# Is the Fixnum size difference a problem? **URL:** \ **Category:** IronRuby\ **Created:** [December 21, 2007, 9:53pm UTC](https://www.ruby-forum.com/t/is-the-fixnum-size-difference-a-problem/123902 "2007-12-21T21:53:49Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![editkid](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/977dab/32.png) [@editkid](https://www.ruby-forum.com/u/editkid)\ **Post date:** [December 21, 2007, 9:53pm UTC](https://www.ruby-forum.com/t/is-the-fixnum-size-difference-a-problem/123902/1 "2007-12-21T21:53:49Z")
The size of Fixnum in a standard C implementation of Ruby is a machine word minus one bit. I.E. For most 32 bit machines that is 31 bits. (The implementation uses the bottom bit as a flag). Therefore the largest value that you can hold in their Fixnum is 0x3fffffff. IronRuby uses System.Int32 to hold its Fixnum values and doesn’t have a flag bit so the size of Fixnum in IronRuby is 32 bits. Therefore the largest value you can hold is 0x7fffffff. This shouldn’t run into any problems since the transition between Fixnum and Bignum is supposedly transparent; you don’t really care (although you may know) what class is representing the value you are working with. Unfortunately, I recently noticed that the div method has different semantics depending upon whether you are a Fixnum or Bignum. some\_fixnum.div(0.0) =\> raises FloatDomainError some\_bignum.div(0.0) =\> Infinity IronRuby has the same semantics, except that of course what is a Bignum for CRuby is not a Bignum for IronRuby. In CRuby: 0x40000000.div(0.0) =\> Infinity In IronRuby: 0x40000000.div(0.0) =\> raises FloatDomainError Now clearly this is a problem with the fundamental implementation of Fixnum/Bignum but Rubinius seems to follow suit and I assume JRuby too (although I haven’t checked). What are your thoughts on dealing with this issue? Should we brush it under the carpet and hope that Ruby 2.0 will fix it or what? Should we force IronRuby’s Fixnum to be 31 bits? Any ideas most welcome. Pete ---
**Author:** ![editkid](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/977dab/32.png) [@editkid](https://www.ruby-forum.com/u/editkid)\ **Post date:** [December 21, 2007, 10:45pm UTC](https://www.ruby-forum.com/t/is-the-fixnum-size-difference-a-problem/123902/2 "2007-12-21T22:45:27Z")
Peter Bacon D.: > Now clearly this is a problem with the fundamental implementation of > Fixnum/Bignum but Rubinius seems to follow suit and I assume JRuby too > (although I haven’t checked). > > What are your thoughts on dealing with this issue? Should we brush it > under the carpet and hope that Ruby 2.0 will fix it or what? Should > we force IronRuby’s Fixnum to be 31 bits? I think we can keep Fixnums at 32 bits, and fix div to throw. Just tried in JRuby: irb(main):001:0\> 0x4000\_0000\_0000\_0000.class =\> Fixnum irb(main):002:0\> 0x4000\_0000.div(0.0) FloatDomainError: Infinity They actually have 64bit Fixnums… wow 🙂 - John ---
**Author:** ![editkid](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/977dab/32.png) [@editkid](https://www.ruby-forum.com/u/editkid)\ **Post date:** [December 21, 2007, 11:08pm UTC](https://www.ruby-forum.com/t/is-the-fixnum-size-difference-a-problem/123902/3 "2007-12-21T23:08:55Z")
John M. wrote: > I think we can keep Fixnums at 32 bits, and fix div to throw. Just tried in JRuby: > > irb(main):001:0\> 0x4000\_0000\_0000\_0000.class > =\> Fixnum > irb(main):002:0\> 0x4000\_0000.div(0.0) > FloatDomainError: Infinity > > They actually have 64bit Fixnums… wow 🙂 Yeah, we realized very recently that at some point we switches Fixnum to 64 bits, and now we don’t care to switch it back. So we’re using a Java long primitive internally to represent it, and I believe we’re stripping off the upper bit or two. This issue actually came up today while I was fixing some spec failures in JRuby. The specs do check that Fixnum is four bytes…but I talked with Rubinius folks and they agree it should be made flexible enough for other size Fixnums, since even Ruby’s own documentation says Fixnum may be different sizes on different hardware (and perhaps we may have used a 64-bit machine to run Ruby at some point, leading to JRuby’s 64-bit Fixnums). I don’t think the precision of Fixnum is a specifiable detail, especially since there’s no loss of precision from a rollover into Bignum…whenever that happens. - Charlie