Replace HeaderInfo.increasing property with a "direction" enum property,

which is computed based on the value of "step" (if possible).

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/loops/HeaderInfo.kt

@@ -12,6 +12,7 @@ import org.jetbrains.kotlin.ir.IrElement
 import org.jetbrains.kotlin.ir.declarations.IrVariable
 import org.jetbrains.kotlin.ir.descriptors.IrBuiltIns
 import org.jetbrains.kotlin.ir.expressions.IrCall
+import org.jetbrains.kotlin.ir.expressions.IrConst
 import org.jetbrains.kotlin.ir.expressions.IrExpression
 import org.jetbrains.kotlin.ir.types.IrType
 import org.jetbrains.kotlin.ir.types.isSubtypeOfClass
@@ -46,24 +47,36 @@ enum class ProgressionType(val numberCastFunctionName: Name) {
     }
 }
 
+internal enum class ProgressionDirection { DECREASING, INCREASING, UNKNOWN }
+
 // Information about loop that is required by HeaderProcessor to build ForLoopHeader
 internal sealed class HeaderInfo(
     val progressionType: ProgressionType,
     val lowerBound: IrExpression,
     val upperBound: IrExpression,
     val step: IrExpression,
-    val increasing: Boolean,
     val closed: Boolean
-)
+) {
+    val direction: ProgressionDirection by lazy {
+        // If step is a constant (either Int or Long), then we can determine the direction.
+        val stepValue = (step as? IrConst<*>)?.value as? Number?
+        val stepValueAsLong = stepValue?.toLong()
+        when {
+            stepValueAsLong == null -> ProgressionDirection.UNKNOWN
+            stepValueAsLong < 0L -> ProgressionDirection.DECREASING
+            stepValueAsLong > 0L -> ProgressionDirection.INCREASING
+            else -> ProgressionDirection.UNKNOWN
+        }
+    }
+}
 
 internal class ProgressionHeaderInfo(
     progressionType: ProgressionType,
     lowerBound: IrExpression,
     upperBound: IrExpression,
     step: IrExpression,
-    increasing: Boolean = true,
     closed: Boolean = true
-) : HeaderInfo(progressionType, lowerBound, upperBound, step, increasing, closed)
+) : HeaderInfo(progressionType, lowerBound, upperBound, step, closed)
 
 internal class ArrayHeaderInfo(
     lowerBound: IrExpression,
@@ -75,7 +88,6 @@ internal class ArrayHeaderInfo(
     lowerBound,
     upperBound,
     step,
-    increasing = true,
     closed = false
 )
 

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/loops/HeaderProcessor.kt

@@ -54,9 +54,10 @@ internal class ProgressionLoopHeader(
 
     val closed = headerInfo.closed
 
-    private val increasing = headerInfo.increasing
+    private val direction = headerInfo.direction
 
-    fun comparingFunction(builtIns: IrBuiltIns) = if (increasing)
+    // TODO: Handle UNKNOWN direction, which requires a complex expression for the emptiness check
+    fun comparingFunction(builtIns: IrBuiltIns) = if (direction == ProgressionDirection.INCREASING)
         builtIns.lessOrEqualFunByOperandType[builtIns.int]?.symbol!!
     else
         builtIns.greaterOrEqualFunByOperandType[builtIns.int]?.symbol!!

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/loops/ProgressionHandlers.kt

@@ -59,8 +59,7 @@ internal class DownToHandler(private val context: CommonBackendContext, private
                 data,
                 lowerBound = call.extensionReceiver!!,
                 upperBound = call.getValueArgument(0)!!,
-                step = irInt(-1),
-                increasing = false
+                step = irInt(-1)
             )
         }
 }