Bug#788681: libc6: Function remquo() is broken

[Jens Schmidt <crashdump@meine-dateien.info>]

Package: libc6
Version: 2.19-19
Severity: normal

Dear Maintainer,

function remquo() in libm 64 bit is broken. You can proof this with the
following C code:

----
#include <stdio.h>
#include <math.h>

int main(void)
{
  double x, y, z;
  int i;

  x = 777777777777.0;
  y = 5.0;

  z = remquo(x, y, &i);

  printf("remquo(%f, %f) = %f; q = %d\n", x, y, z, i);

  return 0;
}

----

Calculating remquo() with:

x = 777777777777.0
y = 5.0

Correct values are:

rem = 2.0
quo = 155555555555 = 0x2437D568E3 = 0x3 (least low order bits)

----
libm 32 bit yields (gcc -O0 -m32 ... -lm):

rem = 2.0
quo = 3

This result is OK.
----

gcc 32/64 bit with builtin remquo() yields (gcc -O2 -m32 ... ):

rem = 2.0
quo = 936732899 = 0x37D568E3

This result is OK, but there are too many bits in quo. No matter...
----

libm 64 bit yields (gcc -O0 -m64 ... -lm):

rem = 777777777737.0
quo = 8

This result ist completely wrong!




-- System Information:
Debian Release: stretch/sid
  APT prefers testing-proposed-updates
  APT policy: (500, 'testing-proposed-updates'), (500, 'stable-updates'), (500, 'proposed-updates'), (500, 'testing'), (500, 'stable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 3.16.0-4-amd64 (SMP w/4 CPU cores)
Locale: LANG=de_DE.utf8, LC_CTYPE=de_DE.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: sysvinit (via /sbin/init)