GNU bug report logs - #11310
wrong conversions of ieee754 from/to foreign endianness

Previous Next

Full log

View this message in rfc822 format

From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Klaus Stehle <klaus.stehle <at> uni-tuebingen.de>
Subject: bug#11310: closed (Re: bug#11310: wrong conversions of ieee754
 from/to foreign endianness)
Date: Sun, 22 Apr 2012 18:46:02 +0000

[Message part 1 (text/plain, inline)]

Your bug report

#11310: wrong conversions of ieee754 from/to foreign endianness

which was filed against the guile package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 11310 <at> debbugs.gnu.org.

-- 
11310: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=11310
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

[Message part 2 (message/rfc822, inline)]

From: ludo <at> gnu.org (Ludovic Courtès)
To: Klaus Stehle <klaus.stehle <at> uni-tuebingen.de>
Cc: 11310-done <at> debbugs.gnu.org
Subject: Re: bug#11310: wrong conversions of ieee754 from/to foreign endianness
Date: Sun, 22 Apr 2012 20:44:54 +0200

Hi Klaus,

Thanks for the report!  Fixed in
398446c7428b3d98d168fcc3ff170829d3e09f9a, and double-checked on HPPA.

Ludo’.

[Message part 3 (message/rfc822, inline)]

From: Klaus Stehle <klaus.stehle <at> uni-tuebingen.de>
To: bug-guile <at> gnu.org
Subject: wrong conversions of ieee754 from/to foreign endianness
Date: Sun, 22 Apr 2012 18:44:44 +0200 (CEST)

Hi,

There is a problem in bytevectors.c - conversions of float and double
to the foreign endianess don't work.


Here is the scheme example:
---------------------------
(use-modules (rnrs bytevectors))

(define bv (make-bytevector 4))

(bytevector-ieee-single-set! bv 0 1.0 (endianness little))
(display bv) (newline)

=> #vu8(0 0 128 63)                     correct!

(bytevector-ieee-single-set! bv 0 1.0 (endianness big))
(display bv) (newline)

=> #vu8(254 0 0 0)                      oops!

The results come from a little-endian machine and therefore the
big-endian result is wrong. We expect #vu8(63 128 0 0).



In bytevectors.c there is the following code:

#ifdef WORDS_BIGENDIAN
  /* Assuming little endian for both byte and word order.  */
  target->little_endian.negative = src.big_endian.negative;
  target->little_endian.exponent = src.big_endian.exponent;
  target->little_endian.mantissa = src.big_endian.mantissa;
#else
  target->big_endian.negative = src.little_endian.negative;
  target->big_endian.exponent = src.little_endian.exponent;
  target->big_endian.mantissa = src.little_endian.mantissa;
#endif

Using the 'FOREIGN' structure of the scm_ieee754_float union
does NOT work, because in this case the structure assumes the
wrong  *BIT* order.



example in C shows better (to run on a little endian machine):
--------------------------------------------------------------
union scm_ieee754_float target;
target.f = 0.0;
target.litte_endian.negative = 1;

=> 00 00 00 80                  correct!

target.f = 0.0;
target.big_endian.negative = 1;

=> 01 00 00 00                  oops!


Maybe this effect depends on the machine and the compiler.
I am using an amd64 with gcc.

The bug should be easy to fix, instead of using the ieee754 unions
and their weird structures, exchanging whole bytes should suffice
and this probably may be faster.

Thanks
Klaus

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.