[Bug tree-optimization/92226] New: [10 Regression] live nul char store to array eliminated

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92226

            Bug ID: 92226
           Summary: [10 Regression] live nul char store to array
                    eliminated
           Product: gcc
           Version: unknown
            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: ---

One of the recent strlen changes (bisection points to r277076) leads to a
miscompilation of the following test case.

The problem is that the nul-over-nul optimization is being applied when it
shouldn't be.  It looks to me like the change above might have exposed a latent
bug here, or the change is incomplete.  Before the change,
compare_nonzero_chars returned -1 in this case which doesn't really correspond
to the documentation of the function -- that SI might not start with OFF
nonzero characters, where SI length is in [0, 7] and OFF is 0 -- but now it
returns 0.  That also doesn't
reflect exactly what's going on. 

$ cat a.c && gcc -O2 -S -Wall -Wextra -fdump-tree-optimized=/dev/stdout a.c

struct A { char a[8], c; };

void f (void*);

void g (const char *s)
{
  if (__builtin_strlen (s) > 7)
    return;

  struct A a;
  __builtin_strcpy (a.a, s);
  a.a[0] = 0;                  // incorrectly eliminated
  f (&a);
}

;; Function g (g, funcdef_no=0, decl_uid=1935, cgraph_uid=1, symbol_order=0)

g (const char * s)
{
  struct A a;
  long unsigned int _1;
  long unsigned int _10;

  <bb 2> [local count: 1073741824]:
  _1 = __builtin_strlen (s_4(D));
  if (_1 > 7)
    goto <bb 3>; [51.12%]
  else
    goto <bb 4>; [48.88%]

  <bb 3> [local count: 548896821]:
  a ={v} {CLOBBER};
  goto <bb 5>; [100.00%]

  <bb 4> [local count: 524845004]:
  _10 = _1 + 1;
  __builtin_memcpy (&a.a, s_4(D), _10);
  f (&a);
  a ={v} {CLOBBER};

  <bb 5> [local count: 1073741824]:
  return;

}