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From: "franke at computer dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/97336] False positive -Wstring-compare warning for
 strncmp()
Date: Fri, 09 Oct 2020 12:08:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97336

--- Comment #2 from Christian Franke <franke at computer dot org> ---
Sorry, but I disagree that this report is INVALID.

Unlike for example -Wstringop-overflow, warnings like -Wstring-compare shou=
ld
IMO only occur if the warning condition holds for *all* function calls
generated by loop unrolling.

BTW, if the partial loop unrolling is done manually, the warning should occ=
ur,
but does not:

int f(const char * p, int n)
{
  char buf[10] =3D {0, };
  int i =3D 0;
  if (n <=3D 0) {
    if (!__builtin_strncmp(buf, "12345", 5) // <=3D=3D no warning
        && (i =3D=3D 5 || buf[5] =3D=3D ' ')) // <=3D=3D warning if removed
      return 1;
  }
  else {
    do {
      buf[i] =3D p[i]; i++;
    } while (i < (int)sizeof(buf)-1 && i < n);
    if (!__builtin_strncmp(buf, "12345", 5)
        && (i =3D=3D 5 || buf[5] =3D=3D ' '))
      return 1;
  }
  return 0;
}

With the above code, the warning occurs if the condition "&& (i =3D=3D 5 ||=
 buf[5]
=3D=3D ' ')" is removed. If this is done in the original testcase, the warn=
ing does
no longer occur.=