CppMem - An Overview

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CppMem is an interactive tool for exploring the behaviour of small code snippets of the C++ memory model. It should, no it has to be in the toolbox of each programmer, who deals seriously with the memory model.

 

The online tool CppMem provides in twofold way very valuable services.

  1. CppMem verifies the well-defined behaviour of small code snippets. The tool performs on the base of the C++ memory model all possible interleavings of threads, visuals each one in a graph and annotates these graphs with additional details
  2. The very accurate analysis of CppMem gives you a deep insight into the C++ memory model. To make it short. CppMem is the tool for a better understanding of the C++ memory model.

Of course, it's in the nature of the things, that you have at first to overcome a few hurdles. The nature of the things is, that CppMem gives you the whole details to the extremely challenging topic and is highly configurable. So, my plan is to present to you the components of the tool, which you can also install on your PC.

The overview

My simplified overview uses the default configuration. This overview should give you the base for further experiments.

 CppMemUeberblickNumbers

For simplicity reasons, I will follow the red numbers in the screenshot. 

  1. Model
    • Specifies the C++ memory model. preferred is the C++ memory model.
  2. Program
    • Is the executable program in C or C++ syntax.
    • CppMem overs a bunch of programs to typical scenarios of multithreading. To get the details of these programs, read the very well written article Mathematizing C++ Concurrency. Of course, you can also use your own code.
    • CppMem is about multithreading, so there are two specialities.
      • You can easily define two thread by the symbols {{{  ... ||| ... }}}. The three dots (...) are the work packages of the two threads.
      • By using x.readvalue(1), you reduce the possible interleavings of the threads to these interleavings, for which the thread execution gives the value 1 for x.
  3. Display Relations
    • Describes the relations between the read, write and read-write modifications on atomic operations, fences and locks. 
    • You can explicitly enable the relations in the annotated graph with the switches.
    • There are three classes of relations. From my perspective, the coarser distinction between original and derived relations is the most interesting. Here are the default values.
      • Original relations:
        • sb: sequenced-before
        • rf: read from
        • mo: modification order
        • sc: sequentially consistent
        • lo: lock order
      • Dervived relations:
        • sw: synchronises-with
        • dob: dependency-ordered-before
        • unsequenced_races: races in a single thread
        • data_races
  4. Display Layout
    • You can choose with this switch, which Doxygraph graph is used.
  5. Model Predicates
    • To be honest, I have no glue, what this switch means. I didn't find anything in the documentation either.

For a deeper insight you have the official documentation. So, that's enough as staring point. Now it's time to press the run button. 

The test run

The run button shows it immediately. There is a data race.

 CppMemUeberblickNumbersRun 

  1. The data race is quite easy to see. A thread writes x (x = 3), another thread unsynchronized reads x (x==3). That can not work.
  2. Two interleavings of threads are possible due to the C++ memory model. Only one of them is consistent. That is the case if in the expression x==3 the value of x is written from the expression int x = 2 in the main function. The graph displays this relation in the edge annotated with rf and sw.
  3. It is extremely interesting to switch between the different interleaving of the threads.
  4. The graph shows in the format display layout all relations, which you enabled in the Display Relations.
    • a: Wna x=2 is in the graphic the a-th statement, which is a not atomic Write.
    • The key edge in the graph is the edge between the writing of x (b:Wna) and the reading of x (C:Rna). That's the data race on x: (data_race(dr)).

What's next?

That was the test run. I will in the next post analyse the simple program with the help of CppMem. You know, this program has undefined behaviour.

 

 

 

Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, Marko, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland, espkk, Louis St-Amour, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Neil Wang, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Tobi Heideman, Daniel Hufschläger, Red Trip, Alexander Schwarz, Tornike Porchxidze, Alessandro Pezzato, Evangelos Denaxas, Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky, Dimitrov Tsvetomir, Leo Goodstadt, Eduardo Velasquez, John Wiederhirn, and Yacob Cohen-Arazi.

 

Thanks in particular to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, Richard Sargeant, Rusty Fleming, and Said Mert Turkal.

 

 

My special thanks to Embarcadero CBUIDER STUDIO FINAL ICONS 1024 Small

 

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