Re: Why C++ is so slow to compile ? <kanze (AT) gabi-soft (DOT) fr> wrote Quote: Walter wrote: But the biggest culprit is the #include system. More and more of the semantics of a C++ program are being shoved into the .h files (macros, constants, templates, inlines), and the natural tendency of .cpp files is to #include every .h file in the project. Those .h files wind up #include'ing most of the system .h files. If you have "a natural tendency of .cpp to #include every .h file in the project", I'd say that you have a serious design problem. I've seen a *lot* of C/C++ code in the last 20 years. You can call it bad design if you want, but the reality is that every source file eventually winds up #include'ing, one way or another, directly or indirectly, every .h file in the project, sometimes multiple times. I've rarely seen a project where the programmers had the discipline to avoid this (or even considered it a problem). Quote: In practice, the expansion due to include files isn't negligible, but it is far from being the problem you seem to suggest (unless someone is making exagerated use of templates). The only "problem" resulting from it is slow compile times. BTW, lots of times it isn't exactly obvious in a project that this is happening. The only way you can tell is if you have a compiler that will put out a list of what files are actually being #include'd, or you run a utility like makedep over the source: www.digitalmars.com/ctg/makedep.html The problem is such that many .h files get #include'd over and over again in the same source file. This is why the practice of wrapping .h files in #ifdef/#define/#endif evolved, and why many compilers support #pragma once. Quote: The end result is that to compile each .cpp source file, the compiler has to process an incredible amount of source text. Multiply that by the number of source files in the project, and a build is going to take a long time. For the system headers, I think most compilers today support some form of precompiled headers. Precompiled headers help a lot. But using them requires some non-standard assumptions. And the reason precompiled headers came about is because most of the compile time is spent munching header files. Implementing p.h. is not easy, and it wouldn't have been done if this wasn't a big problem for customers. Quote: For the project specific headers, you really do need some sort of dependancy management. But not just because of compile times. It's fine to suggest ways to reduce the number of files #include'd. All I'm saying is what I see as the reality of software development in C++, and why the compiles are slow. As a compiler vendor, I work with the reality of how people use them. I've done a lot of profiling of C++ compilation, and I know where the time is spent. Quote: Languages with true modules, such as the D programming language, Java, C#, etc., don't have this problem because each module is compiled only once, and then symbolically referenced (rather than textually #include'd). It's a two edged sword. My only experience is with Java, and I found it a pain that 1) the compiler had to compile all of the modules I "included", including the implementation of the module (which meant I couldn't compile until there were implementations for everything I used), and 2) changing a single character in the implementation of a member function triggered the recompilation of all code which used the module. You're right that this can happen. But if you have n java source files, the worst case is you must compile n times, i.e. each module is only compiled once. In C++, the worst case is n*n. Also consider that different java compilers vary over 10:1 in compile speeds, just like C++ compilers do. To be frank, few people consider compile speed when selecting a compiler. I've had people insist to me that compile speed was the most important issue, while they've standardized on using the slowest compiler on the market <g>. Quote: All in all, the C++ situation is pretty bad, but Java managed too do worse. (What is really needed is something along the lines of Ada.) -Walter www.digitalmars.com C, C++, D compilers "code of the nerds" [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Kanze Guest Posted: Tue Apr 26, 2005 9:34 am    Post subject: Re: Why C++ is so slow to compile ? Dmitry Mityugov wrote: Quote: Yves Dufournaud wrote: This is a general question : from my experience C/C++ are very slow to compile compared to Java or C#. I have for example never see a C compiler that match Java compile time speed for example (even if it's faster to compile than C++). Used of precompiled header can boost things a bit, but figures that come to my mind and that are comming from real projects are for example : 3000 java file in order of 1 min (this is overpriced in fact) and on the same computer for about the same order of number of cpp files in many hours. I have got no personnals stats on C#, but I suspect that they match Java. I'am wondering what cause this slowliness : does it comes from the language itself that impose some different architecture on the compiler ? If so, which parts ? I believe this is because meanings of many C++ keywords heavily depend on the context in which they're using. For example, keyword 'static' may denote at least 5 (*five*) different things, depending on the context: - a static variable defined outside of a function - a static variable defined inside of a function - a static function - a static method of a class - a static data member of a class Which is more a problem for the user than for the compiler. Compilers have been dealing with this sort of problem for ages. (The - sign is overloaded for unary minus and for subtraction. With different precedences, no less. In almost every procedural language. Never caused any problems for the compiler.) -- James Kanze mailto: [email]james.kanze (AT) free (DOT) fr[/email] Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 pl. Pierre 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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