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 CCFB93858D3C; Wed,  1 Sep 2021 00:56:26 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org CCFB93858D3C
From: "jeremy-gcc-bugzilla at sawicki dot us" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/102150] New: Speculative execution of inline
 assembly causes divide error
Date: Wed, 01 Sep 2021 00:56:26 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 11.2.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: jeremy-gcc-bugzilla at sawicki dot us
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-102150-4@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: Wed, 01 Sep 2021 00:56:26 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102150

            Bug ID: 102150
           Summary: Speculative execution of inline assembly causes divide
                    error
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jeremy-gcc-bugzilla at sawicki dot us
  Target Milestone: ---

Created attachment 51391
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D51391&action=3Dedit
Reproducible test case

The attached test case uses inline assembly to wrap the x86_64 DIV instruct=
ion.
 GCC speculatively executes the inline assembly on inputs that the source
program does not, resulting in a divide error.

The GCC documentation says that non-volatile inline assembly may be discard=
ed
or moved out of loops.  It is not obvious whether speculative execution is =
also
permitted.  I asked on gcc-help and was asked to file a report.

A related report points out that many projects currently wrap the DIV
instruction without using volatile:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D82677

Another related report considers the similar issue of whether pure/const
functions must be non-trapping for inputs they don't actually receive:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D93491

If it is determined that volatile is required, it would helpful to clarify =
in
the documentation that speculative execution may occur without volatile:
https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile

gcc version 11.2.0 (GCC)
Target: x86_64-pc-linux-gnu
Configured with: /home/jeremys/gcc-11.2.0/configure
--prefix=3D/home/jeremys/gcc-11.2.0-install --disable-multilib

Command line: g++ -O3 -o divasm divasm.cpp
No compiler errors/warnings are produced
When executed, a divide error occurs=