Re: [PATCH, PR69110] Don't return NULL access_fns in dr_analyze_indices

[Sebastian Pop]

On Sat, Jan 23, 2016 at 12:28 PM, Tom de Vries <tom_devr...@mentor.com> wrote:
> That was my original patch, and Richard commented: 'I think avoiding a NULL
> access_fns is ok but it should be done unconditionally, not only for the
> DECL_P case'. In order words, he asked me to do the exact opposite of the
> change you now propose.
>

In the case of a DECL_P it is correct to say that it has an access
function of 0.
In the graphite testcase it is not correct to say that the access
function for a given data reference is zero:
we only initialize access_fns in the case of a polynomial chrec:

  if (TREE_CODE (ref) == MEM_REF)
    {
      op = TREE_OPERAND (ref, 0);
      access_fn = analyze_scalar_evolution (loop, op);
      access_fn = instantiate_scev (before_loop, loop, access_fn);
      if (TREE_CODE (access_fn) == POLYNOMIAL_CHREC)
        {
[...]
           access_fns.safe_push (access_fn);
        }
    }

In all other cases we may not have a representation of the access functions.
It is incorrect to initialize to "A[0]" all those data references that
cannot be analyzed.
If needed, instead of returning vNULL, one could initialize the vector to empty:

if (access_fns == vNULL)
  access_fns.create (0);

and that would be correct, though it would not teach the dependence analysis
how to deal with the global variable access function in pr69110.
I think the fix is to add the zero subscript only for DECL_P (ref).

Sebastian