[Bug tree-optimization/59124] [4.9/5/6 Regression] Wrong warnings "array subscript is above array bounds"

["manu at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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?
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Subject: [Bug tree-optimization/59124] [4.9/5/6 Regression] Wrong warnings "array subscript is above array bounds"
Date: Fri, 18 Sep 2015 18:53:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idY124

--- Comment #29 from baoshan <pangbw at gmail dot com> ---

> However, it is clear that _14 = baz[_9] is executed only 5 times (not 5
> times + 1). Why is this estimate wrong?

The max value of n is 6, so the max value of i is 5 as "i < n", then the max
value of j is 4 as "j = i - 1" which means the max iterate times is 4.