https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111764
--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
Shorter testcase, fails at -O2 -ftree-vectorize
short b;
int main()
{
b = 2;
for (int a = 1; a <= 9; a++)
b = b * b;
if (b != 0)
__builtin_abort ();
}
it "works" with unsigned short b because the non-path reduction detection
doesn't allow two uses of 'b':
t.c:5:21: note: Analyze phi: b_lsm.6_2 = PHI <powmult_3(7), 2(2)>
t.c:5:21: missed: reduction used in loop.
t.c:5:21: missed: Unknown def-use cycle pattern.
but the path based reduction detection has
/* We want to allow x + x but not x < 1 ? x : 2. */
if (is_gimple_assign (op_use_stmt)
&& gimple_assign_rhs_code (op_use_stmt) == COND_EXPR)
which also allows x * x to slip through. Even when fixing the earlier
issue of the initial value we end up with
vect_b.13_27 = { 2, 1, 1, 1, 1, 1, 1, 1 };
vect_powmult_5.14_28 = vect_b.13_27 * vect_b.13_27;
allowing x + x is only OK because it's 2 * x and we will actually never
see it as x + x.
