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From: "qrzhang at gatech dot edu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug sanitizer/108845] Unnecessary signed integer overflow checks
Date: Sat, 18 Feb 2023 20:27:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108845

--- Comment #3 from Qirun Zhang <qrzhang at gatech dot edu> ---
(In reply to Jakub Jelinek from comment #2)
> I'm not convinced it is a good idea.
> Sure, in the above case it is obvious it will never trigger, but if we say
> use ranger to decide if the operation can or can't overflow, then VRP is =
in
> many cases based on assumptions which only hold for valid code, but
> sanitizers actually want to diagnose invalid code.


Thanks!

Here is another (similar) example.  Earlier versions of GCC will not inject
UBSAN_CHECK_ADD. However, the latest version of GCC will.

the code example:
=3D=3D=3D=3D=3D=3D
void main() {
  int a =3D 0;
  for (; a !=3D 2; a++)
    ;
}
=3D=3D=3D=3D=3D=3D

Compile with "gcc-11 -fsanitize=3Dsigned-integer-overflow -O3=20
-fdump-tree-optimized", we got no UBSAN checks:

=3D=3D=3D=3D=3D=3D
void main ()
{
  int a;

  <bb 2> [local count: 118111600]:

  <bb 3> [local count: 955630225]:
  # a_6 =3D PHI <1(3), 0(2)>
  a_3 =3D a_6 + 1;
  if (a_3 !=3D 2)
    goto <bb 3>; [87.64%]
  else
    goto <bb 4>; [12.36%]

  <bb 4> [local count: 118111600]:
  return;

}
=3D=3D=3D=3D=3D=3D

Compile with "gcc-trunk -fsanitize=3Dsigned-integer-overflow -O3=20
-fdump-tree-optimized", we got one:

=3D=3D=3D=3D=3D=3D
void main ()
{
  int a;

  <bb 2> [local count: 118111600]:

  <bb 3> [local count: 955630225]:
  # a_5 =3D PHI <a_3(3), 0(2)>
  a_3 =3D .UBSAN_CHECK_ADD (a_5, 1);
  if (a_3 !=3D 2)
    goto <bb 3>; [89.00%]
  else
    goto <bb 4>; [11.00%]

  <bb 4> [local count: 118111600]:
  return;

}
=3D=3D=3D=3D=3D=3D

$ gcc-trunk -v
gcc version 13.0.1 20230218 (experimental) [master r13-6132-g32b5875c911] (=
GCC)=