doc/guide/src/Useless.adoc

   1 Useless
   2 =======
   3
   4 <:Useless:> is an optimization pass for the <:SSA:>
   5 <:IntermediateLanguage:>, invoked from <:SSASimplify:>.
   6
   7 == Description ==
   8
   9 This pass:
  10
  11 * removes components of tuples that are constants (use unification)
  12 * removes function arguments that are constants
  13 * builds some kind of dependence graph where
  14 ** a value of ground type is useful if it is an arg to a primitive
  15 ** a tuple is useful if it contains a useful component
  16 ** a constructor is useful if it contains a useful component or is used in a `Case` transfer
  17
  18 If a useful tuple is coerced to another useful tuple, then all of
  19 their components must agree (exactly).  It is trivial to convert a
  20 useful value to a useless one.
  21
  22 == Implementation ==
  23
  24 * <!ViewGitFile(mlton,master,mlton/ssa/useless.fun)>
  25
  26 == Details and Notes ==
  27
  28 It is also trivial to convert a useful tuple to one of its useful
  29 components -- but this seems hard.
  30
  31 Suppose that you have a `ref`/`array`/`vector` that is useful, but the
  32 components aren't -- then the components are converted to type `unit`,
  33 and any primitive args must be as well.
  34
  35 Unify all handler arguments so that `raise`/`handle` has a consistent
  36 calling convention.