DRY-er trees in pattern matcher code gen.

Rather than building a cascade of if/elses, push additional conditions
into a conjunction in the condition of a single if/else. This is
possible when emitting conditions for the list of arguments of a
pattern.

Here's an example of the improvement to post-pattern matcher trees:

  https://gist.github.com/retronym/0d8f7126157061d72b81

While we could try to rely on the optimizer to coalesce the repeated
else clauses, it seems wasteful to emit the code in that way in
the first place.

1 files changed, 9 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/patmat/MatchCodeGen.scala b/src/compiler/scala/tools/nsc/transform/patmat/MatchCodeGen.scala
index 1642613b9b..af536cb2d3 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/MatchCodeGen.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/MatchCodeGen.scala
@@ -55,7 +55,15 @@ trait MatchCodeGen extends Interface {
       def flatMap(prev: Tree, b: Symbol, next: Tree): Tree
       def flatMapCond(cond: Tree, res: Tree, nextBinder: Symbol, next: Tree): Tree
       def flatMapGuard(cond: Tree, next: Tree): Tree
-      def ifThenElseZero(c: Tree, thenp: Tree): Tree = IF (c) THEN thenp ELSE zero
+      def ifThenElseZero(c: Tree, thenp: Tree): Tree = {
+        val z = zero
+        thenp match {
+          case If(c1, thenp1, elsep1) if z equalsStructure elsep1 =>
+            If(c AND c1, thenp1, elsep1) // cleaner, leaner trees
+          case _ =>
+            If(c, thenp, zero)
+        }
+      }
       protected def zero: Tree
     }