[Bug c/36737] New: Wrong results from floating-point multiplication by 10

[Bug c/36737] New: Wrong results from floating-point multiplication by 10

[aremo at ngi dot it]

A C program, compiled without optimization, which does repeated multiplications
by 10 on a float number (0.2008) outputs incorrect results.

I know about the "most often reported non-bug", but since the K&R specifies
that FLT_DIG (decimal digits of precision) must be at least 6, I still consider
this to be a possible bug.

The program was compiled with command line: 'gcc --save-temps ~/mathbug.c'

-----BEGIN PROGRAM OUTPUT-----
nn = 2.008000
nn = 20.079998
nn = 200.799988
nn = 2007.999878
nn = 20079.998047
nn = 200799.984375
nn = 2007999.875000
------END PROGRAM OUTPUT------

-----BEGIN VERSION INFO-----
~> gcc -v
Using built-in specs.
Target: i386-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-libgcj-multifile
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk
--disable-dssi --enable-plugin
--with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre --with-cpu=generic
--host=i386-redhat-linux
Thread model: posix
gcc version 4.1.2 20070626 (Red Hat 4.1.2-14)
~> uname -a
Linux xyzzy.local 2.6.18-92.1.6.el5 #1 SMP Wed Jun 25 12:46:39 EDT 2008 i686
athlon i386 GNU/Linux
------END VERSION INFO------


-- 
           Summary: Wrong results from floating-point multiplication by 10
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: aremo at ngi dot it
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36737

[Bug c/36737] Wrong results from floating-point multiplication by 10

[aremo at ngi dot it]

------- Comment #1 from aremo at ngi dot it  2008-07-05 09:12 -------
Created an attachment (id=15860)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15860&action=view)
The program's source code


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36737

[Bug c/36737] Wrong results from floating-point multiplication by 10

[aremo at ngi dot it]

------- Comment #2 from aremo at ngi dot it  2008-07-05 09:14 -------
Created an attachment (id=15861)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15861&action=view)
The preprocessed source


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36737

[Bug c/36737] Wrong results from floating-point multiplication by 10

[dominiq at lps dot ens dot fr]

------- Comment #3 from dominiq at lps dot ens dot fr  2008-07-05 09:48 -------
Why do you think the results are incorrect? Did you hear aboud round-off
errors? 0.2008 cannot be represented exactly in float (nor in double) and you
cannot expect to have more that six significant digits. If you want to get what
you expect, use the right format for the output (not fluent enough in C to give
it to you).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36737

[Bug c/36737] Wrong results from floating-point multiplication by 10

[schwab at suse dot de]

------- Comment #4 from schwab at suse dot de  2008-07-05 09:54 -------
Not a bug.


-- 

schwab at suse dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36737

[Bug c/36737] Wrong results from floating-point multiplication by 10

[aremo at ngi dot it]

------- Comment #5 from aremo at ngi dot it  2008-07-05 10:06 -------
(In reply to comment #4)

Well it certainly gives wrong results.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36737

[Bug c/36737] Wrong results from floating-point multiplication by 10

[dominiq at lps dot ens dot fr]

------- Comment #6 from dominiq at lps dot ens dot fr  2008-07-05 10:16 -------
(In reply to comment #5)

> Well it certainly gives wrong results.

No! Read comment #3 and learn the primers about floating point numbers.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36737

[Bug c/36737] Wrong results from floating-point multiplication by 10

[pinskia at gcc dot gnu dot org]

------- Comment #7 from pinskia at gcc dot gnu dot org  2008-07-05 19:08 -------
You should also read http://www.validlab.com/goldberg/paper.pdf .

>Well it certainly gives wrong results.

In this case, the number is not directly representable in float.  You will be
able to get better precision with double but it will still not be directly
representable.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36737