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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/102697] [12 Regression] overflow warning missing
 after -O2 vectorization
Date: Tue, 12 Oct 2021 15:44:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102697

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[Diagnostics] overflow      |[12 Regression] overflow
                   |warning missing after O2    |warning missing after -O2
                   |vectorization.              |vectorization
   Last reconfirmed|                            |2021-10-12
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed as a false negative and a GCC 12 regression.

The expected warning is based on the assumption that an unknown offset to a
subobject is non-negative.  This assumption is supported by the constraint =
that
pointer arithmetic must be confined to the [sub]object the pointer points t=
o.=20
I.e., it's not valid to form a pointer to one subobject by adding an offset=
 to
a pointer to another [sub]object (even within the same object).  The same
constraint is enforced by -Warray-bounds.  The missing -Wstringop-overflow =
is
due to the hack mentioned in the following discussion:
  https://gcc.gnu.org/pipermail/gcc-patches/2021-September/580172.html

The warning is issued from the strlen pass which runs at -O2 and above.  It=
's
still issued when -fno-tree-vectorized is specified but should be issued ev=
en
with vectorization, and ideally at all optimization levels, including -O0.=