[petsc-dev] TSComputeIFunction(): User or PETSc responsible for setting residual vector to zero?

[Brad Aagaard]

The PETSc manual pages do not mention whether or not the user is 
responsible for setting the entries in the function vector to zero in 
the user-defined ifunction and rhsfunction.

In looking at the code it is clear TSComputeIFunction() does not set the 
function vector entries to zero. For the function to be computed 
correctly with the current code, the user must zero the function Vec 
passed in to both the ifunction and rhsfunction. This seems like a bug 
to me, because TSComputeIFunction() could potentially use a single 
global Vec in computing the function from the ifunction and rhsfunction, 
so a user zeroing the entries would result in incorrect values.

It seems to be the code should behave like:

TSComputeIFunction calls VecSet(residual, 0.0) BEFOERE calling ifunction 
and rhsfunction.

User ifunction calls scatter with ADD_VALUES
User rhsfunction calls scatter with ADD_VALUES

This way the user code is agnostic as to whether TSComputeIFunction() 
passes in separate vecs to ifunction and rhsfunction or the same vec 
with the proper -1 scaling done in between.

Whether or not TSComputeIFunction() is changed to zero out the function 
Vec, I think the manual page should be updated to indicate to the user 
whether the user function or PETSc function is responsible for zeroing 
the function Vec.

Thanks,
Brad