[Bug 1811798]

[Jakub-gcc]

Yeah, like that problematic source, all gfortran options used to compile that, any needed modules too + stubbed whatever it calls and whatever is needed in MAIN__ or main to reproduce, ideally with minimal dependencies.
If you know the exact problematic routine, see in the debugger how many times it is called and in which invocation of the routine it misbehaves and try to capture on which arguments it is called.  If you don't know the exact problematic routine, one can e.g. play with assembly bisection between -funroll-loops and no -funroll-loops, rename .L* labels in one of them so that it can be merged by hand more easily.  I think gfortran doesn't have optimize (0) attribute yet.

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https://bugs.launchpad.net/bugs/1811798

Title:
  gcc 8 miscompiles scipy/optimize/minpack/qrsolv.f

Status in gcc:
  Incomplete
Status in gcc-8 package in Ubuntu:
  New
Status in imexam package in Ubuntu:
  New
Status in python-scipy package in Ubuntu:
  New
Status in python-scipy package in Debian:
  New

Bug description:
  Originally reported in https://bugs.debian.org/906198

  The following simple test fails on s390x:

  ---------------------------8<------------------------------
  import numpy as np
  from scipy.optimize import leastsq

  y = np.array([0., 1., 1., 2., 1., 1., 0.])
  x = np.arange(len(y))

  def func(par):
      return par[2] * np.exp(-(x - par[0])**2/par[1]) - y

  print(leastsq(func, [0,1,0]))
  ---------------------------8<------------------------------

  The expected result is

  (array([3.        , 4.42280548, 1.67210345]), 1)

  while on S390x, one gets

  (array([0.        , 1.        , 0.34027645]), 3)

  
  Further information from https://bugs.debian.org/915738

  The bug seems to be a miscompilation of scipy/optimize/minpack/qrsolv.f
  with -funroll-loops. Removing -funroll-loops is enough to get the
  leastsq testcase to pass.

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