https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98028
--- Comment #4 from Andrew Macleod <amacleod at redhat dot com> ---
I would expect in gcc 12 we'll be handling it differently than that.
I have equivalences/relations up and running and will eventually get to
applying those to general statements... which I haven't gotten to yet.
the gimple_fold() routine uses fold_range () via range-ops to calculate a
result. I plan to additionally check if there is a relation between the
operands, and if so, invoke another range_ops routine to see if that relation
makes any further adjustments.
in this case, we'd have VARYING - VARYING, so ranges would resolve as expected
to VARYING.
A query if there is a relation between j_1 and i_2
=========== BB 4 ============
j_1(D) unsigned int VARYING
i_2(D) unsigned int VARYING
Equivalence set : [j_1(D), i_2(D)]
<bb 4> :
_3 = i_2(D) - j_1(D);
return _3;
shows they are in an equivalence set, so that would be trivial to fold to 0 for
MINUS_EXPR.
as well, if we see other relations
if (j > i) __builtin_unreachable();
return i - j;
=========== BB 4 ============
j_1(D) unsigned int VARYING
i_2(D) unsigned int VARYING
Relational : (j_1(D) <= i_2(D))
<bb 4> :
_3 = i_2(D) - j_1(D);
return _3;
it would likewise be pretty straightforward to add the restriction that _3 will
be [0, +INF] (or [1, +INF] if it were j_1 < i_2)
If the relation is !=, we could also provide ~[0,0]
I'd expect the routine for MINUS_EXPR to simply switch on the possible
relations and intersect the current result with this reduced possible outcome.
I hope to get most of this in early in stage 1 and we can begin
enhancing/finding opportunities.
.
