[graphite] Use of loops_mappings (SVN r138161)

[Tobias Grosser <tobi-grosser at web dot de>]

Hi Jan, hi Sebastian,

while looking in the graphite backend code I stepped over a commit
(r138161) introducing graphite_loops_mapping.

At the moment I do not understand perfectly for what it is used, as it
seems to mirror a feature I already implemented (see graphite.h):

---------------------------------------------------------------
/* LOOPS contains for every column in the graphite domain the
corresponding
     gimple loop. If there exists no corresponding gimple loop LOOPS
contains
     NULL. 
  
     Example:

     Original code:

     for (i = 0; i <= 20; i++) 
       for (j = 5; j <= 10; j++)
         A

     Original domain:

     (I)eq  i  j  1
     1      1  0  0   # i >= 0
     1     -1  0  20  # i <= 20
     1      0  1  0   # j >= 0
     1      0 -1  10  # j <= 10

     Original loops vector:
     0         1 
     Loop i    Loop j

     After some changes (Exchange i and j, strip-mine i):
     
     Domain:

     (I)eq  j  ii i  k  1
     1      0  0  1  0  0   # i >= 0
     1      0  0 -1  0  20  # i <= 20
     1      1  0  0  0  0   # j >= 0
     1     -1  0  0  0  10  # j <= 10
     1      0 -1  1  0  0   # ii <= i
     1      0  1 -1  0  1   # ii + 1 >= i 
     1      0 -1  0  2  0   # ii <= 2k
     1      0  1  0 -2  0   # ii >= 2k 

     Iterator vector:
     0        1        2         3
     Loop j   NULL     Loop i    NULL
    
     Means the original loop i is now at column two of the domain and
     loop j in the original loop nest is now at column 0.  Column 1 and
     3 are emtpy.  */
  VEC (loop_p, heap) *loops;


/* Returns the gimple loop, that corresponds to the loop_iterator_INDEX.
If there is no corresponding gimple loop, we return NULL.  */
static inline loop_p
gbb_loop_at_index (graphite_bb_p gb, int index);

/* Returns the corresponding loop iterator index for a gimple loop.  */
static inline int
gbb_loop_index (graphite_bb_p gb, loop_p loop);
---------------------------------------------------------------

During your commit you removed my code from the graphite_trans_*
functions.
Can you explain why you decided to replace GBB_LOOPS with loops_mapping?
Where there any shortcomings in my implementation or did you need some
different features?

Is GBB_LOOPS completely replaced by loops_mapping? In this case I will
remove the remaining parts of GBB_LOOPS.

It would be great, if you could add some documentation for
loops_mapping, as I am not sure, if I understand your code correct.

It seems, that you make a mapping from SCOP_LOOPS() to the index in the
cloog matrices. I think, that this is not always possible.

If we have for example a loop A in SCOP_LOOPS (), that we split during
graphite into two loops B1, B2. The first loop, B1, can be blocked the
second one, B2, not.
So we get for the A->B1 a different mapping as for A->B2.

Thanks

Tobi