[JVM IR] Use append(Char) for 1-length string literals in string

templates and plus concatenations.

This is slightly more efficient and mirrors the behavior of the non-IR
backend for templates (but not for plus concatenations).

#KT-36638 Fixed

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

@@ -15,13 +15,12 @@ import org.jetbrains.kotlin.backend.jvm.JvmBackendContext
 import org.jetbrains.kotlin.backend.jvm.ir.JvmIrBuilder
 import org.jetbrains.kotlin.backend.jvm.ir.createJvmIrBuilder
 import org.jetbrains.kotlin.backend.jvm.lower.inlineclasses.unboxInlineClass
-import org.jetbrains.kotlin.ir.builders.IrBuilderWithScope
+import org.jetbrains.kotlin.ir.builders.*
-import org.jetbrains.kotlin.ir.builders.irCall
-import org.jetbrains.kotlin.ir.builders.irImplicitCast
-import org.jetbrains.kotlin.ir.builders.irString
 import org.jetbrains.kotlin.ir.declarations.IrClass
 import org.jetbrains.kotlin.ir.declarations.IrFile
 import org.jetbrains.kotlin.ir.declarations.IrSimpleFunction
+import org.jetbrains.kotlin.ir.expressions.IrConst
+import org.jetbrains.kotlin.ir.expressions.IrConstKind
 import org.jetbrains.kotlin.ir.expressions.IrExpression
 import org.jetbrains.kotlin.ir.expressions.IrStringConcatenation
 import org.jetbrains.kotlin.ir.expressions.IrTypeOperator
@@ -79,16 +78,23 @@ private class JvmStringConcatenationLowering(val context: JvmBackendContext) : F
     private fun typeToAppendFunction(type: IrType): IrSimpleFunction =
         appendFunctions[type] ?: defaultAppendFunction
 
-    // There is no special append or valueOf function for byte and short on the JVM.
+    private fun IrBuilderWithScope.normalizeArgument(expression: IrExpression): IrExpression =
-    private fun IrBuilderWithScope.widenIntegerType(expression: IrExpression): IrExpression =
         if (expression.type.isByte() || expression.type.isShort()) {
+            // There is no special append or valueOf function for byte and short on the JVM.
             irImplicitCast(expression, context.irBuiltIns.intType)
+        } else if (expression is IrConst<*> && expression.kind == IrConstKind.String && (expression.value as String).length == 1) {
+            // PSI2IR generates const Strings for 1-length literals in string templates (e.g., the space between x and y in "$x $y").
+            // We want to use the more efficient `append(Char)` function in such cases. This mirrors the behavior of the non-IR backend.
+            //
+            // In addition, this also means `append(Char)` will be used for the space in the following case: `x + " " + y`. The non-IR
+            // backend will still use `append(String)` in this case.
+            irChar((expression.value as String)[0])
         } else {
             expression
         }
 
     private fun JvmIrBuilder.callToString(expression: IrExpression): IrExpression {
-        val argument = widenIntegerType(expression)
+        val argument = normalizeArgument(expression)
         val argumentType = if (argument.type.isPrimitiveType()) argument.type else context.irBuiltIns.anyNType
 
         return irCall(backendContext.ir.symbols.typeToStringValueOfFunction(argumentType)).apply {
@@ -146,7 +152,7 @@ private class JvmStringConcatenationLowering(val context: JvmBackendContext) : F
                 else -> {
                     var stringBuilder = irCall(constructor)
                     for (arg in arguments) {
-                        val argument = widenIntegerType(arg)
+                        val argument = normalizeArgument(arg)
                         val appendFunction = typeToAppendFunction(argument.type)
                         stringBuilder = irCall(appendFunction).apply {
                             dispatchReceiver = stringBuilder

compiler/testData/codegen/bytecodeText/kt5016.kt

@@ -1,6 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
-// TODO KT-36638 Use 'java/lang/StringBuilder.append (C)Ljava/lang/StringBuilder;' when appending single character in JVM_IR
-
 // KT-5016 wrong StringBuilder append method invoked
 class kt5016 {
     fun f1(name : String) : String {
@@ -10,4 +7,6 @@ class kt5016 {
 
 // 0 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/Object;\)Ljava/lang/StringBuilder
 // 2 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/String;\)Ljava/lang/StringBuilder
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(C\)Ljava/lang/StringBuilder
+// 3 INVOKEVIRTUAL java/lang/StringBuilder.append
 // 1 INVOKEVIRTUAL java/lang/StringBuilder.toString

compiler/testData/codegen/bytecodeText/kt5016int.kt

@@ -1,6 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
-// TODO KT-36638 Use 'java/lang/StringBuilder.append (C)Ljava/lang/StringBuilder;' when appending single character in JVM_IR
-
 // KT-5016 wrong StringBuilder append method invoked
 class kt5016int {
     fun f1(num : Int) : String {
@@ -11,4 +8,6 @@ class kt5016int {
 // 0 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/Object;\)Ljava/lang/StringBuilder
 // 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/String;\)Ljava/lang/StringBuilder
 // 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(I\)Ljava/lang/StringBuilder
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(C\)Ljava/lang/StringBuilder
+// 3 INVOKEVIRTUAL java/lang/StringBuilder.append
 // 1 INVOKEVIRTUAL java/lang/StringBuilder.toString

compiler/testData/codegen/bytecodeText/kt5016intOrNull.kt

@@ -1,6 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
-// TODO KT-36638 Use 'java/lang/StringBuilder.append (C)Ljava/lang/StringBuilder;' when appending single character in JVM_IR
-
 // KT-5016 wrong StringBuilder append method invoked
 class kt5016intOrNull {
     fun f1(num : Int?) : String {
@@ -10,4 +7,6 @@ class kt5016intOrNull {
 
 // 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/Object;\)Ljava/lang/StringBuilder
 // 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/String;\)Ljava/lang/StringBuilder
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(C\)Ljava/lang/StringBuilder
+// 3 INVOKEVIRTUAL java/lang/StringBuilder.append
 // 1 INVOKEVIRTUAL java/lang/StringBuilder.toString

compiler/testData/codegen/bytecodeText/stringOperations/useAppendCharForOneCharStringInTemplate.kt

@@ -0,0 +1,6 @@
+fun test(s: String, i: Int) = "${"x"}${s}${" "}${i}${"y"}"
+
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/String;\)Ljava/lang/StringBuilder
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(I\)Ljava/lang/StringBuilder
+// 3 INVOKEVIRTUAL java/lang/StringBuilder.append \(C\)Ljava/lang/StringBuilder
+// 5 INVOKEVIRTUAL java/lang/StringBuilder.append

compiler/testData/codegen/bytecodeText/stringOperations/useAppendCharForOneCharStringInTemplate_2.kt

@@ -0,0 +1,6 @@
+fun test(s: String, i: Int) = "x${s} ${i}y"
+
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/String;\)Ljava/lang/StringBuilder
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(I\)Ljava/lang/StringBuilder
+// 3 INVOKEVIRTUAL java/lang/StringBuilder.append \(C\)Ljava/lang/StringBuilder
+// 5 INVOKEVIRTUAL java/lang/StringBuilder.append

compiler/testData/codegen/bytecodeText/stringOperations/useAppendCharForOneCharStringUsingPlus.kt

@@ -0,0 +1,16 @@
+fun test(s: String, i: Int) = "x" + s + " " + i + "y"
+
+// The IR is equivalent for this test and "useAppendCharForOneCharStringInTemplate*.kt" because there is an optimization for 1-length
+// string literals in any string concatenation, whether using templates or + operator (see JvmStringConcatenationLowering).
+// However, for the non-IR backend, `append(String)` will still be used for these 1-length strings.
+
+// JVM_TEMPLATES
+// 4 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/String;\)Ljava/lang/StringBuilder
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(I\)Ljava/lang/StringBuilder
+// 5 INVOKEVIRTUAL java/lang/StringBuilder.append
+
+// JVM_IR_TEMPLATES
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/String;\)Ljava/lang/StringBuilder
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(I\)Ljava/lang/StringBuilder
+// 3 INVOKEVIRTUAL java/lang/StringBuilder.append \(C\)Ljava/lang/StringBuilder
+// 5 INVOKEVIRTUAL java/lang/StringBuilder.append

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

@@ -4249,6 +4249,21 @@ public class BytecodeTextTestGenerated extends AbstractBytecodeTextTest {
         public void testStringPlus() throws Exception {
             runTest("compiler/testData/codegen/bytecodeText/stringOperations/stringPlus.kt");
         }
+
+        @TestMetadata("useAppendCharForOneCharStringInTemplate.kt")
+        public void testUseAppendCharForOneCharStringInTemplate() throws Exception {
+            runTest("compiler/testData/codegen/bytecodeText/stringOperations/useAppendCharForOneCharStringInTemplate.kt");
+        }
+
+        @TestMetadata("useAppendCharForOneCharStringInTemplate_2.kt")
+        public void testUseAppendCharForOneCharStringInTemplate_2() throws Exception {
+            runTest("compiler/testData/codegen/bytecodeText/stringOperations/useAppendCharForOneCharStringInTemplate_2.kt");
+        }
+
+        @TestMetadata("useAppendCharForOneCharStringUsingPlus.kt")
+        public void testUseAppendCharForOneCharStringUsingPlus() throws Exception {
+            runTest("compiler/testData/codegen/bytecodeText/stringOperations/useAppendCharForOneCharStringUsingPlus.kt");
+        }
     }
 
     @TestMetadata("compiler/testData/codegen/bytecodeText/toArray")

compiler/tests/org/jetbrains/kotlin/codegen/ir/IrBytecodeTextTestGenerated.java

@@ -4167,6 +4167,21 @@ public class IrBytecodeTextTestGenerated extends AbstractIrBytecodeTextTest {
         public void testStringPlus() throws Exception {
             runTest("compiler/testData/codegen/bytecodeText/stringOperations/stringPlus.kt");
         }
+
+        @TestMetadata("useAppendCharForOneCharStringInTemplate.kt")
+        public void testUseAppendCharForOneCharStringInTemplate() throws Exception {
+            runTest("compiler/testData/codegen/bytecodeText/stringOperations/useAppendCharForOneCharStringInTemplate.kt");
+        }
+
+        @TestMetadata("useAppendCharForOneCharStringInTemplate_2.kt")
+        public void testUseAppendCharForOneCharStringInTemplate_2() throws Exception {
+            runTest("compiler/testData/codegen/bytecodeText/stringOperations/useAppendCharForOneCharStringInTemplate_2.kt");
+        }
+
+        @TestMetadata("useAppendCharForOneCharStringUsingPlus.kt")
+        public void testUseAppendCharForOneCharStringUsingPlus() throws Exception {
+            runTest("compiler/testData/codegen/bytecodeText/stringOperations/useAppendCharForOneCharStringUsingPlus.kt");
+        }
     }
 
     @TestMetadata("compiler/testData/codegen/bytecodeText/toArray")