http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55555
--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> 2012-12-17
12:14:18 UTC ---
Ok, I'm confused by the following:
<bb 3>: (loop 1 header)
# lxp_1 = PHI <0(2), lxp_24(12)>
t_9 = pol_x[lxp_1];
_10 = (long int) lxp_1;
_11 = _10 * 4;
l_12 = _11 + -1;
goto <bb 5>;
<bb 4>:
_15 = S_2 + l_12;
_16 = coef_x[_15];
_17 = S_2 + -1;
_18 = s[_17];
_19 = _18 * t_9;
_20 = _16 + _19;
coef_x[_15] = _20;
S_22 = S_2 + 1;
<bb 5>: (loop 2 header)
# S_2 = PHI <1(3), S_22(4)>
if (S_2 <= 4)
goto <bb 4>;
else
goto <bb 6>;
<bb 6>:
lxp_24 = lxp_1 + 1;
if (lxp_1 != 1)
goto <bb 12>;
else
goto <bb 7>;
<bb 12>:
goto <bb 3>;
and SCEV says _15 is {l_12 + 1, +, 1}_2 which looks correct. But then it
does
(chrec_apply
(varying_loop = 2)
(chrec = {l_12 + 1, +, 1}_2)
(x = 4)
(res = l_12 + 5))
and magically, via l_12 being {-1, +, 4}_1 (also correct) arrives at
(instantiate_scev
(instantiate_below = 2)
(evolution_loop = 1)
(chrec = {4, +, 4}_1)
(res = {4, +, 4}_1))
huh? So to it _15 is {4, +, 4}_1 (not sure what is considered "initial"
in terms of scalar evolution with a value varying in an inner loop).
This is what infer_loop_bounds_from_undefined derives the bogus bound
for loop 1 from. To my eyes _15 should be {0, + 4}_1!
Maybe it doesn't really make sense to ask for the evolution of something
defined in loop N with respect to an outer loop M?
If we change idx_infer_loop_bounds with
Index: tree-ssa-loop-niter.c
===================================================================
--- tree-ssa-loop-niter.c (revision 194552)
+++ tree-ssa-loop-niter.c (working copy)
@@ -2671,7 +2671,12 @@ idx_infer_loop_bounds (tree base, tree *
upper = false;
}
- ev = instantiate_parameters (loop, analyze_scalar_evolution (loop, *idx));
+ struct loop *dloop = loop_containing_stmt (data->stmt);
+ if (!dloop)
+ return true;
+
+ ev = analyze_scalar_evolution (dloop, *idx);
+ ev = instantiate_parameters (loop, ev);
init = initial_condition (ev);
step = evolution_part_in_loop_num (ev, loop->num);
then we obtain via {l_12 + 1, +, 1}_2, {{0, +, 4}_1, +, 1}_2 the correct
solution (init == 0, step == 4).
I am going to bootstrap and regtest that.
