Make List safe to share across threads without synchronization

Author: retronym

src/library/scala/collection/immutable/List.scala

@@ -7,6 +7,7 @@ import java.io.{ObjectInputStream, ObjectOutputStream}
 import scala.annotation.unchecked.uncheckedVariance
 import scala.annotation.tailrec
 import mutable.{Builder, ListBuffer}
+import scala.runtime.ScalaRunTime
 
 /** A class for immutable linked lists representing ordered collections
   *  of elements of type `A`.
@@ -147,6 +148,7 @@ sealed abstract class List[+A]
       t = nx
       rest = rest.tail
     }
+    ScalaRunTime.releaseFence()
     h
   }
 
@@ -216,6 +218,7 @@ sealed abstract class List[+A]
         t = nx
         rest = rest.tail
       }
+      ScalaRunTime.releaseFence()
       h
     }
   }
@@ -242,7 +245,7 @@ sealed abstract class List[+A]
         }
         rest = rest.tail
       } while (rest ne Nil)
-      h
+      {ScalaRunTime.releaseFence(); h}
     }
   }
 
@@ -267,7 +270,7 @@ sealed abstract class List[+A]
         }
         rest = rest.tail
       }
-      if (!found) Nil else h
+      if (!found) Nil else {ScalaRunTime.releaseFence(); h}
     }
   }
 
@@ -435,7 +438,9 @@ sealed abstract class List[+A]
         }
       }
     }
-    loop(null, null, this, this)
+    val result = loop(null, null, this, this)
+    ScalaRunTime.releaseFence()
+    result
   }
 
   override def filter(p: A => Boolean): List[A] = filterImpl(p, isFlipped = false)
@@ -517,7 +522,9 @@ sealed abstract class List[+A]
       newHead
     }
 
-    noneIn(this)
+    val result = noneIn(this)
+    ScalaRunTime.releaseFence()
+    result
   }
 
   final override def toList: List[A] = this
@@ -546,6 +553,7 @@ sealed abstract class List[+A]
 
 final case class :: [+A](override val head: A, private[scala] var next: List[A @uncheckedVariance]) // sound because `next` is used only locally
   extends List[A] {
+  ScalaRunTime.releaseFence()
   override def isEmpty: Boolean = false
   override def headOption: Some[A] = Some(head)
   override def tail: List[A] = next

src/library/scala/collection/mutable/ListBuffer.scala

@@ -5,6 +5,7 @@ import scala.annotation.tailrec
 import scala.collection.immutable.{List, Nil, ::}
 import scala.annotation.tailrec
 import java.lang.{IllegalArgumentException, IndexOutOfBoundsException}
+import scala.runtime.ScalaRunTime
 
 /** A `Buffer` implementation backed by a list. It provides constant time
   *  prepend and append. Most other operations are linear.
@@ -61,6 +62,7 @@ class ListBuffer[A]
   // Avoids copying where possible.
   override def toList: List[A] = {
     aliased = nonEmpty
+    ScalaRunTime.releaseFence()
     first
   }
 

src/reflect/scala/reflect/internal/tpe/FindMembers.scala

@@ -7,6 +7,7 @@ package tpe
 
 import util.StatisticsStatics
 import Flags._
+import scala.runtime.ScalaRunTime
 
 trait FindMembers {
   this: SymbolTable =>
@@ -282,6 +283,7 @@ trait FindMembers {
     } else {
       if (StatisticsStatics.areSomeColdStatsEnabled) statistics.incCounter(multMemberCount)
       lastM.next = Nil
+      ScalaRunTime.releaseFence()
       initBaseClasses.head.newOverloaded(tpe, members)
     }
   }

src/reflect/scala/reflect/internal/util/Collections.scala

@@ -6,9 +6,10 @@
 package scala
 package reflect.internal.util
 
-import scala.collection.{ mutable, immutable }
+import scala.collection.{immutable, mutable}
 import scala.annotation.tailrec
 import mutable.ListBuffer
+import scala.runtime.ScalaRunTime
 
 /** Profiler driven changes.
  *  TODO - inlining doesn't work from here because of the bug that
@@ -58,6 +59,7 @@ trait Collections {
       tail = next
       rest = rest.tail
     }
+    ScalaRunTime.releaseFence()
     head
   }
 
@@ -128,7 +130,9 @@ trait Collections {
         }
       }
     }
-    loop(null, xs, xs, ys)
+    val result = loop(null, xs, xs, ys)
+    ScalaRunTime.releaseFence()
+    result
   }
 
   final def map3[A, B, C, D](xs1: List[A], xs2: List[B], xs3: List[C])(f: (A, B, C) => D): List[D] = {