[Bug tree-optimization/82455] New: missing -Warray-bounds on strcpy offset in an out-of-bounds range

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82455

            Bug ID: 82455
           Summary: missing -Warray-bounds on strcpy offset in an
                    out-of-bounds range
           Product: gcc
           Version: 8.0
            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: ---

The -Warray-bounds warning is only issued for (a subset of) instances of cases
where the index/offset is constant.  The warning could be made more effective
by making use of range information to detect non-constant out-of-bounds offsets
as well.  The test case below shows an instance where the warning is missing. 
(It also shows an instance of missing -Wstringop-overflow warning but that one
is the subject of a separate bug report).

$ cat z.c && gcc -O2 -S -Wall -Wextra z.c
void fcst (char *d)
{
  char a[2] = "0";

  __builtin_strcpy (d, a + 3);   // -Warray-bounds (good)
                                 // missing -Wstringop-overflow
}

void frng (char *d, int i)
{
  char a[2] = "0";

  if (i < 3)
    i = 3;

  __builtin_strcpy (d, a + i);   // both warnings missing
                                 // (array index out of bounds
                                 // and reading past the end)
}