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Debugging and Profiling C++ Template Metaprograms

The document discusses debugging and profiling techniques for C++ template metaprograms, highlighting tools and methods to improve error detection and execution efficiency. It presents a personal motivation for understanding template programming, along with examples of template use in C++. The document emphasizes the need for better tool support for template metaprogramming, outlining various existing tools and their limitations.

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Debugging and Profiling C++ Template Metaprograms Zoltán Porkoláb gsd@elte.hu Ericsson Hungary Eötvös Loránd University, Budapest
2015.02.27. C++ Russia 2015 2 ● C++ Template Metaprogramming ● Possible debugging and profiling techiques ● Templight back-end tool ● Front-end tools ● 3rd party applications – please, contribute! ● Vision Agenda
2015.02.27. C++ Russia 2015 3 C++ Template Metaprograms ● Expression templates (since 1995!) ● Active libraries, compile-time adaption ● Static interface checking ● Simulating language extensions ● DSL embedding ● Many other areas ...
2015.02.27. C++ Russia 2015 4 Motivation – a personal view template <class T, class S> ? max( T a, S b) // How to define the return value? { if ( a > b ) return a; else return b; } int main() { short is = 3; long il = 2; double d = 3.14; cout << max( il, is); cout << max( is, d); }
2015.02.27. C++ Russia 2015 5 Compile-time vs. Run-time Compile-time Run-timeCompile-time
2015.02.27. C++ Russia 2015 6 Compile-time vs. Run-time Compile-time Run-timeCompile-time 3 < 3.14
2015.02.27. C++ Russia 2015 7 Compile-time vs. Run-time Compile-time Run-timeCompile-time 3 > 2L 3 < 3.14 -1.0 < 0
2015.02.27. C++ Russia 2015 8 Compile-time vs. Run-time Compile-time Run-timeCompile-time 3 > 2L 3 < 3.14 -1.0 < 0 int std::string double long
2015.02.27. C++ Russia 2015 9 Motivation template <class T, class S> ? max( T a, S b) // How to define the return value? { if ( a > b ) return a; else return b; } int main() { short is = 3; long il = 2; double d = 3.14; cout << max( il, is); // long is ''better'' than short cout << max( is, d); // double is ''better'' than short }
2015.02.27. C++ Russia 2015 10 Compile-time vs. Run-time Run-time 3 > 2L 3 < 3.14 -1.0 < 0 int std::string double long Template design time Compile-time Template Instantiation
2015.02.27. C++ Russia 2015 11 Compile-time vs. Run-time Run-timeCompile-time Template Instantiation 3 > 2L 3 < 3.14 -1.0 < 0 int std::string double long T S Template design time
2015.02.27. C++ Russia 2015 12 Compile-time vs. Run-time Run-time 3 > 2L 3 < 3.14 -1.0 < 0 int std::string double long T S sizeof(T) < sizeof(S) Template design time Compile-time Template Instantiation
2015.02.27. C++ Russia 2015 13 Compile-time vs. Run-time Run-time 3 > 2L 3 < 3.14 -1.0 < 0 int std::string double long T S sizeof(T) < sizeof(S) Template design time Compile-time Template Instantiation
2015.02.27. C++ Russia 2015 14 Motivation template <class T, class S> ? max( T a, S b) // How to define the return value? { if ( a > b ) return a; else return b; } int main() { short is = 3; long il = 2; double d = 3.14; cout << max( il, is); // long is ''better'' than short cout << max( is, d); // double is ''better'' than short }
2015.02.27. C++ Russia 2015 15 (de)Motivation template <class T, class S> auto max( T a, S b) -> decltype(a+b) // C++11 { if ( a > b ) return a; else return b; } int main() { short is = 3; long il = 2; double d = 3.14; cout << max( il, is); // -> long cout << max( is, d); // -> double }
2015.02.27. C++ Russia 2015 16 (de)Motivation template <class T, class S> typename std::common_type<T,S>::value max( T a, S b) // C++11 { if ( a > b ) return a; else return b; } int main() { short is = 3; long il = 2; double d = 3.14; cout << max( il, is); // -> long cout << max( is, d); // -> double }
2015.02.27. C++ Russia 2015 17 ● Run-time ● Compile-time
2015.02.27. C++ Russia 2015 18 ● Functions ● Values, literals ● Data structures ● If/else ● Loop ● Assignment ● May depend on input ● Run-time ● Compile-time
2015.02.27. C++ Russia 2015 19 ● Functions ● Values, literals ● Data structures ● If/else ● Loop ● Assignment ● May depend on input ● Metafunctions (type) ● Const, enum,constexpr ● Typelist (type) ● Pattern matching ● Recursion ● Ref. Transparency ● Deterministic ● Run-time ● Compile-time
2015.02.27. C++ Russia 2015 20 ● Imperative ● Object-oriented ● Generative ● (some) Functional ● Run-time ● Compile-time
2015.02.27. C++ Russia 2015 21 ● Imperative ● Object-oriented ● Generative ● (some) Functional ● Pure Functional ● Run-time ● Compile-time
2015.02.27. C++ Russia 2015 22 The usual factorial program ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; }; int main() { const int fact5 = Factorial<5>::value; }
2015.02.27. C++ Russia 2015 23 Bugs!!! ...
2015.02.27. C++ Russia 2015 24 The java programmer ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; } //; int main() { const int fact5 = Factorial<5>::value; }
2015.02.27. C++ Russia 2015 25 The java programmer ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; } //; int main() { const int fact5 = Factorial<5>::value; } $ clang++ fact.cpp fact.cpp:14:2: error: expected ';' after class } ^ ; 1 error generated.
2015.02.27. C++ Russia 2015 26 The vim user ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { ivalue = 1 }; }; int main() { const int fact5 = Factorial<5>::value; }
2015.02.27. C++ Russia 2015 27 The vim user ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { ivalue = 1 }; }; int main() { const int fact5 = Factorial<5>::value; } $ clang++ fact.cpp fact.cpp:5:34: error: no member named 'value' in 'Factorial<0>' enum { value = Factorial<N-1>::value * N }; ~~~~~~~~~~~~~~~~^ fact.cpp:5:18: note: in instantiation of template class 'Factorial<1>' requested here enum { value = Factorial<N-1>::value * N }; ^ fact.cpp:5:18: note: in instantiation of template class 'Factorial<2>' requested here enum { value = Factorial<N-1>::value * N }; ^ fact.cpp:5:18: note: in instantiation of template class 'Factorial<3>' requested here enum { value = Factorial<N-1>::value * N }; ^ fact.cpp:5:18: note: in instantiation of template class 'Factorial<4>' requested here enum { value = Factorial<N-1>::value * N }; ^ fact.cpp:16:21: note: in instantiation of template class 'Factorial<5>' requested here const int fact5 = Factorial<5>::value; ^ 1 error generated.
2015.02.27. C++ Russia 2015 28 The negative approach ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; }; int main() { const int fact5 = Factorial<-5>::value; }
2015.02.27. C++ Russia 2015 29 The negative approach ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; }; int main() { const int fact5 = Factorial<-5>::value; } $ clang++ fact4.cpp fact4.cpp:6:18: fatal error: recursive template instantiation exceeded maximum depth of 512 enum { value = Factorial<N-1>::value * N }; ^ fact4.cpp:6:18: note: in instantiation of template class 'Factorial<-517>' requested here enum { value = Factorial<N-1>::value * N }; Fact4.cpp:6:18: note: (skipping 503 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all) fact4.cpp:18:21: note: in instantiation of template class 'Factorial<-5>' requested here const int fact5 = Factorial<-5>::value; ^ fact4.cpp:6:18: note: use -ftemplate-depth=N to increase recursive template instantiation depth enum { value = Factorial<N-1>::value * N }; ^ 1 error generated.
2015.02.27. C++ Russia 2015 30 The greedy ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; }; int main() { const int fact5 = Factorial<-5>::value; } $ clang++ -ftemplate-depth=10000 fact4.cpp
2015.02.27. C++ Russia 2015 31 The greedy ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; }; int main() { const int fact5 = Factorial<-5>::value; } $ clang++ -ftemplate-depth=10000 fact4.cpp$ clang++ -ftemplate-depth=10000 fact4.cpp clang: error: unable to execute command: Segmentation fault clang: error: clang frontend command failed due to signal (use -v to see invocation) clang version 3.2 (branches/release_32 180710) Target: x86_64-unknown-linux-gnu Thread model: posix clang: note: diagnostic msg: PLEASE submit a bug report to http://llvm.org/bugs/ and include the crash backtrace, preprocessed source, and associated run script. clang: note: diagnostic msg: ******************** PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT: Preprocessed source(s) and associated run script(s) are located at: clang: note: diagnostic msg: /tmp/fact4-iy6zKp.cpp clang: note: diagnostic msg: /tmp/fact4-iy6zKp.sh clang: note: diagnostic msg: ********************
2015.02.27. C++ Russia 2015 32 We need tools ● C++ syntax is not designed for metaprogramming ● Compilers are not optimised for detecting and reporting template metaprogram errors ● Compilers are not optimised for template metaprogram execution ● Compiler internals are black box for most programmers ● Programmers have less experience with template metaprograms
2015.02.27. C++ Russia 2015 33 Tool support ● Pretty good support for run-time C++
2015.02.27. C++ Russia 2015 34 Tool support ● Pretty good support for run-time C++ ● Static analyzers, lint-like tools ● Debuggers ● Profilers ● Code comprehension tools ● Style checkers
2015.02.27. C++ Russia 2015 35 Tool support ● Pretty good support for run-time C++ ● Static analyzers, lint-like tools ● Debuggers ● Profilers ● Code comprehension tools ● Style checkers ● Tools for template metaprogramming
2015.02.27. C++ Russia 2015 36 Tool support ● Pretty good support for run-time C++ ● Static analyzers, lint-like tools ● Debuggers ● Profilers ● Code comprehension tools ● Style checkers ● Tools for template metaprogramming ● ?
2015.02.27. C++ Russia 2015 37 Tool support Run-time Compile-time
2015.02.27. C++ Russia 2015 38 Tool support Run-time Compile-time
2015.02.27. C++ Russia 2015 39 Tool support Run-time Compile-time
2015.02.27. C++ Russia 2015 40 Tool support Run-time Compile-time
2015.02.27. C++ Russia 2015 41 Tool support Run-time Compile-time
2015.02.27. C++ Russia 2015 42 Related work ● Debugging ● Static assert/Concept check (Siek-Lumsdaine, McNamara-Smaragdakis, Alexandrescu, others...) ● Warning generation (many attempt) ● Instrumentation ● Profiling ● Measuring full compilation (Gurtovoy-Abrahams) ● Measuring warning apperance (Watanabe) ● Visualize ● Source execution ● Instantiation graph
2015.02.27. C++ Russia 2015 43 Preprocessor Instrumentator Compiler Warning parser Positioner #line mapping Trace file Position correct trace Instrumented source Preprocessed source template<int i> struct Factorial { /* ---------------- begin inserted ----------- */ struct _TEMPLIGHT_0s { int a; }; enum { _TEMPLIGHT_0 = Templight::ReportTemplateBegin<_TEMPLIGHT_0s, &_TEMPLIGHT_0s::a>::Value }; /* ----------------- end inserted ------------ */ enum { value = Factorial<i-1>::value }; /* ---------------- begin inserted ----------- */ struct _TEMPLIGHT_1s { int a; }; enum { _TEMPLIGHT_1 = Templight::ReportTemplateEnd<_TEMPLIGHT_1s, &_TEMPLIGHT_1s::a>::Value }; /* ----------------- end inserted ------------ */ }; template<> struct Factorial<1> { /* ---------------- begin inserted ----------- */ struct _TEMPLIGHT_2s { int a; }; enum { _TEMPLIGHT_2 = Templight::ReportTemplateBegin<_TEMPLIGHT_2s, &_TEMPLIGHT_2s::a>::Value }; /* ----------------- end inserted ------------ */ enum { value = 1 }; /* ---------------- begin inserted ----------- */ struct _TEMPLIGHT_3s { int a; }; enum { _TEMPLIGHT_3 = Templight::ReportTemplateEnd< _TEMPLIGHT_3s, &_TEMPLIGHT_3s::a>::Value }; /* ----------------- end inserted ------------ */ }; GPCE 2006: Porkoláb, Mihalicza, Sipos: Debugging C++ template metaprograms
2015.02.27. C++ Russia 2015 44 Instrumentation ● Advantages ● Light-way approach (compared to compiler hack) ● Grammar support (we used wave) ● Easier to port: just change the warning generator
2015.02.27. C++ Russia 2015 45 Instrumentation ● Advantages ● Light-way approach (compared to compiler hack) ● Grammar support (we used wave) ● Easier to port: just change the warning generator ● Disadvantages ● Complex constructs are hard (e.g. inheritance) ● Serious distortion in profiling information ● Memoization not detected
2015.02.27. C++ Russia 2015 46 Templight 2.0 ● Based on LLVM/Clang compiler infrastructure ● Patch to ● Detect/measure instantiation ● Detect memoization ● Put timestamp on events ● Measure memory consumption (optional) ● Emit trace in various formats (txt, YAML, XML) ● Front-end tools ● Visual debugger ● Profiler data viewer
2015.02.27. C++ Russia 2015 47 Templight 2.0 Trace C++ C++ C++ Templight patch 3rd party tools Clang source Interactive debugger/ visualizer Instantiation time and memory profiler
2015.02.27. C++ Russia 2015 48 Installation ● Visit http://plc.inf.elte.hu/templight ● Download templight-<timestamp>.tar.gz ● Contains clang patch and the two frontends ● Download Clang source ● Patch and build clang ● Build front-end tools (optional) ● >=Qt 4.6 and >=Graphviz 2.28.0 required ● $ qmake; make
2015.02.27. C++ Russia 2015 49 How to use template<int N> struct Fib { static const int value = Fib<N-2>::value + Fib<N-1>::value; }; template<> struct Fib<0> { static const int value = 0; }; template<> struct Fib<1> { static const int value = 1; }; int main() { static const int fib5 = Fib<5>::value; }
2015.02.27. C++ Russia 2015 50 How to use $ clang++ -templight fib.cpp $ ls fib.cpp.trace.xml $ wc fib.cpp.trace.xml 123 275 3838 fib.cpp.trace.xml $ head fib.cpp.trace.xml <?xml version="1.0" standalone="yes"?> <Trace> <TemplateBegin> <Kind>TemplateInstantiation</Kind> <Context context = "Fib&lt;5&gt;"/> <PointOfInstantiation>fib.cpp|22| 14</PointOfInstantiation> <TimeStamp time = "421998401.188854"/> <MemoryUsage bytes = "0"/> </TemplateBegin> <TemplateBegin>
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2015.02.27. C++ Russia 2015 64 Filters #include <iostream> template<int N> struct Fib { static const int value = Fib<N-2>::value + Fib<N-1>::value; }; template<> struct Fib<0> { static const int value = 0; }; template<> struct Fib<1> { static const int value = 1; }; int main() { std::cout << Fib<5>::value << std::endl; return 0; }
2015.02.27. C++ Russia 2015 65 Filters $ clang++ -templight fib.cpp $ ls fib.cpp.trace.xml $ wc fib.cpp.trace.xml 18291 41765 738233 fib.cpp.trace.xml $ head fib.cpp.trace.xml <?xml version="1.0" standalone="yes"?> <Trace> <TemplateBegin> <Kind>DefaultTemplateArgumentInstantiation</Kind> <Context context = "std::basic_string"/> <PointOfInstantiation>/usr/lib64/gcc/x86_64-suse- linux/4.7/../../../../include/c++/4.7/bits/stringfwd.h|64| 11</PointOfInstantiation> <TimeStamp time = "421999330.595354"/>
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2015.02.27. C++ Russia 2015 75 Memory usage $ clang++ -templight-memory fib.cpp $ ls fib.cpp.memory.trace.xml $ wc fib.cpp.memory.trace.xml 18291 41765 756365 fib5.cpp.memory.trace.xml 0$ head fib.cpp.trace.xml <?xml version="1.0" standalone="yes"?> <Trace> <TemplateBegin> <Kind>TemplateInstantiation</Kind> <Context context = "Fib&lt;5&gt;"/> <PointOfInstantiation>fib.cpp|22| 14</PointOfInstantiation> <TimeStamp time = "421998401.188854"/> <MemoryUsage bytes = "647664"/> </TemplateBegin> <TemplateBegin>
2015.02.27. C++ Russia 2015 76 Distortion ● Safe-mode is installed ● Invalid profiling info ● Flush messages even the compiler crashes ● Internal buffer collects events ● Heap allocated, not growing, size = 500.000 ● Flush at end of compilation ● Distorsion < 3% ● clang++ -templight -trace-capacity=1000000
2015.02.27. C++ Russia 2015 77 Forks, Applications ● Martin Schulze modified client tools http://github.com/schulmar/Templar ● Malte Skarupke's blog: comparing instantiation time of unique_ptr, boost::flat_map, etc. http://probablydance.com/2014/04/05/reinventing -the-wheel-for-better-compile-time/ ● Ábel Sinkovics: comparing lazy and greedy metaprograms
2015.02.27. C++ Russia 2015 78 Evaluation of lazyness ● Ábel Sinkovics
2015.02.27. C++ Russia 2015 79 Metashell – interactive TMP REPL ● Ábel Sinkovics and András Kucsma ● Metashell https://github.com/sabel83/metashell ● Online demo: http://abel.web.elte.hu/shell
2015.02.27. C++ Russia 2015 80 Mikael Persson's Templight fork ● https://github.com/mikael-s-persson/templight ● https://github.com/mikael-s-persson/templight-tools ● Refactored and greatly improved Templight ● Patch is under review ● Tools: KCacheGrind format
2015.02.27. C++ Russia 2015 81 Our vision Trace 3rd party tools Debugger/ visualizer Other tools
2015.02.27. C++ Russia 2015 82 Summary ● Tool support for C++ metaprogramming ● Debugger/profiler requires compiler support ● Templight 2.0 based on clang ● Mikael's patch for clang is under review ● Please use it, give us feadback ● Compiler vendors, will you support Templight?
2015.02.27. C++ Russia 2015 83 Debugging and Profiling C++ Template Metaprograms http://plc.inf.elte.hu/templight gsd@elte.hu Thank you!

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In this document
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Introduction to C++ template metaprogramming and its debugging/profiling techniques, the Templight tool, and a call for community contribution.

Explores C++ template metaprograms including expression templates, compile-time adaptability, language extensions, and motivations through examples on templating.

Discusses the differences between compile-time and run-time operations in templates, utilizing code examples to illustrate concepts.

Demonstrates various methods of determining return types in templates using C++11 features, emphasizing on function implementation examples.

Highlights common bugs and errors faced while using templates with examples that illustrate compilation issues during template instantiation.

Emphasizes the lack of effective tools for debugging C++ templates and suggests Templight as a potential solution for better support.

Listing available tools and resources for debugging and profiling C++ templates, advocating for enhanced support and better tooling practices.

Explains instrumentation advantages for template debugging, describing techniques used in Templight to measure instantiation times and memory consumption.

Instructions on using Templight, including installation and tracing functionality with examples to visualize compilation events.

Discusses potential inaccuracies in profiling information due to various issues in template instantiation, proposing guidelines for better practices.

Showcases community contributions to Templight and invites future improvements, emphasizing the need for ongoing support and feedback.

Wraps up the presentation by revisiting the importance of debugging and profiling in C++ metaprogramming and urging viewers to engage with the project.

Debugging and Profiling C++ Template Metaprograms

  • 1.
    Debugging and Profiling C++Template Metaprograms Zoltán Porkoláb gsd@elte.hu Ericsson Hungary Eötvös Loránd University, Budapest
  • 2.
    2015.02.27. C++ Russia2015 2 ● C++ Template Metaprogramming ● Possible debugging and profiling techiques ● Templight back-end tool ● Front-end tools ● 3rd party applications – please, contribute! ● Vision Agenda
  • 3.
    2015.02.27. C++ Russia2015 3 C++ Template Metaprograms ● Expression templates (since 1995!) ● Active libraries, compile-time adaption ● Static interface checking ● Simulating language extensions ● DSL embedding ● Many other areas ...
  • 4.
    2015.02.27. C++ Russia2015 4 Motivation – a personal view template <class T, class S> ? max( T a, S b) // How to define the return value? { if ( a > b ) return a; else return b; } int main() { short is = 3; long il = 2; double d = 3.14; cout << max( il, is); cout << max( is, d); }
  • 5.
    2015.02.27. C++ Russia2015 5 Compile-time vs. Run-time Compile-time Run-timeCompile-time
  • 6.
    2015.02.27. C++ Russia2015 6 Compile-time vs. Run-time Compile-time Run-timeCompile-time 3 < 3.14
  • 7.
    2015.02.27. C++ Russia2015 7 Compile-time vs. Run-time Compile-time Run-timeCompile-time 3 > 2L 3 < 3.14 -1.0 < 0
  • 8.
    2015.02.27. C++ Russia2015 8 Compile-time vs. Run-time Compile-time Run-timeCompile-time 3 > 2L 3 < 3.14 -1.0 < 0 int std::string double long
  • 9.
    2015.02.27. C++ Russia2015 9 Motivation template <class T, class S> ? max( T a, S b) // How to define the return value? { if ( a > b ) return a; else return b; } int main() { short is = 3; long il = 2; double d = 3.14; cout << max( il, is); // long is ''better'' than short cout << max( is, d); // double is ''better'' than short }
  • 10.
    2015.02.27. C++ Russia2015 10 Compile-time vs. Run-time Run-time 3 > 2L 3 < 3.14 -1.0 < 0 int std::string double long Template design time Compile-time Template Instantiation
  • 11.
    2015.02.27. C++ Russia2015 11 Compile-time vs. Run-time Run-timeCompile-time Template Instantiation 3 > 2L 3 < 3.14 -1.0 < 0 int std::string double long T S Template design time
  • 12.
    2015.02.27. C++ Russia2015 12 Compile-time vs. Run-time Run-time 3 > 2L 3 < 3.14 -1.0 < 0 int std::string double long T S sizeof(T) < sizeof(S) Template design time Compile-time Template Instantiation
  • 13.
    2015.02.27. C++ Russia2015 13 Compile-time vs. Run-time Run-time 3 > 2L 3 < 3.14 -1.0 < 0 int std::string double long T S sizeof(T) < sizeof(S) Template design time Compile-time Template Instantiation
  • 14.
    2015.02.27. C++ Russia2015 14 Motivation template <class T, class S> ? max( T a, S b) // How to define the return value? { if ( a > b ) return a; else return b; } int main() { short is = 3; long il = 2; double d = 3.14; cout << max( il, is); // long is ''better'' than short cout << max( is, d); // double is ''better'' than short }
  • 15.
    2015.02.27. C++ Russia2015 15 (de)Motivation template <class T, class S> auto max( T a, S b) -> decltype(a+b) // C++11 { if ( a > b ) return a; else return b; } int main() { short is = 3; long il = 2; double d = 3.14; cout << max( il, is); // -> long cout << max( is, d); // -> double }
  • 16.
    2015.02.27. C++ Russia2015 16 (de)Motivation template <class T, class S> typename std::common_type<T,S>::value max( T a, S b) // C++11 { if ( a > b ) return a; else return b; } int main() { short is = 3; long il = 2; double d = 3.14; cout << max( il, is); // -> long cout << max( is, d); // -> double }
  • 17.
    2015.02.27. C++ Russia2015 17 ● Run-time ● Compile-time
  • 18.
    2015.02.27. C++ Russia2015 18 ● Functions ● Values, literals ● Data structures ● If/else ● Loop ● Assignment ● May depend on input ● Run-time ● Compile-time
  • 19.
    2015.02.27. C++ Russia2015 19 ● Functions ● Values, literals ● Data structures ● If/else ● Loop ● Assignment ● May depend on input ● Metafunctions (type) ● Const, enum,constexpr ● Typelist (type) ● Pattern matching ● Recursion ● Ref. Transparency ● Deterministic ● Run-time ● Compile-time
  • 20.
    2015.02.27. C++ Russia2015 20 ● Imperative ● Object-oriented ● Generative ● (some) Functional ● Run-time ● Compile-time
  • 21.
    2015.02.27. C++ Russia2015 21 ● Imperative ● Object-oriented ● Generative ● (some) Functional ● Pure Functional ● Run-time ● Compile-time
  • 22.
    2015.02.27. C++ Russia2015 22 The usual factorial program ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; }; int main() { const int fact5 = Factorial<5>::value; }
  • 23.
    2015.02.27. C++ Russia2015 23 Bugs!!! ...
  • 24.
    2015.02.27. C++ Russia2015 24 The java programmer ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; } //; int main() { const int fact5 = Factorial<5>::value; }
  • 25.
    2015.02.27. C++ Russia2015 25 The java programmer ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; } //; int main() { const int fact5 = Factorial<5>::value; } $ clang++ fact.cpp fact.cpp:14:2: error: expected ';' after class } ^ ; 1 error generated.
  • 26.
    2015.02.27. C++ Russia2015 26 The vim user ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { ivalue = 1 }; }; int main() { const int fact5 = Factorial<5>::value; }
  • 27.
    2015.02.27. C++ Russia2015 27 The vim user ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { ivalue = 1 }; }; int main() { const int fact5 = Factorial<5>::value; } $ clang++ fact.cpp fact.cpp:5:34: error: no member named 'value' in 'Factorial<0>' enum { value = Factorial<N-1>::value * N }; ~~~~~~~~~~~~~~~~^ fact.cpp:5:18: note: in instantiation of template class 'Factorial<1>' requested here enum { value = Factorial<N-1>::value * N }; ^ fact.cpp:5:18: note: in instantiation of template class 'Factorial<2>' requested here enum { value = Factorial<N-1>::value * N }; ^ fact.cpp:5:18: note: in instantiation of template class 'Factorial<3>' requested here enum { value = Factorial<N-1>::value * N }; ^ fact.cpp:5:18: note: in instantiation of template class 'Factorial<4>' requested here enum { value = Factorial<N-1>::value * N }; ^ fact.cpp:16:21: note: in instantiation of template class 'Factorial<5>' requested here const int fact5 = Factorial<5>::value; ^ 1 error generated.
  • 28.
    2015.02.27. C++ Russia2015 28 The negative approach ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; }; int main() { const int fact5 = Factorial<-5>::value; }
  • 29.
    2015.02.27. C++ Russia2015 29 The negative approach ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; }; int main() { const int fact5 = Factorial<-5>::value; } $ clang++ fact4.cpp fact4.cpp:6:18: fatal error: recursive template instantiation exceeded maximum depth of 512 enum { value = Factorial<N-1>::value * N }; ^ fact4.cpp:6:18: note: in instantiation of template class 'Factorial<-517>' requested here enum { value = Factorial<N-1>::value * N }; Fact4.cpp:6:18: note: (skipping 503 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all) fact4.cpp:18:21: note: in instantiation of template class 'Factorial<-5>' requested here const int fact5 = Factorial<-5>::value; ^ fact4.cpp:6:18: note: use -ftemplate-depth=N to increase recursive template instantiation depth enum { value = Factorial<N-1>::value * N }; ^ 1 error generated.
  • 30.
    2015.02.27. C++ Russia2015 30 The greedy ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; }; int main() { const int fact5 = Factorial<-5>::value; } $ clang++ -ftemplate-depth=10000 fact4.cpp
  • 31.
    2015.02.27. C++ Russia2015 31 The greedy ... template <int N> struct Factorial { enum { value = Factorial<N-1>::value * N }; }; template <> struct Factorial<0> { enum { value = 1 }; }; int main() { const int fact5 = Factorial<-5>::value; } $ clang++ -ftemplate-depth=10000 fact4.cpp$ clang++ -ftemplate-depth=10000 fact4.cpp clang: error: unable to execute command: Segmentation fault clang: error: clang frontend command failed due to signal (use -v to see invocation) clang version 3.2 (branches/release_32 180710) Target: x86_64-unknown-linux-gnu Thread model: posix clang: note: diagnostic msg: PLEASE submit a bug report to http://llvm.org/bugs/ and include the crash backtrace, preprocessed source, and associated run script. clang: note: diagnostic msg: ******************** PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT: Preprocessed source(s) and associated run script(s) are located at: clang: note: diagnostic msg: /tmp/fact4-iy6zKp.cpp clang: note: diagnostic msg: /tmp/fact4-iy6zKp.sh clang: note: diagnostic msg: ********************
  • 32.
    2015.02.27. C++ Russia2015 32 We need tools ● C++ syntax is not designed for metaprogramming ● Compilers are not optimised for detecting and reporting template metaprogram errors ● Compilers are not optimised for template metaprogram execution ● Compiler internals are black box for most programmers ● Programmers have less experience with template metaprograms
  • 33.
    2015.02.27. C++ Russia2015 33 Tool support ● Pretty good support for run-time C++
  • 34.
    2015.02.27. C++ Russia2015 34 Tool support ● Pretty good support for run-time C++ ● Static analyzers, lint-like tools ● Debuggers ● Profilers ● Code comprehension tools ● Style checkers
  • 35.
    2015.02.27. C++ Russia2015 35 Tool support ● Pretty good support for run-time C++ ● Static analyzers, lint-like tools ● Debuggers ● Profilers ● Code comprehension tools ● Style checkers ● Tools for template metaprogramming
  • 36.
    2015.02.27. C++ Russia2015 36 Tool support ● Pretty good support for run-time C++ ● Static analyzers, lint-like tools ● Debuggers ● Profilers ● Code comprehension tools ● Style checkers ● Tools for template metaprogramming ● ?
  • 37.
    2015.02.27. C++ Russia2015 37 Tool support Run-time Compile-time
  • 38.
    2015.02.27. C++ Russia2015 38 Tool support Run-time Compile-time
  • 39.
    2015.02.27. C++ Russia2015 39 Tool support Run-time Compile-time
  • 40.
    2015.02.27. C++ Russia2015 40 Tool support Run-time Compile-time
  • 41.
    2015.02.27. C++ Russia2015 41 Tool support Run-time Compile-time
  • 42.
    2015.02.27. C++ Russia2015 42 Related work ● Debugging ● Static assert/Concept check (Siek-Lumsdaine, McNamara-Smaragdakis, Alexandrescu, others...) ● Warning generation (many attempt) ● Instrumentation ● Profiling ● Measuring full compilation (Gurtovoy-Abrahams) ● Measuring warning apperance (Watanabe) ● Visualize ● Source execution ● Instantiation graph
  • 43.
    2015.02.27. C++ Russia2015 43 Preprocessor Instrumentator Compiler Warning parser Positioner #line mapping Trace file Position correct trace Instrumented source Preprocessed source template<int i> struct Factorial { /* ---------------- begin inserted ----------- */ struct _TEMPLIGHT_0s { int a; }; enum { _TEMPLIGHT_0 = Templight::ReportTemplateBegin<_TEMPLIGHT_0s, &_TEMPLIGHT_0s::a>::Value }; /* ----------------- end inserted ------------ */ enum { value = Factorial<i-1>::value }; /* ---------------- begin inserted ----------- */ struct _TEMPLIGHT_1s { int a; }; enum { _TEMPLIGHT_1 = Templight::ReportTemplateEnd<_TEMPLIGHT_1s, &_TEMPLIGHT_1s::a>::Value }; /* ----------------- end inserted ------------ */ }; template<> struct Factorial<1> { /* ---------------- begin inserted ----------- */ struct _TEMPLIGHT_2s { int a; }; enum { _TEMPLIGHT_2 = Templight::ReportTemplateBegin<_TEMPLIGHT_2s, &_TEMPLIGHT_2s::a>::Value }; /* ----------------- end inserted ------------ */ enum { value = 1 }; /* ---------------- begin inserted ----------- */ struct _TEMPLIGHT_3s { int a; }; enum { _TEMPLIGHT_3 = Templight::ReportTemplateEnd< _TEMPLIGHT_3s, &_TEMPLIGHT_3s::a>::Value }; /* ----------------- end inserted ------------ */ }; GPCE 2006: Porkoláb, Mihalicza, Sipos: Debugging C++ template metaprograms
  • 44.
    2015.02.27. C++ Russia2015 44 Instrumentation ● Advantages ● Light-way approach (compared to compiler hack) ● Grammar support (we used wave) ● Easier to port: just change the warning generator
  • 45.
    2015.02.27. C++ Russia2015 45 Instrumentation ● Advantages ● Light-way approach (compared to compiler hack) ● Grammar support (we used wave) ● Easier to port: just change the warning generator ● Disadvantages ● Complex constructs are hard (e.g. inheritance) ● Serious distortion in profiling information ● Memoization not detected
  • 46.
    2015.02.27. C++ Russia2015 46 Templight 2.0 ● Based on LLVM/Clang compiler infrastructure ● Patch to ● Detect/measure instantiation ● Detect memoization ● Put timestamp on events ● Measure memory consumption (optional) ● Emit trace in various formats (txt, YAML, XML) ● Front-end tools ● Visual debugger ● Profiler data viewer
  • 47.
    2015.02.27. C++ Russia2015 47 Templight 2.0 Trace C++ C++ C++ Templight patch 3rd party tools Clang source Interactive debugger/ visualizer Instantiation time and memory profiler
  • 48.
    2015.02.27. C++ Russia2015 48 Installation ● Visit http://plc.inf.elte.hu/templight ● Download templight-<timestamp>.tar.gz ● Contains clang patch and the two frontends ● Download Clang source ● Patch and build clang ● Build front-end tools (optional) ● >=Qt 4.6 and >=Graphviz 2.28.0 required ● $ qmake; make
  • 49.
    2015.02.27. C++ Russia2015 49 How to use template<int N> struct Fib { static const int value = Fib<N-2>::value + Fib<N-1>::value; }; template<> struct Fib<0> { static const int value = 0; }; template<> struct Fib<1> { static const int value = 1; }; int main() { static const int fib5 = Fib<5>::value; }
  • 50.
    2015.02.27. C++ Russia2015 50 How to use $ clang++ -templight fib.cpp $ ls fib.cpp.trace.xml $ wc fib.cpp.trace.xml 123 275 3838 fib.cpp.trace.xml $ head fib.cpp.trace.xml <?xml version="1.0" standalone="yes"?> <Trace> <TemplateBegin> <Kind>TemplateInstantiation</Kind> <Context context = "Fib&lt;5&gt;"/> <PointOfInstantiation>fib.cpp|22| 14</PointOfInstantiation> <TimeStamp time = "421998401.188854"/> <MemoryUsage bytes = "0"/> </TemplateBegin> <TemplateBegin>
  • 51.
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    2015.02.27. C++ Russia2015 64 Filters #include <iostream> template<int N> struct Fib { static const int value = Fib<N-2>::value + Fib<N-1>::value; }; template<> struct Fib<0> { static const int value = 0; }; template<> struct Fib<1> { static const int value = 1; }; int main() { std::cout << Fib<5>::value << std::endl; return 0; }
  • 65.
    2015.02.27. C++ Russia2015 65 Filters $ clang++ -templight fib.cpp $ ls fib.cpp.trace.xml $ wc fib.cpp.trace.xml 18291 41765 738233 fib.cpp.trace.xml $ head fib.cpp.trace.xml <?xml version="1.0" standalone="yes"?> <Trace> <TemplateBegin> <Kind>DefaultTemplateArgumentInstantiation</Kind> <Context context = "std::basic_string"/> <PointOfInstantiation>/usr/lib64/gcc/x86_64-suse- linux/4.7/../../../../include/c++/4.7/bits/stringfwd.h|64| 11</PointOfInstantiation> <TimeStamp time = "421999330.595354"/>
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    2015.02.27. C++ Russia2015 75 Memory usage $ clang++ -templight-memory fib.cpp $ ls fib.cpp.memory.trace.xml $ wc fib.cpp.memory.trace.xml 18291 41765 756365 fib5.cpp.memory.trace.xml 0$ head fib.cpp.trace.xml <?xml version="1.0" standalone="yes"?> <Trace> <TemplateBegin> <Kind>TemplateInstantiation</Kind> <Context context = "Fib&lt;5&gt;"/> <PointOfInstantiation>fib.cpp|22| 14</PointOfInstantiation> <TimeStamp time = "421998401.188854"/> <MemoryUsage bytes = "647664"/> </TemplateBegin> <TemplateBegin>
  • 76.
    2015.02.27. C++ Russia2015 76 Distortion ● Safe-mode is installed ● Invalid profiling info ● Flush messages even the compiler crashes ● Internal buffer collects events ● Heap allocated, not growing, size = 500.000 ● Flush at end of compilation ● Distorsion < 3% ● clang++ -templight -trace-capacity=1000000
  • 77.
    2015.02.27. C++ Russia2015 77 Forks, Applications ● Martin Schulze modified client tools http://github.com/schulmar/Templar ● Malte Skarupke's blog: comparing instantiation time of unique_ptr, boost::flat_map, etc. http://probablydance.com/2014/04/05/reinventing -the-wheel-for-better-compile-time/ ● Ábel Sinkovics: comparing lazy and greedy metaprograms
  • 78.
    2015.02.27. C++ Russia2015 78 Evaluation of lazyness ● Ábel Sinkovics
  • 79.
    2015.02.27. C++ Russia2015 79 Metashell – interactive TMP REPL ● Ábel Sinkovics and András Kucsma ● Metashell https://github.com/sabel83/metashell ● Online demo: http://abel.web.elte.hu/shell
  • 80.
    2015.02.27. C++ Russia2015 80 Mikael Persson's Templight fork ● https://github.com/mikael-s-persson/templight ● https://github.com/mikael-s-persson/templight-tools ● Refactored and greatly improved Templight ● Patch is under review ● Tools: KCacheGrind format
  • 81.
    2015.02.27. C++ Russia2015 81 Our vision Trace 3rd party tools Debugger/ visualizer Other tools
  • 82.
    2015.02.27. C++ Russia2015 82 Summary ● Tool support for C++ metaprogramming ● Debugger/profiler requires compiler support ● Templight 2.0 based on clang ● Mikael's patch for clang is under review ● Please use it, give us feadback ● Compiler vendors, will you support Templight?
  • 83.
    2015.02.27. C++ Russia2015 83 Debugging and Profiling C++ Template Metaprograms http://plc.inf.elte.hu/templight gsd@elte.hu Thank you!