Add optimization to reduce extra iterations of the safe init checker. (#17057)

If the cache is never accessed on a certain iteration of safe
initializing checking for a class, then another iteration is not needed.
Implementing this optimization and compiling Dotty, we observe the
following improvements:
From 4158 total iterations to 4004.
From 460 extra iterations to 306.

Running the initialization tests in `tests/init` we have the following
improvements:
From 790 total iterations to 708.
From 139 extra iterations to 57.

Author: olhotak

compiler/src/dotty/tools/dotc/transform/init/Cache.scala

@@ -75,11 +75,19 @@ class Cache[Config, Res]:
    */
   protected var changed: Boolean = false
 
+  /** Whether any value in the output cache (this.current) was accessed
+   *  after being added. If no cached values are used after they are added
+   *  for the first time then another iteration of analysis is not needed.
+   */
+  protected var cacheUsed: Boolean = false
+
   /** Used to avoid allocation, its state does not matter */
   protected given MutableTreeWrapper = new MutableTreeWrapper
 
   def get(config: Config, expr: Tree): Option[Res] =
-    current.get(config, expr)
+    val res = current.get(config, expr)
+    cacheUsed = cacheUsed || res.nonEmpty
+    res
 
   /** Evaluate an expression with cache
    *
@@ -124,6 +132,8 @@ class Cache[Config, Res]:
 
   def hasChanged = changed
 
+  def isUsed = cacheUsed
+
   /** Prepare cache for the next iteration
    *
    *  1. Reset changed flag.
@@ -132,6 +142,7 @@ class Cache[Config, Res]:
    */
   def prepareForNextIteration()(using Context) =
     this.changed = false
+    this.cacheUsed = false
     this.last = this.current
     this.current = Map.empty
 end Cache

compiler/src/dotty/tools/dotc/transform/init/Semantic.scala

@@ -1120,7 +1120,7 @@ object Semantic:
       log("checking " + classSym) { eval(tpl, thisRef, classSym) }
       reporter.errors.foreach(_.issue)
 
-      if cache.hasChanged && reporter.errors.isEmpty then
+      if cache.hasChanged && reporter.errors.isEmpty && cache.isUsed then
         // code to prepare cache and heap for next iteration
         cache.prepareForNextIteration()
         iterate()