Copy propagation, remove unused values (closures!) and local variables

Copy propagation uses an AliasingAnalyzer: it replaces a `LOAD n`
instruction by `LOAD m` where m is the smallest alias of n. This
leads to stale STORE instructions.

Stale STOREs are identified using a ProdCons analyzer and replaced by
POPs.

Values that are pushed on the stack by a side-effect free instruction
and consumed by a POP are then removed by `eliminatePushPop`. This
includes elimination of unused closure allocations and unused boxes
and  tuple allocations (*).

A final cleanup eliminates `STORE x; LOADx` pairs where the stored
value is not otherwise used.

Fixes
  - https://github.com/scala/scala-dev/issues/25
  - https://github.com/scala/scala-dev/issues/7
  - https://github.com/scala/scala-dev/issues/14
  - https://github.com/scala/scala-dev/issues/12

(*) We don't yet rewrite reads of boxes and tuples yet. For example,
`val x = (1, 2); x._1` remains a method invocation and the tuple
cannot be eliminated (https://github.com/scala/scala-dev/issues/11).

Inspired in many ways by Miguel's work!

1 files changed, 3 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala b/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala
index a9ec8c7b30..cbd9ee1a55 100644
--- a/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala
+++ b/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala
@@ -230,6 +230,7 @@ trait ScalaSettings extends AbsScalaSettings
     val emptyLineNumbers        = Choice("empty-line-numbers",        "Eliminate unnecessary line number information.")
     val emptyLabels             = Choice("empty-labels",              "Eliminate and collapse redundant labels in the bytecode.")
     val compactLocals           = Choice("compact-locals",            "Eliminate empty slots in the sequence of local variables.")
+    val copyPropagation         = Choice("copy-propagation",          "Eliminate redundant local variables (locals that are copies of others).")
     val nullnessTracking        = Choice("nullness-tracking",         "Track nullness / non-nullness of local variables and apply optimizations.")
     val closureElimination      = Choice("closure-elimination" ,      "Rewrite closure invocations to the implementation method and eliminate closures.")
     val inlineProject           = Choice("inline-project",            "Inline only methods defined in the files being compiled.")
@@ -240,7 +241,7 @@ trait ScalaSettings extends AbsScalaSettings
     private val defaultChoices = List(unreachableCode)
     val lDefault        = Choice("l:default",   "Enable default optimizations: "+ defaultChoices.mkString(","),                                    expandsTo = defaultChoices)
 
-    private val methodChoices = List(unreachableCode, simplifyJumps, emptyLineNumbers, emptyLabels, compactLocals, nullnessTracking, closureElimination)
+    private val methodChoices = List(unreachableCode, simplifyJumps, emptyLineNumbers, emptyLabels, compactLocals, copyPropagation, nullnessTracking, closureElimination)
     val lMethod         = Choice("l:method",    "Enable intra-method optimizations: "+ methodChoices.mkString(","),                                expandsTo = methodChoices)
 
     private val projectChoices = List(lMethod, inlineProject)
@@ -262,6 +263,7 @@ trait ScalaSettings extends AbsScalaSettings
   def YoptEmptyLineNumbers        = Yopt.contains(YoptChoices.emptyLineNumbers)
   def YoptEmptyLabels             = Yopt.contains(YoptChoices.emptyLabels)
   def YoptCompactLocals           = Yopt.contains(YoptChoices.compactLocals)
+  def YoptCopyPropagation         = Yopt.contains(YoptChoices.copyPropagation)
   def YoptNullnessTracking        = Yopt.contains(YoptChoices.nullnessTracking)
   def YoptClosureElimination      = Yopt.contains(YoptChoices.closureElimination)