complex number compiler and libc bugs (cexp+conj) on OSX and with the intel compiler (icc)

Today a bug in complex number handling surfaced in guile which only appeared on OSX.

This is a short note just to make sure that the bug is reported somewhere.

Test-code (written mostly by Mark Weaver who also analyzed the bug - I only ran the code on a few platforms I happened to have access to):

// test.c
// compile with gcc -O0 -o test test.c -lm
// or with icc -O0 -o test test.c -lm
#include <complex.h>
#include <stdio.h>

int
main (int argc, char **argv)
{
  double complex z = conj (1.0);
  double complex result;

  if (argc == 1)
    z = conj (0.0);

  result = cexp (z);

  printf ("cexp (%f + %f i) => %f + %f i\n",
          creal (z), cimag (z), creal (result), cimag (result));
  result = conj(result);
  printf ("conj(cexp (%f + %f i)) => %f + %f i\n",
          creal (z), cimag (z), creal (result), cimag (result));

  return 0;
}

As by the C-11 standard (pages 561 and 216) this should return:

cexp (0.000000 + -0.000000 i) => 1.000000 + -0.000000 i

conj(cexp (0.000000 + -0.000000 i)) => 1.000000 + 0.000000 i

Page 561:

— cexp(conj(z)) = conj(cexp(z)).

Page 216:

The conj functions compute the complex conjugate of z, by reversing the sign of its imaginary part.

On OSX it returns (compiled with GCC):

TODO: Check the second line!

cexp (0.000000 + -0.000000 i) => 1.000000 + 0.000000 i

With the intel compiler it returns:

cexp (0.000000 + 0.000000 i) => 1.000000 + 0.000000 i

conj(cexp (0.000000 + 0.000000 i)) => 1.000000 + 0.000000 i

In short: On OSX cexp seems broken. With the intel compiler conj seems broken.

icc --version
# => icc (ICC) 13.1.3 20130607
# => Copyright (C) 1985-2013 Intel Corporation.  All rights reserved.

The OSX compiler is GCC 4.8.2 from MacPorts.


[taylanub] ArneBab: You might want to add that compiler optimizations can result in cexp() calls where there are none (which is how this bug surfaced in our case).

[mark_weaver] cexp(z) = e^z = e^(a+bi) = e^a * e^(bi) = e^a * (cos(b) + i*sin(b))

[mark_weaver] for real 'b', e^(bi) is a point on the unit circle on the complex plane.

[mark_weaver] so cexp(bi) can be used to compute cos(b) and sin(b) simultaneously, and probably faster than calling 'sin' and 'cos' separately.

Inhalt abgleichen
Willkommen im Weltenwald!
((λ()'Dr.ArneBab))



Beliebte Inhalte

sn.1w6.org news

Kommunikation im Internet ist in Gefahr!

Liebe Besucher,
leider müssen wir euch kurz unterbrechen:

Das EU-Parlament stimmt nächste Woche über eine Urheberrechtsreform ab,
die alle kleinen Kommunikationsplattformen in ihrer Existenz bedroht.

Offener Brief der Forenbetreiber

Wir bitten euch daher darum, dass ihr

Vielen Dank für eure Unterstützung!

Euer Draketo

Wir sind Teil des #321EUOfflineDay