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 C0B16385DC1C; Thu, 16 Apr 2020 15:35:00 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org C0B16385DC1C
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1587051300;
 bh=E+jWqgtv3oCmrg/TH4E2N00s0V+XZoibEd5qoG47Vnk=;
 h=From:To:Subject:Date:In-Reply-To:References:From;
 b=pbNX2ciXP9GS98LBa3jKJXihYMrZXGzfYxIkw5Y0ch9he/XkTe/o282tOR2RVARvV
 3eMISYMx1Th+v3Sj2t/R+ScKVjTsntuhH7dn/nGoaJTxIjqrniSvTq/46PQnLf7Q5X
 yALT87hznza26ynCzzjZ6Q9wauiaak3QVsD9CO3Y=
From: "vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/34678] Optimization generates incorrect code with
 -frounding-math option (#pragma STDC FENV_ACCESS not implemented)
Date: Thu, 16 Apr 2020 15:35:00 +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: 4.3.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: vincent-gcc at vinc17 dot net
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-34678-4-8DhvpvsbZ5@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-34678-4@http.gcc.gnu.org/bugzilla/>
References: <bug-34678-4@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: Thu, 16 Apr 2020 15:35:00 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D34678

--- Comment #43 from Vincent Lef=C3=A8vre <vincent-gcc at vinc17 dot net> -=
--
Note that the effect of changing the rounding mode after a computation, whe=
ther
-frounding-math is used or not, is not just that the change of rounding mode
may not be honored. If can yield inconsistencies in a block where the round=
ing
mode is not changed.

#include <stdio.h>
#include <stdlib.h>
#include <fenv.h>

#pragma STDC FENV_ACCESS ON

#define CST 0x1p-200

int main (void)
{
  volatile double a =3D CST;
  double b =3D a, c =3D a, d;
  printf ("%a\n", 1.0 - b);
  fesetround (FE_DOWNWARD);
  printf ("%a\n", 1.0 - c);

  if (b =3D=3D c && b =3D=3D CST && c =3D=3D CST)
    {
      printf ("%d\n", 1.0 - b =3D=3D 1.0 - c);
      printf ("1: %a\n", 1.0 - b);
      printf ("2: %a\n", 1.0 - c);
      d =3D b =3D=3D CST ? b : (abort (), 1.0);
      printf ("3: %a\n", 1.0 - d);
      d =3D b =3D=3D CST ? b : 1.0;
      printf ("4: %a\n", 1.0 - d);
    }

  return 0;
}

With -std=3Dc17 -frounding-math -O3 -lm, I get:

0x1p+0
0x1.fffffffffffffp-1
0
1: 0x1p+0
2: 0x1.fffffffffffffp-1
3: 0x1p+0
4: 0x1.fffffffffffffp-1=