[Bug 1811798]

[Krebbel]

I've tried building scipy 1.1.0 from github on a Fedora installation.
The build already uses -funroll-loops. But I couldn't reproduce the
problem with the resulting binary.

gcc version 8.0.1 20180324

Aurelien already tracked it down to a miscompilation of
scipy/optimize/minpack/qrsolv.f

This source file appears to contain just a single function (qrsolv)
which is not too big. I think I can work with that after being able to
reproduce the problem.

As Jakub mentioned the exact compiler cmdline would be good.

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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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