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Re: [Patch, Fortran] assumed-rank some bound intrinsics support, fix failures and improve diagnostcs


On Wed, Aug 1, 2012 at 2:37 PM, Tobias Burnus <burnus@net-b.de> wrote:
> On 08/01/2012 01:37 PM, Mikael Morin wrote:
>>>
>>> However, I found another spot where one needs to have a scalarizer;
>>> possibly a poor man's version is enough. Namely INTENT(OUT) handling.
>>
>> Indeed.
>>>
>>> Do you have an idea how to best handle that case?
>>
>> It seems some new code is necessary. I don't know how well it will
>> fit/reuse the existing though.
>
>
> I think we should try to get this working in some way for 4.8 as
> assumed-rank arrays will be used for the finalization wrapper - and it would
> be awesome to have FINAL support in 4.8.
>
> Background: As it is unknown (at compile time) whether a polymorphic
> variable has no final subroutines or one for that rank or an elemental one
> (or some but no suitable ones) - and as there could be a different
> combination for the parent type, the current plan is to add a _final
> proc-pointer to the vtable, which points to a final wrapper procedure for
> that type. It takes (at least for arrays) an assumed-rank array and
> dispatches the calls based on the rank; for an elemental final subroutine,
> it has to "scalarize it". [It's simple to add a special case as the array is
> contiguous - one just needs to "call elemental(base_address + i*elem_size)",
> where i = 1,size(assumed-size-array).]
>
> And for finalization, it would be great if one could use the INTENT(OUT)
> support. One could alternatively implement it manually on the gfortran AST
> level (gfc_code/gfc_expr) by walking through the derived type or one could
> implement a simplified version, making use of the contiguity of the
> finalized variable.
>
>
>
>> I have been thinking about rewriting the scalarizer in a way that would
>> need less bookkeeping to make things work. Nothing near a patch though,
>> and it's not something for 4.8.
>
>
> I think it would be good to base it on the new array descriptor, which we
> hopefully have by that time. Additionally, we should consider to support:
>
> a) ARRAY_RANGE_REF: That's probably somewhat independent of scalarization,
> but replaces it in some cases:
>   A(:,:,5) = B(:)
> can be implemented as ARRAY_RANGE_REF, if the memory is contiguous; one just
> passes an offset and (via the decl) the size of the array (section). See
> trans-expr.c for one example. A range ref is better than a memcpy/memmove or
> a loop - as the first looses the data type and some alias information and
> the second represents the structure in a more convoluted way. Either could
> be recovered by the middle end, but it currently isn't and doing it
> correctly from the beginning makes the ME life easier.
>
> b) Middle-end arrays. Richard made an initial patch, cf.
> http://gcc.gnu.org/wiki/GCCGathering2011Fortran . It probably needs some
> polishing and some optimizations have to be implemented, but then it should
> work and allow for further optimizations. [Description in the wiki might be
> partially wrong; blame me - and correct it, if you find something.]

Well, I wouldn't concentrate on this one ;)

> I think the latter requires also some thinking about how to handle arrays
> internally: In that case, the array has - at least for the scalarization -
> more than one rank (for the ME) while gfortran normally folds everything to
> rank-1 arrays. Additionally, one needs to think about the case where the
> array has nonunit strides, i.e. where the leftmost stride is not
> sizeof(declared type) but larger by a noninteger amount. (e.g. passing a
> polymorphic array to a TYPE.)

c) Do _not_ fold everything to rank-1 arrays (this makes data
dependence analysis
harder).  If you know the rank of an array use an intermediate array
pointer type
to access the data, like the following C example:

void foo (void *data, int n, int m)
{
  int (*a)[n][m] = (int (*)[n][m]) data;
  int i, j;
  for (i = 0; i < n; ++i)
    for (j = 0; j < m; ++j)
      (*a)[i][j] = 0;
}


d) Think about Frontend optimizations again - using the ISL part of GRAPHITE
on the GFortran IL, possibly driving the scalarizer with the result.

Richard.

> Tobias


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