[Bug tree-optimization/96963] -Wstringop-overflow false positive on -O3 or -O2 -ftree-vectorize when assigning consecutive char struct members

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96963

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |diagnostic
   Last reconfirmed|                            |2020-09-07
     Ever confirmed|0                           |1
             Blocks|                            |88443
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=93200
                 CC|                            |msebor at gcc dot gnu.org

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed.  The vectorizer replaces the two character assignments with a store
into f->a.  The fix for pr93200 added a hack to handle some these cases but not
this one.  This instance of the warning is issued from the strlen pass whose
dump shows the cause of the problem:

$ gcc -O3 -S -Wall -fdump-tree-strlen=/dev/stdout pr96963.c

;; Function clr (clr, funcdef_no=0, decl_uid=1937, cgraph_uid=1,
symbol_order=0)

;; 1 loops found
;;
;; Loop 0
;;  header 0, latch 1
;;  depth 0, outer -1
;;  nodes: 0 1 2
;; 2 succs { 1 }
pr96963.c: In function ‘clr’:
pr96963.c:11:14: warning: writing 2 bytes into a region of size 1
[-Wstringop-overflow=]
   11 |         f->a = 0;
      |         ~~~~~^~~
pr96963.c:3:14: note: at offset 0 to object ‘a’ with size 1 declared here
    3 |         char a;
      |              ^
clr (struct foo * f)
{
  vector(2) char * vectp.4;
  vector(2) char * vectp_f.3;
  char * _1;

  <bb 2> [local count: 1073741824]:
  _1 = &f_2(D)->a;
  heh (_1);
  MEM <vector(2) char> [(char *)_1] = { 0, 0 };   <<< _1 points to f_2(D)->a
  return;
}


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88443
[Bug 88443] [meta-bug] bogus/missing -Wstringop-overflow warnings