[Bug 1875181] [NEW] dgesdd gets stuck for size 40+

[Dirk Toewe]

Public bug reported:

On Ubuntu 20.04 (Focal Fossa), the following code:

https://pastebin.com/A0bTqAAJ

gets stuck on the LAPACKE_dgesdd call using liblapacke-dev
(3.9.0-1build1). It seems to get stuck for every matrix size N>=40. I've
unsuccessfully tried out the following variations:

  * Different compilers (gcc-9, gcc-10, clang-10)
  * Linking to different BLAS implementations (libopenblas-dev, libblas-dev)
  * With and without Different optimization settings (-ffast-math, -march=native, -O3)

The CPU is an AMD Ryzen9 3900X (in case that's relevant).

What I've then tried is to build LAPACK(E) 3.9.0 from source directly
using cmake and the "make.inc.example" file included in the LAPACK
source code. And that compiled version works just fine!

Maybe there is bad optimization done by the compiler used for the Ubuntu
package?

My C/C++ (let alone Fortran) knowledge is quite limited, which means I
am going to need some help if I am to provide further debug information.

** Affects: lapack (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1875181

Title:
  dgesdd gets stuck for size 40+

Status in lapack package in Ubuntu:
  New

Bug description:
  On Ubuntu 20.04 (Focal Fossa), the following code:

  https://pastebin.com/A0bTqAAJ

  gets stuck on the LAPACKE_dgesdd call using liblapacke-dev
  (3.9.0-1build1). It seems to get stuck for every matrix size N>=40.
  I've unsuccessfully tried out the following variations:

    * Different compilers (gcc-9, gcc-10, clang-10)
    * Linking to different BLAS implementations (libopenblas-dev, libblas-dev)
    * With and without Different optimization settings (-ffast-math, -march=native, -O3)

  The CPU is an AMD Ryzen9 3900X (in case that's relevant).

  What I've then tried is to build LAPACK(E) 3.9.0 from source directly
  using cmake and the "make.inc.example" file included in the LAPACK
  source code. And that compiled version works just fine!

  Maybe there is bad optimization done by the compiler used for the
  Ubuntu package?

  My C/C++ (let alone Fortran) knowledge is quite limited, which means I
  am going to need some help if I am to provide further debug
  information.

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