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 B3844385B835; Fri, 17 Apr 2020 04:53:22 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org B3844385B835
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1587099202;
 bh=dKy6vWV9ho8U+6cx7+T2UWO3/U5/+4rOsrsK1/rQ9hU=;
 h=From:To:Subject:Date:From;
 b=HP02jjyCh1wvpKd7B7AncFwZ4QPcszZGEogWpDihJr3v0y18mb7p3PKtWOKNPHHV1
 z+2Q1cXAS4tRHOsmEkZ3BqanHvUVeLC/jxkY13fdWNpvdxPXV+SZzDGkfKg0w1wNhl
 E+PXbuohFkg80V2jRIYaZPmNO7Yhn+75oejAfJI4=
From: "bugdal at aerifal dot cx" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/94631] New: Wrong codegen for arithmetic on bitfields
Date: Fri, 17 Apr 2020 04:53:22 +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: unknown
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: bugdal at aerifal dot cx
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
Message-ID: <bug-94631-4@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: <http://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: <http://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Fri, 17 Apr 2020 04:53:22 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94631

            Bug ID: 94631
           Summary: Wrong codegen for arithmetic on bitfields
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bugdal at aerifal dot cx
  Target Milestone: ---

Test case:

struct foo {
        unsigned long long low:12, hi:52;
};
unsigned long long bar(struct foo *p)
{
        return p->hi*4096;
}

Should generate only a mask off of the low bits, but gcc generates code to =
mask
off the low 12 bits and the high 12 bits (reducing the result to 52 bits).
Presumably GCC is interpreting the expression p->hi as having a phantom type
that's only 52 bits wide, rather than having type unsigned long long.

clang/LLVM compiles it correctly.

I don't believe there's any language in the standard supporting what GCC is
doing here.=