Profiling round-off errors with CADNA

    The CADNA (Control of Accuracy and Debugging for Numerical Applications) library estimates round-off errors in numerical simulations.

    See the documentation in doc/ug_cadna.pdf in the CADNA source code distribution for details.

    Codes written with Basilisk can be automatically instrumented using CADNA to control round-off errors.


    1. Get the latest C/C++ version of CADNA from the Download page and untar it.

    2. Apply this patch using:

    cd cadna_c-2.0.2/
    patch -p1 < $BASILISK/cadna.patch

    This is necessary to add missing double/float to bool conversion operators.

    1. Then follow the installation instructions. On most systems, something like:
    sudo make install

    should be enough.

    1. Install the clang C++ compiler. This is necessary since limitations in g++ mean it cannot be used to compile Basilisk. On Debian-based systems, just do:
    sudo apt-get install clang
    1. In the Basilisk source code, check that the CADNA section of your config file is filled properly. In particular, if you have installed CADNA in a non-standard location (e.g. not /usr/local), you will need to add the corresponding -L/path/to/cadna/lib option to the CADNALIBS variable.

    2. Recompile Basilisk.


    To compile and run with CADNA support, you can use the default Makefile recipe:

    make mytest.CADNA.tst

    Alternatively, you can also compile the code manually using:

    qcc -cadna -Wall -O2 mytest.c -o mytest -lcadnaC -lstdc++ -lm

    where the linking arguments follow those given in CADNALIBS.

    Note that both the compilation and the execution of the code are much slower (10 to 100 times) than without CADNA.

    The printf() formatting of the floating-point results is changed to reflect the number of significant digits, and CADNA diagnostics are written on standard output. See the documentation for details.

    Known bugs

    • The ternary operator can cause trouble when compiling with C++. For example one may get a message looking like:
    cyl_axi.c:124:46: error: conditional expression is ambiguous; 'int' can be
          converted to 'double_st' and vice versa
      ...(double_st)0.1 ? 0 : (double_st)0.5*sq((double_st)0.1)*(double_st)0.5/y);

    The solution here is to replace the ‘0’ constant with ‘0.’, to explicitly cast it to a double. Note that this also holds for the min(), max() and clamp() macros which are implemented using the ternary operator.