doc/guide/src/KnownCase.adoc

   1 KnownCase
   2 =========
   3
   4 <:KnownCase:> is an optimization pass for the <:SSA:>
   5 <:IntermediateLanguage:>, invoked from <:SSASimplify:>.
   6
   7 == Description ==
   8
   9 This pass duplicates and simplifies `Case` transfers when the
  10 constructor of the scrutinee is known.
  11
  12 Uses <:Restore:>.
  13
  14 For example, the program
  15 [source,sml]
  16 ----
  17 val rec last =
  18   fn [] => 0
  19    | [x] => x
  20    | _ :: l => last l
  21
  22 val _ = 1 + last [2, 3, 4, 5, 6, 7]
  23 ----
  24
  25 gives rise to the <:SSA:> function
  26
  27 ----
  28 fun last_0 (x_142) = loopS_1 ()
  29   loopS_1 ()
  30     loop_11 (x_142)
  31   loop_11 (x_143)
  32     case x_143 of
  33       nil_1 => L_73 | ::_0 => L_74
  34   L_73 ()
  35     return global_5
  36   L_74 (x_145, x_144)
  37     case x_145 of
  38       nil_1 => L_75 | _ => L_76
  39   L_75 ()
  40     return x_144
  41   L_76 ()
  42     loop_11 (x_145)
  43 ----
  44
  45 which is simplified to
  46
  47 ----
  48 fun last_0 (x_142) = loopS_1 ()
  49   loopS_1 ()
  50     case x_142 of
  51       nil_1 => L_73 | ::_0 => L_118
  52   L_73 ()
  53     return global_5
  54   L_118 (x_230, x_229)
  55     L_74 (x_230, x_229, x_142)
  56   L_74 (x_145, x_144, x_232)
  57     case x_145 of
  58       nil_1 => L_75 | ::_0 => L_114
  59   L_75 ()
  60     return x_144
  61   L_114 (x_227, x_226)
  62     L_74 (x_227, x_226, x_145)
  63 ----
  64
  65 == Implementation ==
  66
  67 * <!ViewGitFile(mlton,master,mlton/ssa/known-case.fun)>
  68
  69 == Details and Notes ==
  70
  71 One interesting aspect of <:KnownCase:>, is that it often has the
  72 effect of unrolling list traversals by one iteration, moving the
  73 `nil`/`::` check to the end of the loop, rather than the beginning.