[Bug tree-optimization/55079] New: [4.8 regeression] false positive -Warray-bounds

["dimhen at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55079

             Bug #: 55079
           Summary: [4.8 regeression] false positive -Warray-bounds
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: dimhen@gmail.com


$ cat tst.c
int f(unsigned len, int buflen)
{
    unsigned taillen;
    unsigned slen;
    unsigned i;
    int b[17];            /* needed <= 17 to trigger Warning */
    int j = 0;            /* needed to trigger Warning */

    b[0] = 0;
    taillen= buflen & 7;    /* taillen [0..7] */

    if(taillen) {        /* taillen [1..7] */
    slen= 8 - taillen;    /* slen    [7..1] */
    if (len<slen)        /* needed to trigger Warning  */
        slen=len;        /* slen' < slen  */
    for(i=0; i<slen; i++) {
      j = b[taillen];    /* taillen + slen = [1..7] + [7..1] = 8 */
      taillen++;
    }
    }
    return j;
}
$ gcc -Warray-bounds -c -O3 tst.c
tst.c: In function 'f':
tst.c:17:9: warning: array subscript is above array bounds [-Warray-bounds]
    j = b[taillen]; /* taillen + slen = [1..7] + [7..1] = 8 */
         ^
gcc version 4.8.0 20121026 (experimental) [trunk revision 192837] (GCC)

- if i remove some code then warning disappeared
- why '18' is safe array top-bound?