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 6D308385840D; Tue, 26 Oct 2021 00:59:19 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 6D308385840D
From: "joseph at codesourcery dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/102930] equal values appear to be different due to
 missing correct rounding in libc
Date: Tue, 26 Oct 2021 00:59:19 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: middle-end
X-Bugzilla-Version: 12.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: joseph at codesourcery 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: 
Message-ID: <bug-102930-4-7rTS5bJWZM@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-102930-4@http.gcc.gnu.org/bugzilla/>
References: <bug-102930-4@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: Tue, 26 Oct 2021 00:59:19 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102930

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery =
dot com> ---
On Mon, 25 Oct 2021, vincent-gcc at vinc17 dot net via Gcc-bugs wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102930
>=20
> --- Comment #3 from Vincent Lef=C3=A8vre <vincent-gcc at vinc17 dot net> =
---
> (In reply to Jakub Jelinek from comment #2)
> > I think there is nothing that can be done about this, and something like
> > this has been there since forever.  While perhaps some double routines =
in
> > glibc used to be at some point correctly roundded, many others (e.g. fl=
oat
> > or long double) never were, there is always a chance you get different
> > results from compile time folding vs. runtime evaluation.
>=20
> In this case, for its optimizations, GCC shouldn't assume that these resu=
lts
> are equal.

Indeed.  It's fine that different executions of a call to such a function=20
*in the abstract machine* return different values.  But each execution of=20
such a call in the abstract machine must return some particular value=20
(possibly different for different executions); it's not valid to optimize=20
the code in a way that's inconsistent with a single return value for each=20
call to the function in the abstract machine.  (Likewise for other cases=20
discussed in other bugs such as out-of-range floating-point-to-integer=20
conversions.)=