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 4BEB5385840B; Wed, 13 Oct 2021 17:54:28 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 4BEB5385840B
From: "vajdaz at protonmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/93934] Unnecessary fld of uninitialized float stack
 variable results in ub of valid C++ code
Date: Wed, 13 Oct 2021 17:54:28 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 7.4.0
X-Bugzilla-Keywords: wrong-code
X-Bugzilla-Severity: normal
X-Bugzilla-Who: vajdaz at protonmail dot com
X-Bugzilla-Status: NEW
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: 
Message-ID: <bug-93934-4-AZWudek7Zq@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-93934-4@http.gcc.gnu.org/bugzilla/>
References: <bug-93934-4@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, 13 Oct 2021 17:54:28 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D93934

--- Comment #15 from Zoltan Vajda <vajdaz at protonmail dot com> ---
In my special case, I have an embedded realtime application with a lot of FP
atithmetic on Intel 32 bit architecture (huge and complex legacy codebase).=
 FPU
exceptions are enabled, so loading an SNaN results in an exception. This is
intended, and we will don't want to change this configuration. In this cont=
ext
the generated ASM code does result in an fld of an uninitialized local
variable, where looking on the C++ code such an access should not be possib=
le.
If the content of the uninitialized local variable happens to be a SNaN by
accident (chances are very small, but not zero), an FPU exception happens. =
And
again, based on the C++ code no FPU exception should be possible (assuming =
d is
not an SNaN).

Here is a synthetic example that triggers the exception by "placing a bomb"=
 on
the stack.

https://gcc.godbolt.org/z/aooex6dcT

Function place_bomb() has an effect on what happens in func(). That should =
not
be the case! This is all valid C++ code.

This may now accidentally happen in our application. The behavior is
unpredictable, because it depends on what previous function calls left on t=
he
stack.

If you change "-march=3Di686" to "-march=3Di386" in the example linked abov=
e,
everything goes fine.=