doc/guide/src/Redundant.adoc

   1 Redundant
   2 =========
   3
   4 <:Redundant:> is an optimization pass for the <:SSA:>
   5 <:IntermediateLanguage:>, invoked from <:SSASimplify:>.
   6
   7 == Description ==
   8
   9 The redundant SSA optimization eliminates redundant function and label
  10 arguments; an argument of a function or label is redundant if it is
  11 always the same as another argument of the same function or label.
  12 The analysis finds an equivalence relation on the arguments of a
  13 function or label, such that all arguments in an equivalence class are
  14 redundant with respect to the other arguments in the equivalence
  15 class; the transformation selects one representative of each
  16 equivalence class and drops the binding occurrence of
  17 non-representative variables and renames use occurrences of the
  18 non-representative variables to the representative variable.  The
  19 analysis finds the equivalence classes via a fixed-point analysis.
  20 Each vector of arguments to a function or label is initialized to
  21 equivalence classes that equate all arguments of the same type; one
  22 could start with an equivalence class that equates all arguments, but
  23 arguments of different type cannot be redundant.  Variables bound in
  24 statements are initialized to singleton equivalence classes.  The
  25 fixed-point analysis repeatedly refines these equivalence classes on
  26 the formals by the equivalence classes of the actuals.
  27
  28 == Implementation ==
  29
  30 * <!ViewGitFile(mlton,master,mlton/ssa/redundant.fun)>
  31
  32 == Details and Notes ==
  33
  34 The reason <:Redundant:> got put in was due to some output of the
  35 <:ClosureConvert:> pass converter where the environment record, or
  36 components of it, were passed around in several places.  That may have
  37 been more relevant with polyvariant analyses (which are long gone).
  38 But it still seems possibly relevant, especially with more aggressive
  39 flattening, which should reveal some fields in nested closure records
  40 that are redundant.