--- Comment #1 from grosser at gcc dot gnu dot org 2008-10-16 05:20 ---
Created an attachment (id=16505)
-- (http://gcc.gnu.org/bugzilla/attachment.cgi?id=16505action=view)
Fix loop_affine_expr to remove limit_scops () - This is a HACK looking for
smarter solution
Problem 1:
Scalar evolution functions in loop limits and conditions have to be constant or
affine to be allowed in a SCoP.
During SCoP detection we check this with:
if (evolution_function_is_invariant_p (scev, n)
|| evolution_function_is_affine_multivariate_p (scev, n));
in loop_affine_expr().
If this expression is true, it is allowed in a SCoP.
But e.g. the constant expr p_1 * p_2 is allow in a SCoP, but we can not
represent it using the polyhedron model.
The matrix would be
p_1 p_2 Const
xyz
with x*p_1 + y*p_2 + z
There is no way to represent p_1 * p_2, so we fail in scan_tree_for_params
trying to generate the matrix.
There are two solutions:
1. This expression is representable, if we introduce a new parameter p_3 = p_1
* p_2 and calculate it before the SCoP.
2. We detect this cases and do not allow them in any SCoP.
I would like to start with solution two. Later on we can implement solution
one.
I attached a patch, that simply copies scan_tree_for_params() and returns true,
if a expression can be handled by scan_tree_for_params() otherwise false.
This works on polyhedron using -fgraphite, but seems not to be the cleanest
solution. I would prefer to keep
if (evolution_function_is_invariant_p (scev, n)
|| evolution_function_is_affine_multivariate_p (scev, n));
and add only
(no_multiplication_in_parameters (scev)).
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37705
