This pass duplicates and simplifies Case transfers when the constructor of the scrutinee is known.
For example, the program
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)
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.