From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id DF4953857C65; Sun, 15 Nov 2020 04:16:54 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org DF4953857C65
From: "sven.koehler at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/97833] New: -Wconversion behaves erratic
Date: Sun, 15 Nov 2020 04:16:54 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 10.2.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: sven.koehler at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
 attachments.created
Message-ID: <bug-97833-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Sun, 15 Nov 2020 04:16:55 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97833

            Bug ID: 97833
           Summary: -Wconversion behaves erratic
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sven.koehler at gmail dot com
  Target Milestone: ---

Created attachment 49559
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D49559&action=3Dedit
non-working example

Find attached an example for which -Wconversion behaves uncomprehensible.

Why does it yields warning for test2 but not for test1 and test3?
This happens with gcc for 32bit arm and gcc for x86_64.

In all 3 functions, we have 2 shift operations. The operand is uint16_t, wh=
ich
is promoted to int. The result of the shift operations is cast to uint16_t.=
 So
the operands of the bit-wise or are again uint16_t. So both operands of the
bitwise or are promoted to int.

So basically, in all 3 cases the code is returning an int. However, -W
conversion warns only in 1 case.

Also, why does it matter whether x is shifted by 0 or 1 ? Why does a shift =
by 0
result in an error, and a shift by 1 does not?

Why does it matter whether x and y are originally uint8_t being cast to
uint16_t (test2) or a uint16_t (test3) originally? In both cases, the resul=
t of
the shifts is cast to uint16_t.


Is gcc trying to keep track of the range of the individual expressions? Is =
gcc
somehow failing when the a shift by 0 occurs? I believe that a shift by zer=
o is
defined behavior.=