Re: Bug in Numeric#divmod

Dirk Traulsen wrote:
> Hi all!
> 
> I submitted the following bug report to 
> http://rubyforge.org/tracker/?func=detail&atid=1698&aid=14540&
> group_id=426
> and reported it on ruby-talk.
> 
> But Eric Hodel wrote:
>> You may want to bring this up on ruby-core (maybe you have?  There  
>> was also a documentation patch for divmod, so I may be confused).
> 
> I take his advice and report it here too together with all the 
> additional comments sent.
> (By the way, the documentation patch, which is also from me, is for the 
> same method, but unrelated.)
> 
> ===================================================
>> There is a calculating bug in Numeric#divmod.
>> The divisor is reproducibly calculated wrongly for certain numbers.
>>
>> See the following example to illustrate the problem:
>>
>> x=[0.58, 0.59, 0.63, 0.66, 0.67]
>> y=1/12.0
>>
>> z1=x.map {|a| a/y}
>> z2=x.map {|a| a.divmod(y).inspect}
>>
>> a=[RUBY_VERSION, RUBY_RELEASE_DATE, RUBY_PATCHLEVEL, RUBY_PLATFORM]
>> puts "ruby #{a[0]} (#{a[1]} patchlevel #{a[2].to_s}) [#{a[3]}]"
>> puts
>> print 'x='
>> p x
>> puts 'y=1/12.0'
>> puts
>> puts "x/y\n---"
>> puts z1
>> puts
>> puts "x.divmod(y)\n-----------"
>> puts z2
>>
>>
>> Output:
>> -------------------------------------------------
>> ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32]
>>
>> x=[0.58, 0.59, 0.63, 0.66, 0.67]
>> y=1/12.0
>>
>> x/y
>> ---
>> 6.96
>> 7.08
>> 7.56
>> 7.92
>> 8.04
>>
>> x.divmod(y)
>> -----------
>> [6, 0.08]
>> [6, 0.00666666666666667]
>> [6, 0.0466666666666667]
>> [6, 0.0766666666666667]
>> [8, 0.00333333333333341]
>> -------------------------------------------------
>>
>> As you can see the divisor 7 is wrongly displayed as 6, while the 
>> modulos are correct.
>>
>> Dirk Traulsen
> ===================================================
> 
> Here are the additional comments:
> 
> By me:
> ===================================================
>> I tested Numeric#divmod on two more systems: 
>> SUSE 10.1 and Kubuntu 6.10.
>> They both run Ruby 1.8.4 and under both systems Numeric#divmod
>> works correctly! So this bug was either introduced between 
>> Ruby 1.8.4 and 1.8.6 or it is a problem depending on the OS.
> (...)
>> SUSE 10.1:   ruby 1.8.4 (2005-12-24) [i586-linux]
>>
>> x.divmod(y)
>> -----------
>> [6.0, 0.08]
>> [7.0, 0.00666666666666667]
>> [7.0, 0.0466666666666667]
>> [7.0, 0.0766666666666667]
>> [8.0, 0.00333333333333341]
>>
>>
>> Kubuntu 6.10:   ruby 1.8.4 (2005-12-24) [i586-linux]
>>
>> x.divmod(y)
>> -----------
>> [6.0, 0.08]
>> [7.0, 0.00666666666666667]
>> [7.0, 0.0466666666666667]
>> [7.0, 0.0766666666666667]
>> [8.0, 0.00333333333333341]
> ===================================================
> 
> 
> On ruby-talk:
> =================================================
> Am 14 Oct 2007 um 17:03 hat Eric Hodel geschrieben:
> 
>> I get the same results for 1.8.6 and 1.9.0:
>>
>> ruby 1.8.6 (2007-09-23) [powerpc-darwin8.10.0]
>> ruby 1.9.0 (2007-10-13) [powerpc-darwin8.10.0]
> (...) 
>> x.divmod(y)
>> -----------
>> [6, 0.08]
>> [6, 0.00666666666666667]
>> [6, 0.0466666666666667]
>> [6, 0.0766666666666667]
>> [8, 0.00333333333333341]
> =================================================
> Am 14 Oct 2007 um 17:02 hat Konrad Meyer geschrieben:
>  
>> I can't confirm this (incorrect) behaviour.
>>
>> 0.58.divmod(1/12.0) # => [6.0, 0.08]
>> 0.59.divmod(1/12.0) # => [7.0, 0.00666666666666667]
>>
>> Ruby 1.8.6, Fedora 7.
> =================================================
> Am 14 Oct 2007 um 18:15 hat Alex Shulgin geschrieben:
> 
>> It's correct on my system:
>>
>> Debian lenny (testing)
>> Linux 2.6.21-2-686 #1 SMP Wed Jul 11 03:53:02 UTC 2007 i686
>> GNU/Linux 
>> ruby 1.8.6 (2007-06-07) [i486-linux]
> (...)
>> x.divmod(y)
>> -----------
>> [6, 0.08]
>> [7, 0.00666666666666667]
>> [7, 0.0466666666666667]
>> [7, 0.0766666666666667]
>> [8, 0.00333333333333341]
> ==========================================
> Am 14 Oct 2007 um 19:44 hat M. Edward (Ed) Borasky geschrieben:
> 
>> And on Gentoo Linux with ruby-1.8.6-p110-r1
>>
>> ruby 1.8.6 (2007-09-24) [i686-linux]
> (...)
>> x.divmod(y)
>> -----------
>> [6, 0.08]
>> [7, 0.00666666666666667]
>> [7, 0.0466666666666667]
>> [7, 0.0766666666666667]
>> [8, 0.00333333333333341]
>>
>> If it matters, the hardware is an Athlon Thunderbird and Ruby was
>> compiled from source, as most of Gentoo is, using gcc 4.2.1 and CFLAGS
>> "-march athlon-tbird -O2". So ... where is it broken again? 
>>
>> Dirk says "ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32] is
>> broken" Dirk says "Kubuntu 6.10 and SuSE 10.1 using Ruby 1.8.4 works."
>> Konrad says "two test cases worked on Fedora 7 with 1.8.6" And I have
>> it working on Ruby 1.8.6-p110 on Gentoo, compiled from source with GCC
>> 4.2.1 And Eric has it broken on PowerPC Macs for both Ruby 1.8.6 and
>> Ruby 1.9.0 
>>
>> Eric, what compiler made the Ruby interpreters on your PowerPC?
>>
>> And everybody -- recompile ruby-1.8.6-p111 and see whether it works or
>> not. I say it's a compiler problem at this point -- maybe
>> over-aggressive floating-point optimization?? I doubt if the Ruby
>> source is broken. 
> ==========================================
> 
> 
> Taken together:
> 
> incorrect = bug:
> ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32]
> ruby 1.8.6 (2007-09-23) [powerpc-darwin8.10.0]
> ruby 1.9.0 (2007-10-13) [powerpc-darwin8.10.0]
>  
> correct:
> Fedora 7:  Ruby 1.8.6
> Kubuntu 6.10:  ruby 1.8.4 (2005-12-24) [i586-linux]
> SUSE 10.1:  ruby 1.8.4 (2005-12-24) [i586-linux]
> Debian Lenny:  ruby 1.8.6 (2007-06-07) [i486-linux]
> Gentoo Linux with ruby-1.8.6-p110-r1
> 
> Please help to confirm these findings. Especially with not tested 
> system and Ruby version combinations this would be really helpful
> to nail down this massive bug. A computer should really calculate 
> correctly...
> 
> Dirk Traulsen
> 
> 
> 

Gosh ... just when I was getting all smug and complacent, I wandered 
over to my fancy-schmancy dual-core Athlon64 X2 and found that *it* is 
broken!

$ ruby divmod.rb
ruby 1.8.6 (2007-09-24) [x86_64-linux]

x=[0.58, 0.59, 0.63, 0.66, 0.67]
y=1/12.0

x/y
---
6.96
7.08
7.56
7.92
8.04

x.divmod(y)
-----------
[6, 0.08]
[6, 0.00666666666666667]
[6, 0.0466666666666667]
[6, 0.0766666666666667]
[8, 0.00333333333333341]

Again, this is Ruby 1.8.6-p110-r1, although I don't recall which 
compiler I used -- probably GCC 4.2.0 and definitely "-march=athlon64 
-O2". I've got lots of other Ruby versions on that system -- I'm testing 
GCC optimization, etc., on it. So I think I'm going to stuff this into 
my benchmark suite and see if it's optimization-level dependent. That 
might or might not make it a GCC problem. ;)