Control of IEEE compliant FPUs rounding mode and precision
Ruby C
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Ruby gem for the control of IEEE compliant FPUs rounding mode and precision (if available).

IEEE_FPU has two writeable properties:

  • rounding which has values :even, :up, :down and :zero

  • precision with values :single, :double and :extended

The latter property is not available in all IEEE FPUs

The scope method can be used to set up an scope in which FPU properties are modified; when the scope is exited the FPU state is restored.


puts IEEE_FPU.rounding                # -> even
puts "%.20f"%(1.0+Float::EPSILON/2)   # -> 1.00000000000000000000
IEEE_FPU.scope {
  IEEE_FPU.rounding = :down
  puts IEEE_FPU.rounding              # -> down
  puts "%.20f"%(1.0+Float::EPSILON/2) # -> 1.00000000000000000000
IEEE_FPU.scope { |fpu|
  fpu.rounding = :up
  puts IEEE_FPU.rounding              # -> up
  puts "%.20f"%(1.0+Float::EPSILON/2) # -> 1.00000000000000022204
puts IEEE_FPU.rounding                # -> even
puts "%.20f"%(1.0+Float::EPSILON/2)   # -> 1.00000000000000000000

expr = '(3.2-2.0)-1.2'
IEEE_FPU.scope {
  IEEE_FPU.precision = :single           # -> 0.0
  puts eval(expr)
IEEE_FPU.scope {
  IEEE_FPU.precision = :double
  puts eval(expr)                        # -> 2.22044604925031e-16

Note: IEEE_FPU.precision=:extended has a very limited influence in Ruby because all results are casted to Float (double) values. Its effect is more noticeable in C/C++ extensions.

FPU parameters can be assigneed passign a hash to the scope method:

IEEE_FPU.scope(:precision=>:single) {
  puts eval(expr)

An array can be passed for one (only one) of the parameters; the block will be repeated with every value of the parameter and the results will be collected in an array.

For example this checks the result of an expression in all the rounding modes:

puts IEEE_FPU.scope(:rounding=>[:even,:up,:down,:zero]) {
  1.0 + Float::EPSILON/2

Different parameters can be passed to each call of the block by assigning an array to :parameters

For example, interval arithmetic could be implementes like this:

def sum_intervals(i1,i2)
  IEEE_FPU.scope(:rounding=>[:down,:up],:parameters=>[0,1]) do |fpu,i|
i = sum_intervals([1.0]*2, [Float::EPSILON/2]*2)


  • Gem with the C extension; avoid compiling under mswin32 if possible (the extension is not needed)

  • Binary gems for mingw32

  • Rename IEEE_FPU, review API design

  • Flt interoperatibility

Contributing to ieee-fpu

  • Check out the latest master to make sure the feature hasn't been implemented or the bug hasn't been fixed yet.

  • Check out the issue tracker to make sure someone already hasn't requested it and/or contributed it.

  • Fork the project.

  • Start a feature/bugfix branch.

  • Commit and push until you are happy with your contribution.

  • Make sure to add tests for it. This is important so I don't break it in a future version unintentionally.

  • Please try not to mess with the Rakefile, version, or history. If you want to have your own version, or is otherwise necessary, that is fine, but please isolate to its own commit so I can cherry-pick around it.


Copyright © 2007, 2012 Javier Goizueta. See LICENSE.txt for further details.