doc/guide/src/PolyEqual.adoc

   1 PolyEqual
   2 =========
   3
   4 <:PolyEqual:> is an optimization pass for the <:SSA:>
   5 <:IntermediateLanguage:>, invoked from <:SSASimplify:>.
   6
   7 == Description ==
   8
   9 This pass implements polymorphic equality.
  10
  11 == Implementation ==
  12
  13 * <!ViewGitFile(mlton,master,mlton/ssa/poly-equal.fun)>
  14
  15 == Details and Notes ==
  16
  17 For each datatype, tycon, and vector type, it builds and equality
  18 function and translates calls to `MLton_equal` into calls to that
  19 function.
  20
  21 Also generates calls to `Word_equal`.
  22
  23 For tuples, it does the equality test inline; i.e., it does not create
  24 a separate equality function for each tuple type.
  25
  26 All equality functions are created only if necessary, i.e., if
  27 equality is actually used at a type.
  28
  29 Optimizations:
  30
  31 * for datatypes that are enumerations, do not build a case dispatch,
  32 just use `MLton_eq`, as the backend will represent these as ints
  33
  34 * deep equality always does an `MLton_eq` test first
  35
  36 * If one argument to `=` is a constant and the type will get
  37 translated to an `IntOrPointer`, then just use `eq` instead of the
  38 full equality.  This is important for implementing code like the
  39 following efficiently:
  40 +
  41 ----
  42 if x = 0  ...    (* where x is of type IntInf.int *)
  43 ----
  44
  45 * Also convert pointer equality on scalar types to type specific
  46 primitives.