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From: "aldyh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/104986] [12 Regression] bogus writing 1 byte into a
 region of size 0 with -fwrapv and -O2 -fpeel-loops since
 r12-4698-gf6d012338bf87f42
Date: Wed, 23 Mar 2022 09:45:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104986

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amacleod at redhat dot com,
                   |                            |siddhesh at gcc dot gnu.org

--- Comment #2 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
[Siddhesh, I don't know if this is something you're looking at.  If not, fe=
el
free to unsubscribe.]

This may have first been visible with r12-4698-gf6d012338bf87f42, but it is=
n't
threader related as it can be reproduced with -fno-thread-jumps.

I can actually reproduce it without VRP[12] (but not without evrp).

+ ./cc1 a.c -fdump-tree-all-details -quiet -I/tmp -O2 -fpeel-loops -fwrapv
-fno-thread-jumps -fdisable-tr
ee-vrp2 -fdisable-tree-vrp1
cc1: note: disable pass tree-vrp2 for functions in the range of [0, 4294967=
295]
cc1: note: disable pass tree-vrp1 for functions in the range of [0, 4294967=
295]
a.c: In function =E2=80=98inetnot=E2=80=99:
a.c:12:14: warning: writing 1 byte into a region of size 0
[-Wstringop-overflow=3D]
etc
etc

Disabling evrp causes the problem to go away, but all these middle end warn=
ings
are very sensitive to better ranges, so who knows.

For that matter, the only transformation evrp is doing is correct:

@@ -74,7 +90,7 @@
   <bb 9> :
   # nb_8 =3D PHI <iftmp.0_9(7), nb_23(8)>
   nb_23 =3D nb_8 + -1;
-  if (nb_8 > 0)
+  if (nb_8 !=3D 0)
     goto <bb 8>; [INV]
   else
     goto <bb 10>; [INV]

...which is merely changing the conditional in the decreasing loop from > 0
into !=3D 0.

It looks like a problem elsewhere.=