LAPACK/ScaLAPACK Development

For empty matrices the workspace query returns 0.0 but 0 is an illegal value for lwork. See the following program where I also show the result from dsyev

program test

implicit none
integer*8 :: lwork, info, ipiv(0)
double precision :: a(0,0), w(0), work(1)

lwork = -1_8
call dsyev('N', 'U', 0_8, a, 1_8, w, work, lwork, info)

write(*,*) work

lwork = -1_8
call dsytrf('U', 0_8, a, 1_8, ipiv, work, lwork, info)

write(*,*) work

end program test

Hi Andreas,

Thanks for reporting this. (1) The correct behavior would indeed be to return WORK(1) = 1 when N=0. (2) I have checked a few LAPACK subroutines and unfortunately we are quite inconsistent with respect to this. You reported DSYTRF. DGEQRF has the same issue. DGELS and DSYSV are fine. So I will try to correct DGEQRF and DSYTRF and we will take it from there. Not much point in starting a systematic check.

Cheers,
Julien.

This may be down to interpretation.
Routines may say something like "immediate return when N=0"
and may also say "WORK is not referenced N=0".

To be consistent you would have to have a rule as to which takes precedence:
the workspace query or the immediate return.
If the former then you would have to remove the "WORK not referenced comments"
from routines with workspace query mechanisms.

There might be different solutions, but what I wanted to avoid is illustrated in the the program below. If I run a work space query first and then use it I get the error message

** On entry to DSYTRF, parameter number 7 had an illegal value

program test

implicit none
integer :: lwork, info, ipiv(0)
double precision :: a(0,0), w(0), work(1)

lwork = -1
call dsyev('N', 'U', 0, a, 1, w, work, lwork, info)
lwork = work(1)
call dsyev('N', 'U', 0, a, 1, w, work, lwork, info)

lwork = -1
call dsytrf('U', 0, a, 1, ipiv, work, lwork, info)
lwork = work(1)
call dsytrf('U', 0, a, 1, ipiv, work, lwork, info)

end program test