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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/95794] New: strnlen of a constant string plus
 variable offset not folded when bound exceeds size
Date: Sat, 20 Jun 2020 23:19:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95794

            Bug ID: 95794
           Summary: strnlen of a constant string plus variable offset not
                    folded when bound exceeds size
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

GCC folds comparisons of the results of calls to strlen with constant strin=
gs
and variable offsets but it doesn't do the same for the equivalent strnlen =
with
a bound in excess of the string size (strnlen with a bound greater than the
string length are equivalent to strlen with the same argument).

$ cat z.c && gcc -O2 -S -Wall -fdump-tree-optimized=3D/dev/stdout z.c
const char a[] =3D "12345";

void f (int i)
{
  if (__builtin_strlen (&a[i]) > 5)       // folded to false
    __builtin_abort ();
}

void g (int i)
{
  if (__builtin_strnlen (&a[i], 7) > 5)   // not folded
    __builtin_abort ();
}


;; Function f (f, funcdef_no=3D0, decl_uid=3D1931, cgraph_uid=3D1, symbol_o=
rder=3D1)

f (int i)
{
  <bb 2> [local count: 1073741824]:
  return;

}



;; Function g (g, funcdef_no=3D1, decl_uid=3D1934, cgraph_uid=3D2, symbol_o=
rder=3D2)

g (int i)
{
  const char * _1;
  long unsigned int _2;
  sizetype _6;

  <bb 2> [local count: 1073741824]:
  _6 =3D (sizetype) i_3(D);
  _1 =3D &a + _6;
  _2 =3D __builtin_strnlen (_1, 7);
  if (_2 > 5)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [100.00%]

  <bb 3> [count: 0]:
  __builtin_abort ();

  <bb 4> [local count: 1073741824]:
  return;

}=