Cache array length in for-in-array loop if possible

If the range expression is not a local variable (which can be updated in
the loop body affecting loop behavior, see KT-21354), we can cache the
array length, thus turning a for-in-array loop into a simple optimizable
counter loop.

 #KT-21321 In Progress

compiler/backend/src/org/jetbrains/kotlin/codegen/range/ArrayRangeValue.kt

@@ -23,9 +23,9 @@ import org.jetbrains.kotlin.codegen.range.inExpression.InExpressionGenerator
 import org.jetbrains.kotlin.psi.KtForExpression
 import org.jetbrains.kotlin.psi.KtSimpleNameExpression
 
-class ArrayRangeValue : RangeValue {
+class ArrayRangeValue(private val canCacheArrayLength: Boolean) : RangeValue {
     override fun createForLoopGenerator(codegen: ExpressionCodegen, forExpression: KtForExpression) =
-            ForInArrayLoopGenerator(codegen, forExpression)
+            ForInArrayLoopGenerator(codegen, forExpression, canCacheArrayLength)
 
     override fun createInExpressionGenerator(codegen: ExpressionCodegen, operatorReference: KtSimpleNameExpression): InExpressionGenerator =
             CallBasedInExpressionGenerator(codegen, operatorReference)

compiler/backend/src/org/jetbrains/kotlin/codegen/range/RangeValues.kt

@@ -19,6 +19,7 @@ package org.jetbrains.kotlin.codegen.range
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.codegen.*
 import org.jetbrains.kotlin.descriptors.CallableDescriptor
+import org.jetbrains.kotlin.descriptors.impl.LocalVariableDescriptor
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.resolve.BindingContext
@@ -44,7 +45,7 @@ fun ExpressionCodegen.createRangeValueForExpression(rangeExpression: KtExpressio
 
     return when {
         asmRangeType.sort == Type.ARRAY ->
-            ArrayRangeValue()
+            ArrayRangeValue(!isLocalVarReference(rangeExpression, bindingContext))
         isPrimitiveRange(rangeType) ->
             PrimitiveRangeRangeValue()
         isPrimitiveProgression(rangeType) ->
@@ -56,6 +57,12 @@ fun ExpressionCodegen.createRangeValueForExpression(rangeExpression: KtExpressio
     }
 }
 
+fun isLocalVarReference(rangeExpression: KtExpression, bindingContext: BindingContext): Boolean {
+    if (rangeExpression !is KtSimpleNameExpression) return false
+    val resultingDescriptor = rangeExpression.getResolvedCall(bindingContext)?.resultingDescriptor ?: return false
+    return resultingDescriptor is LocalVariableDescriptor && resultingDescriptor.isVar
+}
+
 private fun isSubtypeOfCharSequence(type: KotlinType, builtIns: KotlinBuiltIns) =
         KotlinTypeChecker.DEFAULT.isSubtypeOf(type, builtIns.getBuiltInClassByName(Name.identifier("CharSequence")).defaultType)
 

compiler/backend/src/org/jetbrains/kotlin/codegen/range/forLoop/ForInArrayLoopGenerator.kt

@@ -17,21 +17,25 @@
 package org.jetbrains.kotlin.codegen.range.forLoop
 
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
+import org.jetbrains.kotlin.codegen.AsmUtil.boxType
+import org.jetbrains.kotlin.codegen.ExpressionCodegen
+import org.jetbrains.kotlin.codegen.StackValue
 import org.jetbrains.kotlin.psi.KtForExpression
+import org.jetbrains.kotlin.resolve.jvm.AsmTypes.OBJECT_TYPE
 import org.jetbrains.kotlin.types.KotlinType
 import org.jetbrains.org.objectweb.asm.Label
 import org.jetbrains.org.objectweb.asm.Type
 
-import org.jetbrains.kotlin.codegen.AsmUtil.boxType
-import org.jetbrains.kotlin.codegen.ExpressionCodegen
-import org.jetbrains.kotlin.codegen.StackValue
-import org.jetbrains.kotlin.resolve.jvm.AsmTypes.OBJECT_TYPE
-
-class ForInArrayLoopGenerator(codegen: ExpressionCodegen, forExpression: KtForExpression)
-    : AbstractForLoopGenerator(codegen, forExpression)
-{
+class ForInArrayLoopGenerator(
+        codegen: ExpressionCodegen,
+        forExpression: KtForExpression,
+        // We can cache array length if the corresponding range expression is not a local var.
+        // See https://youtrack.jetbrains.com/issue/KT-21354.
+        private val canCacheArrayLength: Boolean
+) : AbstractForLoopGenerator(codegen, forExpression) {
     private var indexVar: Int = 0
     private var arrayVar: Int = 0
+    private var arrayLengthVar: Int = 0
     private val loopRangeType: KotlinType = bindingContext.getType(forExpression.loopRange!!)!!
 
     override fun beforeLoop() {
@@ -51,6 +55,13 @@ class ForInArrayLoopGenerator(codegen: ExpressionCodegen, forExpression: KtForEx
             v.store(arrayVar, OBJECT_TYPE)
         }
 
+        if (canCacheArrayLength) {
+            arrayLengthVar = createLoopTempVariable(Type.INT_TYPE)
+            v.load(arrayVar, OBJECT_TYPE)
+            v.arraylength()
+            v.store(arrayLengthVar, Type.INT_TYPE)
+        }
+
         v.iconst(0)
         v.store(indexVar, Type.INT_TYPE)
     }
@@ -59,8 +70,13 @@ class ForInArrayLoopGenerator(codegen: ExpressionCodegen, forExpression: KtForEx
 
     override fun checkPreCondition(loopExit: Label) {
         v.load(indexVar, Type.INT_TYPE)
-        v.load(arrayVar, OBJECT_TYPE)
-        v.arraylength()
+        if (canCacheArrayLength) {
+            v.load(arrayLengthVar, Type.INT_TYPE)
+        }
+        else {
+            v.load(arrayVar, OBJECT_TYPE)
+            v.arraylength()
+        }
         v.ificmpge(loopExit)
     }