JVM_IR: fold constant string concatenations

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/JvmLower.kt

@@ -105,8 +105,8 @@ val jvmPhases = namedIrFilePhase(
             tailrecPhase then
             toArrayPhase then
             jvmTypeOperatorLoweringPhase then
-            foldConstantLoweringPhase then
             flattenStringConcatenationPhase then
+            foldConstantLoweringPhase then
             jvmBuiltinOptimizationLoweringPhase then
             additionalClassAnnotationPhase then
 

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/FoldConstantLowering.kt

@@ -10,10 +10,9 @@ import org.jetbrains.kotlin.backend.common.phaser.makeIrFilePhase
 import org.jetbrains.kotlin.backend.jvm.JvmBackendContext
 import org.jetbrains.kotlin.ir.declarations.IrFile
 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.expressions.*
 import org.jetbrains.kotlin.ir.expressions.impl.IrConstImpl
+import org.jetbrains.kotlin.ir.expressions.impl.IrStringConcatenationImpl
 import org.jetbrains.kotlin.ir.types.*
 import org.jetbrains.kotlin.ir.util.fqNameWhenAvailable
 import org.jetbrains.kotlin.ir.visitors.IrElementTransformerVoid
@@ -176,6 +175,26 @@ class FoldConstantLowering(private val context: JvmBackendContext) : IrElementTr
                     else -> expression
                 }
             }
+
+            override fun visitStringConcatenation(expression: IrStringConcatenation): IrExpression {
+                expression.transformChildrenVoid(this)
+                val folded = mutableListOf<IrExpression>()
+                for (next in expression.arguments) {
+                    val last = folded.lastOrNull()
+                    when {
+                        next !is IrConst<*> -> folded += next
+                        last !is IrConst<*> -> folded += IrConstImpl.string(
+                            next.startOffset, next.endOffset, context.irBuiltIns.stringType, next.value.toString()
+                        )
+                        else -> folded[folded.size - 1] = IrConstImpl.string(
+                            last.startOffset, next.endOffset, context.irBuiltIns.stringType,
+                            last.value.toString() + next.value.toString()
+                        )
+                    }
+                }
+                return folded.singleOrNull() as? IrConst<*>
+                    ?: IrStringConcatenationImpl(expression.startOffset, expression.endOffset, expression.type, folded)
+            }
         })
     }
 }