Optimize recursive string concatenation

compiler/backend/src/org/jetbrains/kotlin/codegen/ExpressionCodegen.java

@@ -3184,7 +3184,9 @@ public class ExpressionCodegen extends KtVisitor<StackValue, StackValue> impleme
     }
 
     public void invokeAppend(KtExpression expr) {
-        if (expr instanceof KtBinaryExpression) {
+        ConstantValue<?> compileTimeConstant = getPrimitiveOrStringCompileTimeConstant(expr, bindingContext);
+
+        if (compileTimeConstant == null && expr instanceof KtBinaryExpression) {
             KtBinaryExpression binaryExpression = (KtBinaryExpression) expr;
             if (binaryExpression.getOperationToken() == KtTokens.PLUS) {
                 KtExpression left = binaryExpression.getLeft();
@@ -3198,8 +3200,13 @@ public class ExpressionCodegen extends KtVisitor<StackValue, StackValue> impleme
                 }
             }
         }
+
         Type exprType = expressionType(expr);
-        gen(expr, exprType);
+        if (compileTimeConstant != null) {
+            StackValue.constant(compileTimeConstant.getValue(), exprType).put(exprType, v);
+        } else {
+            gen(expr, exprType);
+        }
         genInvokeAppendMethod(v, exprType.getSort() == Type.ARRAY ? OBJECT_TYPE : exprType);
     }
 

compiler/testData/codegen/bytecodeText/constants/partialString.kt

@@ -0,0 +1,7 @@
+fun box(): String {
+    val z = 1;
+    return "O" + "K".toString() + z
+}
+
+// 1 LDC "OK"
+// 1 LDC

compiler/tests/org/jetbrains/kotlin/codegen/BytecodeTextTestGenerated.java

@@ -625,6 +625,12 @@ public class BytecodeTextTestGenerated extends AbstractBytecodeTextTest {
             doTest(fileName);
         }
 
+        @TestMetadata("partialString.kt")
+        public void testPartialString() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/constants/partialString.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("short.kt")
         public void testShort() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/constants/short.kt");