https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59124
--- Comment #28 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to baoshan from comment #27)
> > It seems GCC at some moment unrolls the loop and creates such block with
> > those ranges. Probably, the block is unreachable, but it would be better to
> > not create it in the first place. Finding out where and why it is created
> > would help to figure out a fix.
>
>
> At pass "cunrolli", it would unroll the loops according the estimated
> iterate times. The problem is at this time it use array
> ref(infer_loop_bounds_from_ref) to infer the iterate times which is not
> accurate. The inaccurate iterate times result the extra blocks.
The question is why is it inaccurate? Note that cunrolli says:
Statement _14 = baz[_9];
is executed at most 5 (bounded by 5) + 1 times in loop 2.
Loop 2 iterates at most 6 times.
Analyzing # of iterations of loop 2
exit condition [i_2 + 4294967295, + , 4294967295] != 0
bounds on difference of bases: -4294967295 ... 0
Applying pattern match.pd:71, generic-match.c:9500
result:
# of iterations i_2 + 4294967295, bounded by 4294967295
Estimating sizes for loop 2
BB: 7, after_exit: 0
size: 2 if (j_3 != 0)
Exit condition will be eliminated in last copy.
BB: 6, after_exit: 1
size: 1 _9 = j_3 + 4294967295;
size: 0 _10 = (long unsigned int) _9;
Applying pattern match.pd:71, generic-match.c:9500
Applying pattern match.pd:136, generic-match.c:6019
size: 1 _11 = _10 * 4;
Applying pattern match.pd:71, generic-match.c:9500
Applying pattern match.pd:136, generic-match.c:6019
size: 1 _13 = bar_12(D) + _11;
size: 1 _14 = baz[_9];
size: 1 *_13 = _14;
size: 7-0, last_iteration: 2-2
Loop size: 7
Estimated size after unrolling: 28
pr59124.c:8:5: note: loop turned into non-loop; it never loops.
pr59124.c:8:5: note: loop with 7 iterations completely unrolled
Last iteration exit edge was proved true.
Forced statement unreachable: _14 = baz[_9];
However, it is clear that _14 = baz[_9] is executed only 5 times (not 5 times +
1). Why is this estimate wrong?
