Bug#569436: blas: zgesvd seems to give incorrect results

[Kumar Appaiah]

(Also CCing #571572)

Dear Denis,

On Sat, Feb 27, 2010 at 05:44:45PM +0100, Denis Barbier wrote:
> Hi,
> 
> I played with the example provided by Kumar Appaiah, and narrowed the
> problem down to zdrot; after copying zdrot.f into the same directory
> as zgesvd_ex.f:
>   $ gfortran -O2 -c zgesvd_ex.f
>   $ gfortran -O2 -c zdrot.f
>   $ gfortran -o zgesvd_ex zgesvd_ex.o zdrot.o -llapack
>   $ ./zgesvd_ex
> gives the expected result (with libblas3gf 1.2-4), but
>   $ gfortran -O2 -ftree-vectorize -c zdrot.f
>   $ gfortran -o zgesvd_ex zgesvd_ex.o zdrot.o -llapack
>   $ ./zgesvd_ex
> gives the wrong result.
> This looks like a bug in the gcc vectorizer, and it cannot be
> reproduced with gcc 4.5 from experimental.

This was fantastic analysis. I actually would like to know how you
zeroed in onto zdrot to find the problem.

I shall now try to play around with zdrot to see if I can create a
test case which reproduces the bug, so that I can file a bug report
with GCC.

Thanks so much!

Kumar
-- 
In this playhouse of infinite forms I have had my play, and here have
I caught sight of him that is formless.
- Rabindranath Tagore (Gitanjali, 1912)
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