https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108845
--- Comment #4 from Qirun Zhang <qrzhang at gatech dot edu> ---
(In reply to Qirun Zhang from comment #3)
> (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:
> ======
> void main() {
> int a = 0;
> for (; a != 2; a++)
> ;
> }
> ======
>
> Compile with "gcc-11 -fsanitize=signed-integer-overflow -O3
> -fdump-tree-optimized", we got no UBSAN checks:
>
> ======
> void main ()
> {
> int a;
>
> <bb 2> [local count: 118111600]:
>
> <bb 3> [local count: 955630225]:
> # a_6 = PHI <1(3), 0(2)>
> a_3 = a_6 + 1;
> if (a_3 != 2)
> goto <bb 3>; [87.64%]
> else
> goto <bb 4>; [12.36%]
>
> <bb 4> [local count: 118111600]:
> return;
>
> }
> ======
>
> Compile with "gcc-trunk -fsanitize=signed-integer-overflow -O3
> -fdump-tree-optimized", we got one:
>
> ======
> void main ()
> {
> int a;
>
> <bb 2> [local count: 118111600]:
>
> <bb 3> [local count: 955630225]:
> # a_5 = PHI <a_3(3), 0(2)>
> a_3 = .UBSAN_CHECK_ADD (a_5, 1);
> if (a_3 != 2)
> goto <bb 3>; [89.00%]
> else
> goto <bb 4>; [11.00%]
>
> <bb 4> [local count: 118111600]:
> return;
>
> }
> ======
>
> $ gcc-trunk -v
> gcc version 13.0.1 20230218 (experimental) [master r13-6132-g32b5875c911]
> (GCC)
I did a bisect. For the testcase provided in comment #3, the behavior was
introduced in the following commit:
commit 502ffb1f389011b28ee51815242c7397790802d5
Author: Andrew MacLeod <amacl...@redhat.com>
Date: Tue Nov 2 21:26:44 2021 -0400
Switch vrp2 to ranger.
This patch flips the default for the VRP2 pass to execute ranger vrp rather
than the assert_expr version of VRP.
* params.opt (param_vrp2_mode): Make ranger the default for VRP2.
