[Bug c/65461] New: -Warray-bounds warnings in the linux kernel (free_area_init_nodes)

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65461

            Bug ID: 65461
           Summary: -Warray-bounds warnings in the linux kernel
                    (free_area_init_nodes)
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marxin at gcc dot gnu.org

Hello.

After reducing following test case, I see false positive VRP warning for:

enum zone_type {
    ZONE_NORMAL, 
    ZONE_MOVABLE,
    __MAX_NR_ZONES
};
unsigned long arch_zone_lowest_possible_pfn[2];
unsigned long arch_zone_highest_possible_pfn[2];

void __next_mem_pfn_range(int *a, unsigned long *b, unsigned long *c, int *d);

void free_area_init_nodes(unsigned long *max_zone_pfn) {
    unsigned long start_pfn, end_pfn;
    int i, nid;
    for (i = 1; i < 2; i++) {
        if (i == ZONE_MOVABLE)
          continue;

        arch_zone_lowest_possible_pfn[i] = arch_zone_highest_possible_pfn[i-1];
    }

   for (i = -1; i >= 0; __next_mem_pfn_range(&i, &start_pfn, &end_pfn, &nid))
     {}
}

testcase.i: In function ‘free_area_init_nodes’:
testcase.i:18:74: warning: array subscript is below array bounds
[-Warray-bounds]
         arch_zone_lowest_possible_pfn[i] =
arch_zone_highest_possible_pfn[i-1];

Well, it looks the issue is similar (duplicate) to PR1006, but maybe it worth
for analysis.
VPR dump shows following code:

Value ranges after VRP:

.MEM_1: VARYING
.MEM_2: VARYING
.MEM_3: VARYING
i.0_6: VARYING
_7: [-INF(OVF), -1]
_8: VARYING
i.1_10: [-2147483647, 2]
i.0_13: VARYING
i.0_19: [1, 1]  EQUIVALENCES: { i.0_6 i.0_21 } (2 elements)
i.0_20: [-INF, 0]  EQUIVALENCES: { i.0_6 i.0_21 } (2 elements)
i.0_21: [-INF, 1]  EQUIVALENCES: { i.0_6 } (1 elements)

For:
free_area_init_nodes (long unsigned int * max_zone_pfn)
{
  int nid;
  int i;
  long unsigned int end_pfn;
  long unsigned int start_pfn;
  int i.0_6;
  int _7;
  long unsigned int _8;
  int i.1_10;
  int i.0_13;

  <bb 2>:
  i = 1;
  goto <bb 6>;

  <bb 3>:
  if (i.0_6 == 1)
    goto <bb 5>;
  else
    goto <bb 4>;

  <bb 4>:
  _7 = i.0_6 + -1;
  _8 = arch_zone_highest_possible_pfn[_7];
  arch_zone_lowest_possible_pfn[i.0_6] = _8;

  <bb 5>:
  i.1_10 = i.0_6 + 1;
  i = i.1_10;

  <bb 6>:
  i.0_6 = i;
  if (i.0_6 <= 1)
    goto <bb 3>;
  else
    goto <bb 7>;

  <bb 7>:
  i = -1;
  goto <bb 9>;

  <bb 8>:
  __next_mem_pfn_range (&i, &start_pfn, &end_pfn, &nid);

  <bb 9>:
  i.0_13 = i;
  if (i.0_13 >= 0)
    goto <bb 8>;
  else
    goto <bb 10>;

  <bb 10>:
  start_pfn ={v} {CLOBBER};
  end_pfn ={v} {CLOBBER};
  i ={v} {CLOBBER};
  nid ={v} {CLOBBER};
  return;

}

Thanks,
Martin