= KnownCase

<<KnownCase#>> is an optimization pass for the <<SSA#>>
<<IntermediateLanguage#>>, invoked from <<SSASimplify#>>.

== Description

This pass duplicates and simplifies `Case` transfers when the
constructor of the scrutinee is known.

Uses <<Restore#>>.

For example, the program
[source,sml]
----
val rec last =
  fn [] => 0
   | [x] => x
   | _ :: l => last l

val _ = 1 + last [2, 3, 4, 5, 6, 7]
----

gives rise to the <<SSA#>> function

----
fun last_0 (x_142) = loopS_1 ()
  loopS_1 ()
    loop_11 (x_142)
  loop_11 (x_143)
    case x_143 of
      nil_1 => L_73 | ::_0 => L_74
  L_73 ()
    return global_5
  L_74 (x_145, x_144)
    case x_145 of
      nil_1 => L_75 | _ => L_76
  L_75 ()
    return x_144
  L_76 ()
    loop_11 (x_145)
----

which is simplified to

----
fun last_0 (x_142) = loopS_1 ()
  loopS_1 ()
    case x_142 of
      nil_1 => L_73 | ::_0 => L_118
  L_73 ()
    return global_5
  L_118 (x_230, x_229)
    L_74 (x_230, x_229, x_142)
  L_74 (x_145, x_144, x_232)
    case x_145 of
      nil_1 => L_75 | ::_0 => L_114
  L_75 ()
    return x_144
  L_114 (x_227, x_226)
    L_74 (x_227, x_226, x_145)
----

== Implementation

* https://github.com/MLton/mlton/blob/master/mlton/ssa/known-case.fun[`known-case.fun`]

== Details and Notes

One interesting aspect of <<KnownCase#>>, is that it often has the
effect of unrolling list traversals by one iteration, moving the
`nil`/`::` check to the end of the loop, rather than the beginning.
