[SciPy-Dev] ODE solvers

[Sturla Molden]

On 12/06/15 12:39, Benny Malengier wrote:

> Warning
> This integrator is not re-entrant. You cannot have two ode
> <http://docs.scipy.org/doc/scipy-0.14.0/reference/generated/scipy.integrate.ode.html#scipy.integrate.ode>
> instances using the “lsoda” integrator at the same time.

By the way, no Fortran code can be assumed re-entrant because Fortran 
compilers are allowed to use an implicit SAVE attribute on local 
variables. One of the compilers which uses this misfeature is gfortran.

In gfortran, local variables larger than 32768 bytes have by default the 
SAVE attribute in gfortran. This is from the gfortran documentation:

-fmax-stack-var-size=n
This option specifies the size in bytes of the largest array that will 
be put on the stack; if the size is exceeded static memory is used 
(except in procedures marked as RECURSIVE). Use the option -frecursive 
to allow for recursive procedures which do not have a RECURSIVE 
attribute or for parallel programs. Use -fno-automatic to never use the 
stack. This option currently only affects local arrays declared with 
constant bounds, and may not apply to all character variables. Future 
versions of GNU Fortran may improve this behavior.

The default value for n is 32768.


And this is what happens when you try to use gfortran with pthreads and 
don't know about this "feature":

http://icl.cs.utk.edu/lapack-forum/viewtopic.php?t=1930



There is also a terrible WTF in the language which mandates that all 
local variables initialized within a procedure have the SAVE attribute. 
So if you write something like

    real :: foo(3) = [1., 2., 3.]
    real :: bar = 4.

the subroutine is no longer re-entrant. OuchFortran mandates that foo 
and bar are kept in static storage. Ouch!

In fact, there should be no difference between

    real, save :: bar = 4.

and

    real :: bar = 4.

Can you spot a bug like this in thousands of lines of Fortran code 
before you declare it thread-safe?


If we are to assume a piece of Fortran code is reentrant we should check 
it in the test suite. Because implicit SAVE is allowed, this is actually 
compiler dependent.



Sturla