OT: negative zeros with ATLAS 3.8.4.x

[John W. Eaton]

On 11-Apr-2011, Marco Caliari wrote:

| Dear maintainers,
| 
| the following code
| 
| clear all
| m = 45;
| y = linspace(0,1,m+1)';
| y = y(1:m);
| F = [0:m-1].'*2*pi*y.';
| F(:,1)
| 
| produces some strange negative zeros (in random positions) with Octave 
| 3.4.0 and ATLAS 3.8.4.x (x=1 or x=2, Core232SSE3), whereas everything is 
| fine with ATLAS 3.8.3. So, I suspect a bug in ATLAS (uninitialized 
| values?). Maybe someone is able to write a neat C code to submit to ATLAS 
| developers and/or to check that there is surely no fault in Octave.

Can you try to make the expression a little simpler?

Do you get the same result for

  m = 45;
  y = linspace(0,1,m+1);
  y = y(1:m);
  F = [0:m-1].'*2*pi*y;
  F(:,1)

?  What about

  m = 45;
  y = linspace(0,1,m+1);
  y = y(1:m);
  t = (0:m-1)'*2*pi;
  F = t*y;
  F(:,1)

?  Or

  m = 45;
  y = linspace(0,1,m+1);
  y = y(1:m);
  t = (0:m-1)';
  F = t*y;
  F(:,1)

?  If you can narrow it down to one matrix multiplication operation
then it should be simple to write the equivalent Fortran program
calling LAPACK.

jwe