Guard fuel with a private final val

Author: DarkDimius

compiler/src/dotty/tools/dotc/transform/localopt/Simplify.scala

@@ -67,10 +67,26 @@ class Simplify extends MiniPhaseTransform with IdentityDenotTransformer {
    */
   var fuel: Int = -1
 
+
+  /** using fuel stops any inlining and prevents optimizations from triggering.
+   * on my tests it gives 20% slowdown, so it is going to be disabled in public builds.
+   */
+  private final val useFuel = false
+
+  private var optimisations: List[Optimisation] = _
+
   override def prepareForUnit(tree: Tree)(implicit ctx: Context) = {
     val maxFuel = ctx.settings.YoptFuel.value
+    if (!useFuel && maxFuel != ctx.settings.YoptFuel.default)
+      ctx.error("Optimization fuel-debugging requires enabling in source, see Simplify.scala::useFuel")
     if (fuel < 0 && maxFuel > 0) // Both defaults are at -1
       fuel = maxFuel
+
+    optimisations = {
+      val o = if (ctx.erasedTypes) afterErasure else beforeErasure
+      val p = ctx.settings.YoptPhases.value
+      if (p.isEmpty) o else o.filter(x => p.contains(x.name))
+    }
     this
   }
 
@@ -79,17 +95,12 @@ class Simplify extends MiniPhaseTransform with IdentityDenotTransformer {
     val ctx0 = ctx
     if (ctx.settings.optimise.value && !tree.symbol.is(Label)) {
       implicit val ctx: Context = ctx0.withOwner(tree.symbol(ctx0))
-      val optimisations = {
-        val o = if (ctx.erasedTypes) afterErasure else beforeErasure
-        val p = ctx.settings.YoptPhases.value
-        if (p.isEmpty) o else o.filter(x => p.contains(x.name))
-      }
-
+      
       var rhs0 = tree.rhs
       var rhs1: Tree = null
       while (rhs1 ne rhs0) {
         rhs1 = rhs0
-        val context = ctx.withOwner(tree.symbol)
+//        val context = ctx.withOwner(tree.symbol)
         optimisations.foreach { optimisation => // TODO: fuse for performance
           // Visit
           rhs0.foreachSubTree(optimisation.visitor)
@@ -100,12 +111,12 @@ class Simplify extends MiniPhaseTransform with IdentityDenotTransformer {
               val innerCtx = if (tree.isDef && tree.symbol.exists) ctx.withOwner(tree.symbol) else ctx
               val childOptimizedTree = super.transform(tree)(innerCtx)
 
-              if (fuel == 0)
+              if (useFuel && fuel == 0)
                 childOptimizedTree
               else {
                 val fullyOptimizedTree = optimisation.transformer(ctx)(childOptimizedTree)
 
-                if (tree ne fullyOptimizedTree) {
+                if (useFuel && (tree ne fullyOptimizedTree)) {
                   if (fuel > 0) fuel -= 1
                   if (fuel != -1 && fuel < 10) {
                     println(s"${tree.symbol} was simplified by ${optimisation.name} (fuel=$fuel): ${tree.show}")