GenBCode: Compact local variable slots

After eliminating unreachable code, there may be unused local varaible
slots. Such gaps increase the method's frame size unnecessarily.

The optimization does not attempt to re-use the same local variable
slot for non-overlapping locals (proper register allocation):

    def f(b: Boolean) = if (b) { val x = e; x } else { val y = e; y }

x and y will not use the same slot, even though they could.

This was originally implemented by Miguel in
  https://github.com/lrytz/scala/commit/09c40359338f8770e4f99d045999af062112973e

1 files changed, 7 insertions, 5 deletions

diff --git a/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala b/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala
index 2406b9e66b..2c9d101106 100644
--- a/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala
+++ b/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala
@@ -215,20 +215,21 @@ trait ScalaSettings extends AbsScalaSettings
     val recurseUnreachableJumps = Choice("recurse-unreachable-jumps", "Recursively apply unreachable-code and simplify-jumps (if enabled) until reaching a fixpoint.")
     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 lNone           = Choice("l:none",      "Don't enable any optimizations.")
 
     private val defaultChoices = List(unreachableCode)
-    val lDefault        = Choice("l:default",   "Enable default optimizations: "+ defaultChoices.mkString(","),                                   expandsTo = defaultChoices)
+    val lDefault        = Choice("l:default",   "Enable default optimizations: "+ defaultChoices.mkString(","),                                    expandsTo = defaultChoices)
 
-    private val methodChoices = List(unreachableCode, simplifyJumps, recurseUnreachableJumps, emptyLineNumbers, emptyLabels)
-    val lMethod         = Choice("l:method",    "Enable intra-method optimizations: "+ methodChoices.mkString(","),                               expandsTo = methodChoices)
+    private val methodChoices = List(unreachableCode, simplifyJumps, recurseUnreachableJumps, emptyLineNumbers, emptyLabels, compactLocals)
+    val lMethod         = Choice("l:method",    "Enable intra-method optimizations: "+ methodChoices.mkString(","),                                expandsTo = methodChoices)
 
     private val projectChoices = List(lMethod)
-    val lProject        = Choice("l:project",   "Enable cross-method optimizations within the current project: "+ projectChoices.mkString(","),   expandsTo = projectChoices)
+    val lProject        = Choice("l:project",   "Enable cross-method optimizations within the current project: "+ projectChoices.mkString(","),    expandsTo = projectChoices)
 
     private val classpathChoices = List(lProject)
-    val lClasspath      = Choice("l:classpath", "Enable cross-method optmizations across the entire classpath: "+ classpathChoices.mkString(","), expandsTo = classpathChoices)
+    val lClasspath      = Choice("l:classpath", "Enable cross-method optimizations across the entire classpath: "+ classpathChoices.mkString(","), expandsTo = classpathChoices)
   }
 
   val Yopt = MultiChoiceSetting(
@@ -242,6 +243,7 @@ trait ScalaSettings extends AbsScalaSettings
   def YoptRecurseUnreachableJumps = Yopt.contains(YoptChoices.recurseUnreachableJumps)
   def YoptEmptyLineNumbers        = Yopt.contains(YoptChoices.emptyLineNumbers)
   def YoptEmptyLabels             = Yopt.contains(YoptChoices.emptyLabels)
+  def YoptCompactLocals           = Yopt.contains(YoptChoices.compactLocals)
 
   private def removalIn212 = "This flag is scheduled for removal in 2.12. If you have a case where you need this flag then please report a bug."