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 296633858026; Mon, 27 Sep 2021 14:27:35 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 296633858026
From: "zimmerma+gcc at loria dot fr" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/102498] New: wrong output of printf with long double constant
 and non-default rounding mode
Date: Mon, 27 Sep 2021 14:27:34 +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: 10.2.1
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: zimmerma+gcc at loria dot fr
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-102498-4@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: Mon, 27 Sep 2021 14:27:35 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102498

            Bug ID: 102498
           Summary: wrong output of printf with long double constant and
                    non-default rounding mode
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zimmerma+gcc at loria dot fr
  Target Milestone: ---

with the following program:

#include <stdio.h>
#include <math.h>
#include <fenv.h>
const long double a =3D 0xc.90fdaa22168c235p-1l;
const long double b =3D 0xc.90fdaa22168c235p-2l;
int main()
{
  fesetround (FE_TONEAREST);
  printf ("FE_TONEAREST:  a=3D%La b=3D%La\n", a, b);
  fesetround (FE_TOWARDZERO);
  printf ("FE_TOWARDZERO: a=3D%La b=3D%La\n", a, b);
  fesetround (FE_UPWARD);
  printf ("FE_UPWARD:     a=3D%La b=3D%La\n", a, b);
  fesetround (FE_DOWNWARD);
  printf ("FE_DOWNWARD:   a=3D%La b=3D%La\n", a, b);
}

I get:

$ gcc -frounding-math -O0 -fno-builtin e.c -lm
$ ./a.out=20
FE_TONEAREST:  a=3D0xc.90fdaa22168c235p-1 b=3D0xc.90fdaa22168c235p-2
FE_TOWARDZERO: a=3D0xc.90fdaa22168c235p-1 b=3D0xc.90fdaa22168c234p-2
FE_UPWARD:     a=3D0xc.90fdaa22168c235p-1 b=3D0xc.90fdaa22168c235p-2
FE_DOWNWARD:   a=3D0xc.90fdaa22168c235p-1 b=3D0xc.90fdaa22168c234p-2

The value 'a' is printed the same whatever the rounding mode, but 'b' (which
approximates pi to nearest) is wrongly printed for FE_TOWARDZERO and
FE_DOWNWARD.

According to Alexander Monakov, the same problem should happen with og_2 10,
log_2 e, log_10 2, and log_e 2, see
https://sourceware.org/pipermail/libc-alpha/2021-September/131411.html=