[Bug tree-optimization/86614] New: duplicate -Warray-bounds for a strncpy call with out-of-bounds offset

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86614

            Bug ID: 86614
           Summary: duplicate -Warray-bounds for a strncpy  call with
                    out-of-bounds offset
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

With the recent enhancement to -Warray-bounds (bug 84047) GCC issues duplicate
instances of the warning for calls to some built-in functions with out-of-bound
offsets.  The following test case was isolated from
c-c++-common/Warray-bounds-2.c (r262906 prunes these duplicates from the test
to avoid spurious failures).

$ cat c.c && gcc -O2 -S -Warray-bounds c.c
extern char* strncpy (char*, const char*, __SIZE_TYPE__);

void sink (void*);

struct { char b[17]; } a[2];

void g (const char *s, unsigned n)
{
  int i = (char*)a[1].b - (char*)a + 1;
  char *d = a[1].b;
  strncpy (d + i, s, n);
}
c.c: In function ‘g’:
c.c:11:3: warning: array subscript 35 is outside array bounds of ‘struct
<anonymous>[2]’ [-Warray-bounds]
   strncpy (d + i, s, n);
   ^~~~~~~~~~~~~~~~~~~~~
c.c:11:3: warning: ‘strncpy’ offset 35 is out of the bounds [0, 34] of object
‘a’ with type ‘struct <anonymous>[2]’ [-Warray-bounds]
c.c:5:24: note: ‘a’ declared here
 struct { char b[17]; } a[2];
                        ^