warnin about 'info' is a bug in eigs-base.cc

[Daniel J Sebald]

There are a string of warnings related to 'info' that I noted a couple 
posts ago:

In file included from ../../octave/libinterp/dldfcn/eigs.cc:39:0:
../../octave/liboctave/sparse-base-chol.h: In function 'bool 
make_cholb(SparseMatrix&, SparseMatrix&, ColumnVector&)':
../../octave/liboctave/sparse-base-chol.h:56:9: warning: 'info' may be 
used uninitialized in this function
../../octave/liboctave/eigs-base.cc:368:19: note: 'info' was declared here
[snip]

Those don't have anything to do with "octave.info".  'info' is a 
variable here.

First, I assume the following inclusion in eigs.c is intended:

#include "eigs-base.cc"

as I don't see an associated "eigs-base.h" header file.  But it is 
somewhat unconventional to include C++ code via the header route.

Anyway, these warnings are actually a bug.  In the constructor for 
sparse_base_chol_rep is variable "info" defined as a reference:

    sparse_base_chol_rep (const chol_type& a, octave_idx_type& info,
                          const bool natural)
      : count (1), Lsparse (0), Common (), is_pd (false), minor_p (0),
        perms (), cond (0)
      {
        info = init (a, natural, info);
      }

but the init member function definition is not a reference, but passed 
in via the stack and is called 'nargout'.

    octave_idx_type init (const chol_type& a, bool natural = true,
                          octave_idx_type nargout = 0);

So nargout = info as an input variable.  (info is also used as an output 
variable.)  I'm not sure what nargout stands for--possibly it is in 
reference to the command outputs, i.e., the usual scenario.

I do know there is some conditional code in sparse-base-chol.cc:

  if (is_pd || nargout > 1)
    {

So this is a bug in the sense that "info", being uninitialized, could be 
greater than one (or not) based on what's sitting randomly on the stack.

I believe what is happening is that the sparse code is doing some work 
in the background, and if at the point of the conditional test either 
the sparse matrix is positive definite (is_pd) or a wrong number of 
output arguments was given in the command (nargout), the background 
processing will be abandoned.

Judging from the conditional, the number of arguments can be 0 or 1 and 
after that there is no further use of nargout.  Therefore, I picked a 
value of 1 to initialize 'info'.  (See patch.)

If there is anyone who thinks there should be some way to determine 
'nargout' before issuing the constructor in these two cases (i.e., set 
'info' to something other than 1), and this is relevant at this point in 
the code (e.g., there isn't already something ahead of it 
sanity-checking the number of input and output arguments), please 
describe.  Right now, nargout seems like something extraneous given it 
will never be greater than 1 in the cases I've seen; I'm not 100% sure 
though.  It can be left as is, so long as something valid is passed into 
the constructor.

Dan

octave-sparsechol_info-2012aug29.patch
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