Null out local vars created by the inliner

This should fix scala/bug#9137

src/compiler/scala/tools/nsc/backend/jvm/analysis/NullnessAnalyzer.scala

@@ -26,11 +26,6 @@ import scala.tools.nsc.backend.jvm.opt.BytecodeUtils._
  * call). However, the receiver is an value on the stack and consumed while interpreting the
  * instruction - so we can only gain some knowledge if we know that the receiver was an alias of
  * some other local variable or stack slot. Therefore we use the AliasingFrame class.
- *
- * TODO:
- * Finally, we'd also like to exploit the knowledge gained from `if (x == null)` tests: x is known
- * to be null in one branch, not null in the other. This will make use of alias tracking as well.
- * We still have to figure out how to do this exactly in the analyzer framework.
  */
 
 /**

src/compiler/scala/tools/nsc/backend/jvm/opt/Inliner.scala

@@ -671,6 +671,7 @@ abstract class Inliner {
 
     // add a STORE instruction for each expected argument, including for THIS instance if any
     val argStores = new InsnList
+    val nullOutArgLocals = new InsnList
     var nextLocalIndex = callsiteMethod.maxLocals
     if (!isStaticMethod(callee)) {
       if (!receiverKnownNotNull) {
@@ -682,6 +683,8 @@ abstract class Inliner {
         argStores.add(nonNullLabel)
       }
       argStores.add(new VarInsnNode(ASTORE, nextLocalIndex))
+      nullOutArgLocals.add(new InsnNode(ACONST_NULL))
+      nullOutArgLocals.add(new VarInsnNode(ASTORE, nextLocalIndex))
       nextLocalIndex += 1
     }
 
@@ -692,9 +695,15 @@ abstract class Inliner {
     for(argTp <- calleeParamTypes) {
       val opc = argTp.getOpcode(ISTORE) // returns the correct xSTORE instruction for argTp
       argStores.insert(new VarInsnNode(opc, nextLocalIndex)) // "insert" is "prepend" - the last argument is on the top of the stack
+      if (opc == ASTORE) {
+        nullOutArgLocals.add(new InsnNode(ACONST_NULL))
+        nullOutArgLocals.add(new VarInsnNode(ASTORE, nextLocalIndex))
+      }
       nextLocalIndex += argTp.getSize
     }
 
+    val hasNullOutInsn = nullOutArgLocals.size > 0
+
     clonedInstructions.insert(argStores)
 
     // label for the exit of the inlined functions. xRETURNs are replaced by GOTOs to this label.
@@ -714,6 +723,8 @@ abstract class Inliner {
       }
     }
 
+    clonedInstructions.add(nullOutArgLocals)
+
     // replace xRETURNs:
     //   - store the return value (if any)
     //   - clear the stack of the inlined method (insert DROPs)
@@ -794,7 +805,10 @@ abstract class Inliner {
       // When adding a null check for the receiver, a DUP is inserted, which might cause a new maxStack.
       // If the callsite has other argument values than the receiver on the stack, these are pop'ed
       // and stored into locals before the null check, so in that case the maxStack doesn't grow.
-      val stackSlotForNullCheck = if (!isStaticMethod(callee) && !receiverKnownNotNull && calleeParamTypes.isEmpty) 1 else 0
+      val stackSlotForNullCheck =
+        if (!isStaticMethod(callee) && !receiverKnownNotNull && calleeParamTypes.isEmpty) 1
+        else if (hasNullOutInsn) 1
+        else 0
       callsiteStackHeight + stackSlotForNullCheck
     }
 

test/benchmarks/src/main/scala/scala/tools/nsc/backend/jvm/ProdConsBenchmark.scala

@@ -9,6 +9,7 @@ import org.openjdk.jmh.infra.Blackhole
 
 import scala.collection.JavaConverters.asScalaIteratorConverter
 import scala.tools.asm.tree.ClassNode
+import scala.tools.nsc.backend.jvm.analysis.ProdConsAnalyzer
 
 @BenchmarkMode(Array(Mode.AverageTime))
 @Fork(2)

test/junit/scala/tools/nsc/backend/jvm/opt/InlinerTest.scala

@@ -80,6 +80,7 @@ class InlinerTest extends BytecodeTesting {
         VarOp(ALOAD, 0), VarOp(ASTORE, 1), // store this
         Op(ICONST_1), VarOp(ISTORE, 2), Jump(GOTO, Label(10)), // store return value
         Label(10), VarOp(ILOAD, 2), // load return value
+        Op(ACONST_NULL), VarOp(ASTORE, 1), // null out inliner local
         VarOp(ALOAD, 0), Invoke(INVOKEVIRTUAL, "C", "f", "()I", false), Op(IADD), Op(IRETURN)))
 
     // line numbers are kept, so there's a line 2 (from the inlined f)
@@ -117,6 +118,7 @@ class InlinerTest extends BytecodeTesting {
       Jump(GOTO, Label(11)),
       Label(11),
       VarOp(ALOAD, 2),
+      Op(ACONST_NULL), VarOp(ASTORE, 1),
       Op(ATHROW))
 
     assertSameCode(convertMethod(g), gBeforeLocalOpt)