Issue 26 in libfixmath: fix16_div bug due to __builtin_clzl

[libfi...@googlecode.com]

Status: New
Owner: ----
Labels: Type-Defect Priority-Medium

New issue 26 by mark.kat...@gmail.com: fix16_div bug due to __builtin_clzl
http://code.google.com/p/libfixmath/issues/detail?id=26

To reproduce issue:

#include <iostream>
#include "fixmath.h"

std::cout <<
fix16_to_float(
fix16_div(
fix16_from_float(10.0f),
fix16_from_float(1.1f)
)
);
std::cout << std::endl;

Expected versus actual output:

expected: 9.09087
actual: 0

Version/operating system/compiler details:

libfixmath version: r64
operating system: Linux Raspberry 3.5.0-22-generic #34-Ubuntu SMP Tue
Jan 8 21:47:00 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
compiler: gcc version 4.7.2 (Ubuntu/Linaro 4.7.2-2ubuntu1)
compilation flags include: -O0 -g -std=c++0x

Details:

Bug seems to be due to __builtin_clzl, which returns 44 for
fix16_from_int(10), causing all sorts of problems. I guess you may be
falsely assuming this function counts the number of leading zeros of a
32-bit integer, whereas really an int may have more than 32 bits.

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[libfi...@googlecode.com]

Updates:
Owner: Petteri.Aimonen

Comment #1 on issue 26 by Petteri.Aimonen: fix16_div bug due to
__builtin_clzl
http://code.google.com/p/libfixmath/issues/detail?id=26

Hmm, isn't this a duplicate of issue #17, which is already fixed? The macro
should be based on sizeof(long) so that bitcount is taken into account:
https://code.google.com/p/libfixmath/source/browse/trunk/libfixmath/fix16.c#278

Running the attached test.cc on my computer:

petteri@oddish:~/libfixmath/unittests$ uname -a
Linux oddish 3.11.0-12-generic #19-Ubuntu SMP Wed Oct 9 16:20:46 UTC 2013
x86_64 x86_64 x86_64 GNU/Linux
petteri@oddish:~/libfixmath/unittests$ g++ -I ../libfixmath -o test
test.cc ../libfixmath/fix16.c
petteri@oddish:~/libfixmath/unittests$ ./test
sizeof(int) = 4
sizeof(long) = 8
9.09087


Attachments:
test.cc 403 bytes

[libfi...@googlecode.com]

Updates:
Owner: Flat...@googlemail.com

Comment #2 on issue 26 by Petteri.Aimonen: fix16_div bug due to
__builtin_clzl
http://code.google.com/p/libfixmath/issues/detail?id=26

Ah, just saw the revision number r64. Upgrade to newest svn and it should
work.

flatmush: maybe we should put a new release on the downloads tab some
time? :)

[libfi...@googlecode.com]

Comment #3 on issue 26 by mark.kat...@gmail.com: fix16_div bug due to
__builtin_clzl
http://code.google.com/p/libfixmath/issues/detail?id=26

I would second that suggestion. If this is a known bug that has been fixed
then maybe the default suggested download shouldn't be a version of the
library which still contains this bug.

[libfi...@googlecode.com]

Updates:
Status: Done
Cc: Flat...@googlemail.com

Comment #4 on issue 26 by Petteri.Aimonen: fix16_div bug due to
__builtin_clzl
http://code.google.com/p/libfixmath/issues/detail?id=26

flatmush: I took the liberty of marking the downloads as deprecated. Google
Code will discontinue the supports for downloads soon anyway, so maybe it
is better to only have svn.