Hello,
> I am trying to understand the usage of some functions in tree-affine.c
> file and I appreciate your help.
>
> For example; for the two memory accesses
> arr[b+8].X and arr[b+9].X, how does their affine combinations
> will look like after executing the following sequence of operation?
> (taken from http://gcc.gnu.org/ml/gcc-patches/2007-01/msg02605.html)
>
> get_inner_reference_aff (mem1, &off1, &size1);
> get_inner_reference_aff (mem2, &off2, &size2);
off1 will be affine combination 1 * &arr + 8 * b + 64,
off2 will be 1 * &arr + 8 * b + 72 (assuming that size of the element of arr
is 8),
> aff_combination_expand (&off1, ttae_cache);
> aff_combination_expand (&off2, ttae_cache);
In these cases, off1 and off2 will be unchanged, since b is not defined
in terms of an affine expression.
> aff_combination_scale (&off1, double_int_minus_one);
> aff_combination_add (&off2, &off1);
This subtracts the two affine combinations, so off2 will be 8 in the
end.
> Also, why does diff->n is an indication for the fact that two memory
> accesses can overlap in the following snippet taken from the above link?
n is the number of terms of the affine combination, excluding the
constant offset. For example, off1 above has n = 2, the terms of the affine
combination (&arr, b) and their coefficients (1, 8) are kept in the
elts array, the constant offset (64) is kept in the offset field.
So, if there is any non-constant term (i.e., n > 0), we cannot be sure
that the difference of the addresses is non-zero (actually, this could
be improved -- we could consider the value of the offset modulo the
greatest common divisor of the coefficients, and in some cases derive
that there cannot be an overlap).
Zdenek
> static bool
> cannot_overlap_p (aff_tree *diff, double_int size1, double_int size2)
> {
> double_int d, bound;
>
> /* Unless the difference is a constant, we fail. */
> if (diff->n != 0)
> return false;
>
> Thanks in advance,
> Revital
