Re: [gentoo-science] Atlas giving wrong results on matrix multiplication

["Xavier Fernández i Marín" <xfim.ll@gmail.com>]

Andrea Arteaga vas escriure el dia dl, 19 set 2011:

> Dear Xavier,
> 
> thank you for your email and sorry for the late reply.
> In theory it should be useless to provide optimization flags when
> compiling ATLAS, because it automatically uses and tests many flags,
> but I could be wrong.

Yes, Atlas tries to find the best options. However, I was worried because
I had -Ofast in CFLAGS, and thought that maybe some GCC optimizations were
harming the binary.

> 
> How did you install ATLAS? From the science overlay? I suggest that
> you update it. Now in the overlay we have sci-libs/atlas-3.9.49
> (~keyworded) [BTW, I will probably bump the version to 3.9.51 today].
> Could you install it and redo the test? Otherwise, could you shortly
> explain how to reproduce the problem, so that I can try?

No, I simply use the atlas provided in the official gentoo branch.

François sent an email pointing that there was a problem with 3.9-xx, and
suggesting going bach to 3.8. This is what I did and it is working fine,
without errors. So I assume that, effectively, it was something related
to the 3.9 branch, rather than compiling options.

A simplification of the code that was giving trouble is:

-----8<---------------
B <- c(0, 4.9039)
for (l in 10:100) {
  X <- cbind(1, matrix(rnorm(l, 0, 3), ncol=1))
  X <- ics
  for (i in 1:1000) {
    M <- X %*% B
    w <- which(is.nan(M))
    l <- length(w) 
    if (l!=0) {
      print(paste("Length", l, "error(s) on", X[w,2], sep=" "))
    }
  }
}
-----8<---------------

Thank you,
-- 
-  Xavier  -