[bug] [minuit] FPE/crash in Minuit2

[andriish]

Describe the bug

FPE when calling Simplex with no free parameters.

Expected behavior

No FPE/crash

To Reproduce

simplex.cxx

#include "Minuit2/FunctionMinimum.h"
#include "Minuit2/MnUserParameterState.h"
#include "Minuit2/MnPrint.h"
#include "Minuit2/MnMigrad.h"
#include "Minuit2/MnSimplex.h"
#include "Minuit2/FCNBase.h"

#include <fenv.h>


using namespace ROOT::Minuit2;

class TestChi2 : public ROOT::Minuit2::FCNBase
{
public:
    TestChi2() {}
    ~TestChi2() {}
    virtual double Up() const {return 1.0;}
    virtual double operator () (const std::vector<double> &a) const
    {
        return std::pow(a.at(0)-0.11,2)+std::pow(a.at(1)-0.5,2)/100.0;
    }
};

int main(int argc, char ** argv)
{
     feenableexcept(FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW);
     MnUserParameters fit_parameters;
     fit_parameters.Add("A",0.118,0.01,0,1);
     fit_parameters.Add("B",0.5,0.1,0.1,100.0);
     fit_parameters.Fix("A");
     fit_parameters.Fix("B");
     TestChi2* FULL= new TestChi2();
     MnUserParameters temp_fit_parameters=fit_parameters;
     MnMigrad migrad0(*FULL, temp_fit_parameters);           
     FunctionMinimum min_migrad0 = migrad0();                // OK
     MnSimplex simplex0(*FULL, temp_fit_parameters);
     FunctionMinimum min_simplex0 = simplex0();              // FPE
     exit(0);
}

g++ simplex.cc -o simplex.exe  $(root-config --glibs --cflags)  -lMinuit2
./simplex.exe

results in

The lines below might hint at the cause of the crash.
You may get help by asking at the ROOT forum http://root.cern.ch/forum
Only if you are really convinced it is a bug in ROOT then please submit a
report at http://root.cern.ch/bugs Please post the ENTIRE stack trace
from above as an attachment in addition to anything else
that might help us fixing this issue.
===========================================================
#5  ROOT::Minuit2::SimplexBuilder::Minimum (this=0x7ffcf9555c58, mfcn=..., seed=..., maxfcn=200, minedm=0.10000000000000001) at /usr/src/debug/root-6.22.02/math/minuit2/src/SimplexBuilder.cxx:48
#6  0x00007f045c93e1cd in ROOT::Minuit2::ModularFunctionMinimizer::Minimize (this=<optimized out>, mfcn=..., gc=..., seed=..., strategy=..., maxfcn=200, toler=<optimized out>) at /usr/src/debug/root-6.22.02/math/minuit2/src/ModularFunctionMinimizer.cxx:166
#7  0x00007f045c93c737 in ROOT::Minuit2::ModularFunctionMinimizer::Minimize (this=0x7ffcf9555c48, fcn=..., st=..., strategy=..., maxfcn=200, toler=0.10000000000000001) at /usr/src/debug/root-6.22.02/math/minuit2/src/ModularFunctionMinimizer.cxx:120
#8  0x00007f045c905c51 in ROOT::Minuit2::MnApplication::operator() (this=0x7ffcf9555b00, maxfcn=200, toler=0.10000000000000001) at /usr/src/debug/root-6.22.02/math/minuit2/src/MnApplication.cxx:57
#9  0x000000000041068c in main ()
===========================================================

Setup

CentOS7/gcc-4.8.5 root 6.22 from EPEL

Additional context

Not a personal priority of me, so it is unlikely that I will provode a patch in the next days.
However, I can briefly describe how the FPE occurs.
It occurs on line 48 in SimplexBuilder when all parameters are fixed and 1./double(n) is always 1/0. So it is just plain crash, not an FPE.
To avoid it the Simplex builder should not be called for the case when there are no free parameters but instead, ModularFunctionMinimizer::Minimize() should call FCn just once and return "current state".

And yes, just preempting a question about a fit with zero free parameters. Yes, it is very useful.
Imagine a set of different models fitted to the data using the same codebase. Some models have 2 free parameters, others 3, but some have 0. And for the last category because if this bug one cannot use simplex from ROOT. (I use migrad).

[andriish]

Hi @eguiraud ,

this is still present in 6.26.

Andrii

[eguiraud]

Hi @andriish , that is likely expected as the issue is still open. It is a question for @lmoneta what the strategy here is, let's ping him.

[andriish]

Present in 6.28