Performance improvement: Avoid most operations in interpolateUndetVars

Perform the operation only if there are qualifying type variables, which is rarely the case.
Reverted variances optimization to simpler and shorter previous implementation, because variances
is no longer hot.

1 files changed, 24 insertions, 22 deletions

diff --git a/src/dotty/tools/dotc/typer/Inferencing.scala b/src/dotty/tools/dotc/typer/Inferencing.scala
index 763904ac4..986ddf570 100644
--- a/src/dotty/tools/dotc/typer/Inferencing.scala
+++ b/src/dotty/tools/dotc/typer/Inferencing.scala
@@ -602,31 +602,33 @@ object Inferencing {
    *  approximate it by its lower bound. Otherwise, if it appears contravariantly
    *  in type `tp` approximate it by its upper bound.
    */
-  def interpolateUndetVars(tree: Tree)(implicit ctx: Context): Unit = Stats.track("interpolateUndetVars") {
-    val tp = tree.tpe.widen
+  def interpolateUndetVars(tree: Tree)(implicit ctx: Context): Unit = {
     val constraint = ctx.typerState.constraint
-
-    constr.println(s"interpolate undet vars in ${tp.show}, pos = ${tree.pos}, mode = ${ctx.mode}, undets = ${constraint.uninstVars map (tvar => s"${tvar.show}@${tvar.owningTree.pos}")}")
-    constr.println(s"qualifying undet vars: ${constraint.uninstVars filter qualifies map (tvar => s"$tvar / ${tvar.show}")}, constraint: ${constraint.show}")
-
-    def qualifies(tvar: TypeVar) = tree contains tvar.owningTree
-    val vs = tp.variances(qualifies)
-    var changed = false
-    vs foreachBinding { (tvar, v) =>
-      if (v != 0) {
-        typr.println(s"interpolate ${if (v == 1) "co" else "contra"}variant ${tvar.show} in ${tp.show}")
-        tvar.instantiate(fromBelow = v == 1)
-        changed = true
+    val qualifies = (tvar: TypeVar) => tree contains tvar.owningTree
+    def interpolate() = Stats.track("interpolateUndetVars") {
+      val tp = tree.tpe.widen
+      constr.println(s"interpolate undet vars in ${tp.show}, pos = ${tree.pos}, mode = ${ctx.mode}, undets = ${constraint.uninstVars map (tvar => s"${tvar.show}@${tvar.owningTree.pos}")}")
+      constr.println(s"qualifying undet vars: ${constraint.uninstVars filter qualifies map (tvar => s"$tvar / ${tvar.show}")}, constraint: ${constraint.show}")
+
+      val vs = tp.variances(qualifies)
+      var changed = false
+      vs foreachBinding { (tvar, v) =>
+        if (v != 0) {
+          typr.println(s"interpolate ${if (v == 1) "co" else "contra"}variant ${tvar.show} in ${tp.show}")
+          tvar.instantiate(fromBelow = v == 1)
+          changed = true
+        }
       }
+      if (changed) // instantiations might have uncovered new typevars to interpolate
+        interpolateUndetVars(tree)
+      else
+        for (tvar <- constraint.uninstVars)
+          if (!(vs contains tvar) && qualifies(tvar)) {
+            typr.println(s"instantiating non-occurring ${tvar.show} in ${tp.show}")
+            tvar.instantiate(fromBelow = true)
+          }
     }
-    if (changed) // instantiations might have uncovered new typevars to interpolate
-      interpolateUndetVars(tree)
-    else
-      for (tvar <- constraint.uninstVars)
-        if (!(vs contains tvar) && qualifies(tvar)) {
-          typr.println(s"instantiating non-occurring ${tvar.show} in ${tp.show}")
-          tvar.instantiate(fromBelow = true)
-        }
+    if (constraint.uninstVars exists qualifies) interpolate()
   }
 
   /** Instantiate undetermined type variables to that type `tp` is