[Bug 1811798] [NEW] gcc 8 miscompiles scipy/optimize/minpack/qrsolv.f

[Graham Inggs]

Public bug reported:

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.

** Affects: gcc-8 (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: imexam (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: python-scipy (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: python-scipy (Debian)
     Importance: Unknown
         Status: Unknown

** Bug watch added: Debian Bug tracker #915738
   https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=915738

** Also affects: python-scipy (Debian) via
   https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=915738
   Importance: Unknown
       Status: Unknown

** Also affects: imexam (Ubuntu)
   Importance: Undecided
       Status: New

** Also affects: gcc-8 (Ubuntu)
   Importance: Undecided
       Status: New

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

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

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:
  Unknown

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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