[Bug tree-optimization/96447] False positive -Wstringop-overflow with -O3 due to loop unrolling

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96447

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |diagnostic,
                   |                            |missed-optimization
          Component|other                       |tree-optimization
                 CC|                            |msebor at gcc dot gnu.org
            Summary|False positive              |False positive
                   |-Wstringop-overflow with    |-Wstringop-overflow with
                   |-O3                         |-O3 due to loop unrolling
   Last reconfirmed|                            |2020-08-03
     Ever confirmed|0                           |1

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning works as designed.  The false positive is caused by the assignment
below in the unrolled loop with the index in RANGE [4, 5].  GCC doesn't
understand that the constraint on bpp implies that i < 4, likely because the
range of bpp is discontiguous.

;;   basic block 19, loop depth 0, count 0 (precise), probably never executed
;;    prev block 18, next block 20, flags: (NEW, REACHABLE, VISITED)
;;    pred:       18 [80.0% (guessed)]  count:0 (precise)
(TRUE_VALUE,EXECUTABLE)
  # .MEM_3 = VDEF <.MEM_50>
  dataD.1941[i_49] = 0;
  # RANGE [4, 5] NONZERO 5
  i_2 = i_49 + 1;
  # RANGE [32, 32] NONZERO 32
  _1 = _6 + 8;
  if (bpp_25 > 32)
    goto <bb 20>; [80.00%]
  else
    goto <bb 16>; [20.00%]
;;    succ:       20 [80.0% (guessed)]  count:0 (precise)
(TRUE_VALUE,EXECUTABLE)
;;                16 [20.0% (guessed)]  count:0 (precise)
(FALSE_VALUE,EXECUTABLE)

;;   basic block 20, loop depth 0, count 0 (precise), probably never executed
;;    prev block 19, next block 21, flags: (NEW, REACHABLE, VISITED)
;;    pred:       19 [80.0% (guessed)]  count:0 (precise)
(TRUE_VALUE,EXECUTABLE)
  # .MEM_45 = VDEF <.MEM_3>
  dataD.1941[i_2] = 0;   <<< warning


The warning can be avoided by constraining the range of bpp to [1, 4] instead
(by shifting it to the right by 3 bits) and changing the loop test to i < bpp. 
That also leads to GCC emitting better code.

int load(input * s)
{
    unsigned char bpp = get8u (s);

    if(bpp != 8 && bpp != 16 && bpp != 24 && bpp != 32) {
        return 0;
    }

    bpp >>= 3;

    unsigned char data[4];
    for(int i = 0; i < bpp; ++i) {
        data[i] = get8u (s);
    }