http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55555
--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> 2012-12-14
14:11:59 UTC ---
The unrolling puts __builtin_unreachable ()s into the inner duplicated loops:
First iteration, good:
<bb 16>:
# lxp_30 = PHI <0(2)>
t_32 = pol_x[lxp_30];
_33 = (long int) lxp_30;
_34 = _33 * 4;
l_35 = _34 + -1;
<bb 17>:
# S_36 = PHI <1(16), S_45(18)>
if (S_36 <= 4)
goto <bb 18>;
else
goto <bb 19>;
<bb 18>:
_38 = S_36 + l_35;
_39 = coef_x[_38];
_40 = S_36 + -1;
_41 = s[_40];
_42 = _41 * t_32;
_43 = _39 + _42;
coef_x[_38] = _43;
S_45 = S_36 + 1;
goto <bb 17>;
<bb 19>:
lxp_46 = lxp_30 + 1;
<bb 20>:
Peeled iteration, bad:
<bb 3>:
# lxp_1 = PHI <lxp_46(20)>
t_9 = pol_x[lxp_1];
_10 = (long int) lxp_1;
_11 = _10 * 4;
l_12 = _11 + -1;
goto <bb 5>;
<bb 4>:
_13 = S_3 + l_12;
__builtin_unreachable ();
_14 = coef_x[_13];
_15 = S_3 + -1;
_16 = s[_15];
_17 = _16 * t_9;
_18 = _14 + _17;
__builtin_unreachable ();
coef_x[_13] = _18;
S_20 = S_3 + 1;
<bb 5>:
# S_3 = PHI <1(3), S_20(4)>
if (S_3 <= 4)
goto <bb 4>;
else
goto <bb 6>;
<bb 6>:
lxp_21 = lxp_1 + 1;
if (1 == 0)
goto <bb 15>;
else
goto <bb 7>;
<bb 15>:
<bb 22>:
__builtin_unreachable ();
<bb 7>:
__asm__ __volatile__("" : : "r" &coef_x : "memory");
goto <bb 13>;
Note that the outer loop body looks ok - it is the inner body that get's
mangled in a bogus way.
That's because the loop bounds derived from the inner loop accesses
are bogus:
remove_exits_and_undefined_stmts (loop=0x7ffff67ec770, npeeled=1)
at /space/rguenther/src/svn/trunk/gcc/tree-ssa-loop-ivcanon.c:481
481 bool changed = false;
(gdb) n
483 for (elt = loop->bounds; elt; elt = elt->next)
(gdb)
488 if (!elt->is_exit
(gdb) p elt
$8 = (nb_iter_bound *) 0x7ffff6925e38
(gdb) p *elt
$9 = {stmt = 0x7ffff6905c80, bound = {low = 0, high = 0}, is_exit = false,
next = 0x7ffff6925dc0}
(gdb) p elt->stmt
$10 = (gimple) 0x7ffff6905c80
(gdb) call debug_gimple_stmt ($10)
# .MEM_19 = VDEF <.MEM_6>
coef_x[_13] = _18;
as far as I can see even with lxp == 1 we have at most an index of 4 + 1*4 - 1.
Statement _14 = coef_x[_13];
is executed at most 0 (bounded by 0) + 1 times in loop 1.
_that's_ bogus.
