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Increase initialCapacity for HashSet in ExtractDependencies.scala #18219

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Increase initialCapacity for HashSet in ExtractDependencies.scala #18219

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Increase initialCapacity for HashSet in ExtractDependencies.scala
lolgab Jul 16, 2023
26630e8
Reduce initialCapacity to 64
lolgab Jul 17, 2023
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2 changes: 1 addition & 1 deletion compiler/src/dotty/tools/dotc/sbt/ExtractDependencies.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -447,7 +447,7 @@ private class ExtractDependenciesCollector extends tpd.TreeTraverser { thisTreeT
// Avoid cycles by remembering both the types (testcase:
// tests/run/enum-values.scala) and the symbols of named types (testcase:
// tests/pos-java-interop/i13575) we've seen before.
val seen = new mutable.HashSet[Symbol | Type]
val seen = new mutable.HashSet[Symbol | Type](initialCapacity = 128, loadFactor = mutable.HashSet.defaultLoadFactor)
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@nicolasstucki nicolasstucki Jul 17, 2023 •
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128 seems a bit too excessive. If we look at the histogram of maximum sizes we see that 64 is enough to cover most cases (99%). Even 32 we would cover 87% of cases and have an extra allocation for the 13% (larger cases).

Screenshot 2023-07-17 at 10 08 39

I measured it when running scala3-bootstrapped/compile after a clean.

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@nicolasstucki Nice histogram! I wanted to build one too but gave up eventually. How did you do it?
Do your percentages consider the loadFactor of 75%? Should I update the initialCapacity to 64?

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  def addTypeDependency(tpe: Type)(using Context): Unit = {
    val traverser = new TypeDependencyTraverser {
      def addDependency(symbol: Symbol) = addMemberRefDependency(symbol)
    }
    traverser.traverse(tpe)
    println("seen: " + traverser.seen.size)
  }

  def addPatMatDependency(tpe: Type)(using Context): Unit = {
    val traverser = new TypeDependencyTraverser {
      def addDependency(symbol: Symbol) =
        if (!ignoreDependency(symbol) && symbol.is(Sealed)) {
          val usedName = symbol.zincMangledName
          addUsedName(usedName, UseScope.PatMatTarget)
        }
    }
    traverser.traverse(tpe)
    println("seen: " + traverser.seen.size)

  }

sbt "clean; scala3-bootstrapped/compile" > sbtOutput.txt then filter out seen: from that file and import the numbers in google sheets.

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I did not consider the load factor. My percentages only take into account the size of the set. I would go for 64 if we want to minimize those allocations.

def traverse(tp: Type): Unit = if (!seen.contains(tp)) {
seen += tp
tp match {
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