MLton 20070826 SimplifyTypes
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SimplifyTypes is an optimization pass for the SSA IntermediateLanguage, invoked from SSASimplify.

Description

This pass computes a "cardinality" of each datatype, which is an abstraction of the number of values of the datatype.

This pass removes all datatypes whose cardinality is Zero or One and removes:

which are such datatypes.

This pass marks constructors as one of:

This pass also removes Useless and Transparent constructors.

Implementation

[WWW]simplify-types.sig [WWW]simplify-types.fun

Details and Notes

This pass must happen before polymorphic equality is implemented because

  1. it will make polymorphic equality faster because some types are simpler

  2. it removes uses of polymorphic equality that must return true

We must keep track of Transparent constructors whose argument type uses array because of datatypes like the following:

  datatype t = T of t vector
Such a datatype has Cardinality.Many, but we cannot eliminate the datatype and replace the lhs by the rhs, i.e. we must keep the circularity around.

Must do similar things for vectors.

Also, to eliminate as many Transparent constructors as possible, for something like the following,

  datatype t = T of u array
       and u = U of t vector
we (arbitrarily) expand one of the datatypes first. The result will be something like
  datatype u = U of u array array
where all uses of t are replaced by u array.


Last edited on 2006-11-02 17:39:32 by MatthewFluet.