[m-dev.] Re: tabled_for_io interacts badly with c code which contains labels

[Ian MacLarty]

On 9 Aug 2005, at 12:30, Zoltan Somogyi wrote:

> On 08-Aug-2005, Ian MacLarty <maclarty at cs.mu.OZ.AU> wrote:
>> Zoltan, could we wrap the original code in a procedure,
>
> In many cases, the body is much cheaper to execute than a function 
> call,
> so that would be a significant performance loss. However, there is no 
> reason
> why the author of the foreign_proc can't use that technique where it is
> needed.
>

I don't agree with requiring the author to change their code to make it 
work with I/O tabling.  This might mean existing  libraries which make 
use of foreign code would need to be modified to work with the 
declarative debugger which is undesirable.

>> or change the
>> I/O tabling transformation somehow to overcome this problem?
>
> We could modify the transformation to put something like
> "#define MR_IO_TABLING_COPY_NUM 1" before one copy and
> "#define MR_IO_TABLING_COPY_NUM 2" before the other, and let the C code
> include that macro in the label name.
>

This has the same problem where the implementor of the foreign proc 
must be aware of I/O tabling and the limitations it imposes.

What about changing the transformation to something like the following?

p(A, B, S0, S) :-
   (if
           % Get the global I/O table, the global I/O
           % counter, and the starting point for tabling
           % I/O actions, if we are in the tabled range.
       table_io_in_range(T0, Counter, Start)
   then
           % Look up the input arguments.
       impure table_lookup_insert_start_int(T0, Counter,
           Start, T),
       (if
           semipure table_io_has_occurred(T)
       then
           semipure table_memo_get_answer_block(T, Block),
           impure table_restore_string_answer(Block, 0, B),

           RunOrigCode = no,
           SaveAnswers = no
       else
           RunOrigCode = yes,
           SaveAnswers = yes
       )
   else
       RunOrigCode = yes,
       SaveAnswers = no
   ),
   (
       RunOrigCode = yes,
       <original code>,
       (
           SaveAnswers = yes,
           % Save the answers in the table.
           impure table_io_create_answer_block(T, 1, Block),
           impure table_save_string_answer(Block, 0, B)
       ;
           SaveAnswers = no
       )
   ;
       RunOrigCode = no,
       table_io_copy_io_state(S0, S)
   ).

Surely not copying the code would be more cache efficient?

Ian.

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