[Bug tree-optimization/92683] New: [10 Regression] strncmp incorrect result with equal substrings and non-const bound

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92683

            Bug ID: 92683
           Summary: [10 Regression] strncmp incorrect result with equal
                    substrings and non-const bound
           Product: gcc
           Version: 10.0
            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: ---

r278621 introduced a bug where a non-constant bound in calls to strncmp is
treated incorrectly as if it were a constant greater than the length of either
argument.

$ cat t.c && gcc -O1 -S -Wall -Wextra -Wunused
-fdump-tree-forwprop1=/dev/stdout t.c
int f (void)
{
  return __builtin_strncmp ("123", "1234", 3);   // correctly folded to zero
}

int g (void)
{
  int n = 3;
  return __builtin_strncmp ("123", "1234", n);   // incorrectly folded to -1
}


;; Function f (f, funcdef_no=0, decl_uid=1930, cgraph_uid=1, symbol_order=0)

f ()
{
  <bb 2> :
  return 0;

}



;; Function g (g, funcdef_no=1, decl_uid=1933, cgraph_uid=2, symbol_order=1)

g ()
{
  int n;

  <bb 2> :
  return -1;

}