https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101501
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
# of iterations (a_lsm.5_10 * 52) / 52, bounded by 63
is obviously wrong with a_lsm.5_10 being a 'char', that boils down to zero
iterations when computed at runtime. The loop body looks like
<bb 3> [local count: 1073741824]:
# a_3 = PHI <a_5(2), a_6(5)>
d:
a_6 = a_3 + 255;
c_7 = a_3 * 52;
if (c_7 != 0)
goto <bb 5>; [89.00%]
else
goto <bb 4>; [11.00%]
<bb 5> [local count: 955630224]:
goto <bb 3>; [100.00%]
and SCEV computes
(scalar = c_7)
(scalar_evolution = {a_5 * 52, +, 204}_1))
which looks correct.
niter compute does
static bool
number_of_iterations_ne (class loop *loop, tree type, affine_iv *iv,
tree final, class tree_niter_desc *niter,
bool exit_must_be_taken, bounds *bnds)
{
...
/* Compute no-overflow information for the control iv. This can be
proven when below two conditions are satisfied:
1) IV evaluates toward FINAL at beginning, i.e:
base <= FINAL ; step > 0
base >= FINAL ; step < 0
2) |FINAL - base| is an exact multiple of step.
...
if (!niter->control.no_overflow
&& (integer_onep (s) || multiple_of_p (type, c, s)))
{
and there we have the issue of multiple_of_p not handling overflow (there's
a duplicate bug, PR100499 about this issue). The 1) condition ends up
as unsigned-val >= 0 which is trivially true.
