Bug#569436: [Pkg-scicomp-devel] Bug#569436: Bug#569436: lapack: zgesvd seems to give incorrect results

[Kumar Appaiah]

Dear Sylvestre,

On Tue, Feb 16, 2010 at 03:35:28PM +0100, Sylvestre Ledru wrote:
> 
> > > Since SVD is something I consider very important, I would request you
> > > to please re-upload Blas with an optimization level of -O2, so that
> > > things remain sane with all the reverse dependencies.
> > OK, thank for your investigation. I will do that then.
> I had a look and the problem is that blas is compiled by default with
> -03. I am afraid of switching to -02 decreases the performance on a key
> component...
> However, I have a few leads:
> * disable optimisation for just this file

But which file? From what I can see, zgesvd in Lapack seems to
indirectly call several Blas functions. So, which file would you
compile with -O2?

> * see if forcing gcc-4.3 brings back the initial behavior

True, but this is not a permanent solution, since it is unlikely that
Squeeze will be shipped with two gcc's.

> * report upstream (which I will probably have to do anyway)

That is true. There is a fourth option:

* Use -O2 on amd64, and -O3 on the rest: this is a sane option, since
  Atlas is available anyway on amd64, and Atlas provides better
  performance than the corresponding Blas/Lapack libraries
  anyway. Would you be agreeable to this?

Thanks for looking into this.

Kumar
-- 
Bigotry tries to keep truth safe in its hand
With a grip that kills it.
- Rabindranath Tagore (Fireflies, 1928)