Don't allocate Val.Zero for reference types

[densh]

No description provided.

[densh]

Note: this change does change the NIR but doesn't affect the binary representation.

[densh]

Binary compatibility hacks are the best.

[densh]

This considerably improves linking performance on 1G heaps. As discovered by @xuwei-k in https://twitter.com/xuwei_k/status/857620328624701440.

[xuwei-k]

• https://github.com/scalaprops/scalaprops/tree/106cfeae9331a2a9c2d1cd935cf593e42cee426c
• https://travis-ci.org/scalaprops/scalaprops/jobs/226839323#L1628

git clone -b scala-native https://github.com/scalaprops/scalaprops
cd scalaprops
sbt nativeTest/run

[info] Compiling 101 Scala sources to /home/travis/build/scalaprops/scalaprops/nativeTest/target/scala-2.11/classes...
[info] Linking (7498 ms)
[info] Discovered 7661 classes and 43896 methods
[info] Optimizing (78139 ms)
java.util.concurrent.ExecutionException: java.lang.OutOfMemoryError: Java heap space
	at java.util.concurrent.FutureTask.report(FutureTask.java:122)
	at java.util.concurrent.FutureTask.get(FutureTask.java:192)
	at sbt.ConcurrentRestrictions$$anon$4.take(ConcurrentRestrictions.scala:188)
	at sbt.Execute.next$1(Execute.scala:85)
	at sbt.Execute.processAll(Execute.scala:88)
	at sbt.Execute.runKeep(Execute.scala:68)
	at sbt.EvaluateTask$.liftedTree1$1(EvaluateTask.scala:359)
	at sbt.EvaluateTask$.run$1(EvaluateTask.scala:358)
	at sbt.EvaluateTask$.runTask(EvaluateTask.scala:378)
	at sbt.Aggregation$$anonfun$3.apply(Aggregation.scala:69)
	at sbt.Aggregation$$anonfun$3.apply(Aggregation.scala:67)
	at sbt.EvaluateTask$.withStreams(EvaluateTask.scala:314)
	at sbt.Aggregation$.timedRun(Aggregation.scala:67)
	at sbt.Aggregation$.runTasks(Aggregation.scala:76)
	at sbt.Aggregation$$anonfun$applyDynamicTasks$1.apply(Aggregation.scala:117)
	at sbt.Aggregation$$anonfun$applyDynamicTasks$1.apply(Aggregation.scala:115)
	at sbt.Command$$anonfun$applyEffect$2$$anonfun$apply$3.apply(Command.scala:61)
	at sbt.Command$$anonfun$applyEffect$2$$anonfun$apply$3.apply(Command.scala:61)
	at sbt.Act$$anonfun$sbt$Act$$actParser0$1$$anonfun$sbt$Act$$anonfun$$evaluate$1$1$$anonfun$apply$10.apply(Act.scala:253)
	at sbt.Act$$anonfun$sbt$Act$$actParser0$1$$anonfun$sbt$Act$$anonfun$$evaluate$1$1$$anonfun$apply$10.apply(Act.scala:250)
	at sbt.Command$.process(Command.scala:93)
	at sbt.MainLoop$$anonfun$1$$anonfun$apply$1.apply(MainLoop.scala:96)
	at sbt.MainLoop$$anonfun$1$$anonfun$apply$1.apply(MainLoop.scala:96)
	at sbt.State$$anon$1.doX$1(State.scala:183)
	at sbt.State$$anon$1.process(State.scala:190)
	at sbt.MainLoop$$anonfun$1.apply(MainLoop.scala:96)
	at sbt.MainLoop$$anonfun$1.apply(MainLoop.scala:96)
	at sbt.ErrorHandling$.wideConvert(ErrorHandling.scala:17)
	at sbt.MainLoop$.next(MainLoop.scala:96)
	at sbt.MainLoop$.run(MainLoop.scala:89)
	at sbt.MainLoop$$anonfun$runWithNewLog$1.apply(MainLoop.scala:68)
	at sbt.MainLoop$$anonfun$runWithNewLog$1.apply(MainLoop.scala:63)
	at sbt.Using.apply(Using.scala:24)
	at sbt.MainLoop$.runWithNewLog(MainLoop.scala:63)
	at sbt.MainLoop$.runAndClearLast(MainLoop.scala:46)
	at sbt.MainLoop$.runLoggedLoop(MainLoop.scala:30)
	at sbt.MainLoop$.runLogged(MainLoop.scala:22)
	at sbt.StandardMain$.runManaged(Main.scala:109)
	at sbt.xMain.run(Main.scala:38)
	at xsbt.boot.Launch$$anonfun$run$1.apply(Launch.scala:109)
	at xsbt.boot.Launch$.withContextLoader(Launch.scala:128)
	at xsbt.boot.Launch$.run(Launch.scala:109)
	at xsbt.boot.Launch$$anonfun$apply$1.apply(Launch.scala:35)
	at xsbt.boot.Launch$.launch(Launch.scala:117)
	at xsbt.boot.Launch$.apply(Launch.scala:18)
	at xsbt.boot.Boot$.runImpl(Boot.scala:41)
	at xsbt.boot.Boot$.main(Boot.scala:17)
	at xsbt.boot.Boot.main(Boot.scala)
Caused by: java.lang.OutOfMemoryError: Java heap space
	at java.lang.StringCoding$StringEncoder.encode(StringCoding.java:300)
	at java.lang.StringCoding.encode(StringCoding.java:344)
	at java.lang.String.getBytes(String.java:918)
	at scala.scalanative.codegen.CodeGen$Impl.gen(CodeGen.scala:79)
	at scala.scalanative.codegen.CodeGen$$anonfun$apply$1$$anonfun$debug$1$2.apply(CodeGen.scala:43)
	at scala.scalanative.codegen.CodeGen$$anonfun$apply$1$$anonfun$debug$1$2.apply(CodeGen.scala:38)
	at scala.collection.Iterator$class.foreach(Iterator.scala:727)
	at scala.collection.parallel.mutable.ParHashTable$EntryIterator.foreach(ParHashTable.scala:31)
	at scala.collection.parallel.ParIterableLike$Foreach.leaf(ParIterableLike.scala:975)
	at scala.collection.parallel.Task$$anonfun$tryLeaf$1.apply$mcV$sp(Tasks.scala:54)
	at scala.collection.parallel.Task$$anonfun$tryLeaf$1.apply(Tasks.scala:53)
	at scala.collection.parallel.Task$$anonfun$tryLeaf$1.apply(Tasks.scala:53)
	at scala.collection.parallel.Task$class.tryLeaf(Tasks.scala:56)
	at scala.collection.parallel.ParIterableLike$Foreach.tryLeaf(ParIterableLike.scala:972)
	at scala.collection.parallel.AdaptiveWorkStealingTasks$WrappedTask$class.internal(Tasks.scala:183)
	at scala.collection.parallel.AdaptiveWorkStealingForkJoinTasks$WrappedTask.internal(Tasks.scala:514)
	at scala.collection.parallel.AdaptiveWorkStealingTasks$WrappedTask$class.compute(Tasks.scala:162)
	at scala.collection.parallel.AdaptiveWorkStealingForkJoinTasks$WrappedTask.compute(Tasks.scala:514)
	at scala.concurrent.forkjoin.RecursiveAction.exec(RecursiveAction.java:160)
	at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
	at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
	at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
	at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)
[error] java.util.concurrent.ExecutionException: java.lang.OutOfMemoryError: Java heap space

[xuwei-k]

when succeeded (-Xmx8G)

$ ll -h nativeTest/target/scala-2.11/nativetest-out 
-rwxr-xr-x  1 kenji   174M Apr 28 23:46 nativeTest/target/scala-2.11/nativetest-out
$ ll -hS nativeTest/target/scala-2.11/native/ | head
total 1486560
-rw-r--r--   1 kenji   413M Apr 28 23:46 __dispatch.ll
-rw-r--r--   1 kenji   156M Apr 28 23:46 __dispatch.ll.o
-rw-r--r--   1 kenji    56M Apr 28 23:45 __class_has_trait.ll
-rw-r--r--   1 kenji    26M Apr 28 23:45 scalaz.ll
-rw-r--r--   1 kenji    11M Apr 28 23:45 scalaprops.ll
-rw-r--r--   1 kenji   7.4M Apr 28 23:45 __const.ll
-rw-r--r--   1 kenji   7.2M Apr 28 23:45 __trait_has_trait.ll
-rw-r--r--   1 kenji   6.9M Apr 28 23:46 scalaz.ll.o
-rw-r--r--   1 kenji   5.6M Apr 28 23:46 __class_has_trait.ll.o

[densh]

-rw-r--r-- 1 kenji 413M Apr 28 23:46 __dispatch.ll
-rw-r--r-- 1 kenji 156M Apr 28 23:46 __dispatch.ll.o

This looks like #180 strikes again, thanks for the detailed reproduction information, @xuwei-k !