blas-f77 compatibility check

[Jaroslav Hajek]

hello,

there have been recently some issues concerning building Octave with
BLAS libraries incompatible with the Fortran compiler used.
The problem in question was triggered by a call to ZDOTC from
qrupdate/zqrqhv.f. Currently, this is fixed by providing replacements
for ZDOTU and ZDOTC in blas-xtra/xzdotc.f and blas-xtra/xzdotu.f, and
changing the call in qrupdate/zqrhqhv.f to XZDOTC instead.

The attached changeset provides an alternative solution, that uses a
new ACX_BLAS_WITH_F77_FUNC macro from Peter Simons'
archive (see http://autoconf-archive.cryp.to/acx_blas_f77_func.html)
instead of plain ACX_BLAS in configure.in, imposing the constraint
that the BLAS library used for building Octave *must* be compatible
with the Fortran 77 compiler, otherwise it is rejected.

The advantages of this solution over the current one are:

1. It is possible to build Octave with user-supplied BLAS but no
LAPACK. In such case, the LAPACK subset in libcruft/lapack will be
used, and, to quote John's phrase (which for some reason I like a
lot), "all hell will break loose". This is currently not fixed, as it
would involve turning a lot of LAPACK's calls to ZDOTC and ZDOTU into
XZDOTU and XZDOTC. That would be of course an error-prone task, and it
would complicate LAPACK updates.

2. It does not involve modifying BLAS calls in other (ported or
written) Fortran sources. Currently the only other source in question
is qrupdate/zqrqhv.f, but more may be ported/written in the future.
The replacements also have a good chance of being slower than BLAS
versions (and they are used even in the compatible case).

3. When building packages containing Fortran sources with mkoctfile, I
consider it sort-of natural to assume that the Fortran compiler used
by mkoctfile is compatible with the BLAS_LIBS given by mkoctfile. (At
least that's what I had always assumed before this issue appeared). I
think that there should be some warning somewhere if this is not the
case, but I have no good idea where it should be.

The obvious drawback of adding this additional constraint is that it
would reduce the number of configurations Octave can be built with.
(But, given the above thoughts, these are problematic configurations anyway).


-- 
RNDr. Jaroslav Hajek
computing expert
Aeronautical Research and Test Institute (VZLU)
Prague, Czech Republic
url: www.highegg.matfyz.cz

blas_f77_func.diff
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