wiki:ReplacingGMPNotes/HaskellLibrary

Replacing GMP with a Haskell integer arithmetic library

One possiblity is to replace GMP with an arbitrary-precision integer arithmetic library written entirely in Haskell. There is at least one available:

The downside is speed, of course. The upside is that (a) you don't need GMP and (b) it's relatively easy to do.

Because of the speed issue we can't abandon GMP. So there are two possibilities:

  • Easy: add a build-time option so that you can build GHC with GMP, or instead build GHC with a Haskell Integer library. Disadvantage: there would be two versions of (say) ghc-6.8:
    • vanilla ghc-6.8 using GMP
    • ghc-6.8-special with Haskell Integer library
  • Harder: give a new flag to GHC, say -integer-package=my-integer-2.0. The intention would be that the compiler would then use the Integer library in package my-integer-2.0 instead of GMP. Advantage: only one version of GHC.

Here are some notes on what you'd need to do to achieve this. You'll have to change three bits:

  • The libraries
  • The compiler
  • The runtime system

The libraries

Currently Integer is defined in the module GHC.Num. It would not be hard to pull out the Integer stuff, into a separate module GHC.Integer (exporting a well-defined interface, and not exposing the representation of Integer) that GHC.Num imported. GHC.Num is where class Num is defined, and Num uses Integer in the type of the class method fromInteger; that's why GHC.Num has to import Integer and not the other way round.

Then for the "easy" plan, you could simply change the import so that instead of GHC.Num importing GHC.Integer it imported MyIntegerModule or whatever. Now recompile all libraries.

The tricky bit about the "harder" plan is that you'll need to build the libraries twice; once with GHC.Num importing GHC.Integer, and once with GHC.Num importing your MyIntegerModule instead. You'd need to ship GHC with both sets of libraries; the -integer-package flag would select which set of libraries you use when compiling and linking. It's rather unclear exactly how to do this so that it works nicely with Cabal etc.

The compiler

GHC knows relatively little about the Integer type. Look in prelude/PrelNames and search for Integer. These Names "know" which module the functions are defined in; look at the defn of plusIntegerName for example.

Currently, then, GHC expects the type Integer and the value plusInteger to be defined in GHC.Num. That's fine: just add defns like this:

module GHC.Num where
  import GHC.Integer( Integer, plusInteger, ... ) as I
    -- or import MyIntegerModule(...) as I

  type Integer = I.Integer
  plusInteger = I.plusInteger
  ...etc...

Now GHC will still find GHC.Num.Integer but it'll find a type synonym that will tell it where Integer is really defined. Similarly plusInteger.

Integer literals are built by the desugarer in DsUtils.mkIntegerExpr. All it assumes is that there are functions:

  smallIntegerDataCon :: Int -> Integer
  plusInteger, timesInteger :: Integer -> Integer -> Integer

It uses these functions to construct big integer literals using arithmetic. Despite the name, it's not important that smallIntegerDataCon is bound to a data constructor. All the library needs to provide is a way to convert an Int to an Integer. So that can be handled just the same way as above.

In short, the compiler can be untouched EXCEPT for any jiggery pokery to do with finding the right libraries when using the "harder" approach.

The runtime system

You'd need to build two versions of the runtime system too, one omitting all the GMP goop.

Last modified 7 years ago Last modified on Sep 13, 2007 3:37:48 PM