Optimize important case of string appending to the builder. (#1591)

runtime/src/main/cpp/KString.cpp

@@ -840,6 +840,30 @@ OBJ_GETTER(Kotlin_String_plusImpl, KString thiz, KString other) {
   RETURN_OBJ(result->obj());
 }
 
+KInt Kotlin_StringBuilder_insertString(KRef builder, KInt position, KString fromString) {
+  auto toArray = builder->array();
+  RuntimeAssert(toArray->count_ >= fromString->count_ + position, "must be true");
+  memcpy(CharArrayAddressOfElementAt(toArray, position),
+         CharArrayAddressOfElementAt(fromString, 0),
+         fromString->count_ * sizeof(KChar));
+  return fromString->count_;
+}
+
+KInt Kotlin_StringBuilder_insertInt(KRef builder, KInt position, KInt value) {
+  auto toArray = builder->array();
+  RuntimeAssert(toArray->count_ >= 11 + position, "must be true");
+  char cstring[12];
+  auto length = konan::snprintf(cstring, sizeof(cstring), "%d", value);
+  auto* from = &cstring[0];
+  auto* to = CharArrayAddressOfElementAt(toArray, position);
+  auto* end = from + length;
+  while (from != end) {
+    *to++ = *from++;
+  }
+  return length;
+}
+
+
 KBoolean Kotlin_String_equals(KString thiz, KConstRef other) {
   if (other == nullptr || other->type_info() != theStringTypeInfo) return false;
   // Important, due to literal internalization.

runtime/src/main/kotlin/kotlin/text/StringBuilder.kt

@@ -65,6 +65,12 @@ private external fun String.toUtf8OrThrowImpl(start: Int, size: Int) : ByteArray
 @SymbolName("Kotlin_String_fromCharArray")
 external fun fromCharArray(array: CharArray, start: Int, size: Int) : String
 
+@SymbolName("Kotlin_StringBuilder_insertString")
+private external fun insertString(array: CharArray, start: Int, value: String): Int
+
+@SymbolName("Kotlin_StringBuilder_insertInt")
+private external fun insertInt(array: CharArray, start: Int, value: Int): Int
+
 /**
  * Builds new string by populating newly created [StringBuilder] using provided [builderAction]
  * and then converting it to [String].
@@ -253,8 +259,15 @@ class StringBuilder private constructor (
         return this
     }
 
-    fun insert(index: Int, string: String): StringBuilder = insert(index, string.toCharArray())
+    fun insert(index: Int, string: String): StringBuilder {
+        checkInsertIndex(index)
+        ensureExtraCapacity(string.length)
+        array.copyRangeTo(array, index, length, index + string.length)
+        length += insertString(array, index, string)
+        return this
+    }
 
+    // TODO: optimize those!
     fun insert(index: Int, value: Boolean) = insert(index, value.toString())
     fun insert(index: Int, value: Byte)    = insert(index, value.toString())
     fun insert(index: Int, value: Short)   = insert(index, value.toString())
@@ -296,12 +309,21 @@ class StringBuilder private constructor (
         return this
     }
 
-    fun append(it: String): StringBuilder = append(it.toCharArray())
+    fun append(it: String): StringBuilder {
+        ensureExtraCapacity(it.length)
+        length += insertString(array, length, it)
+        return this
+    }
 
+    // TODO: optimize those!
     fun append(it: Boolean) = append(it.toString())
     fun append(it: Byte) = append(it.toString())
     fun append(it: Short) = append(it.toString())
-    fun append(it: Int) = append(it.toString())
+    fun append(it: Int): StringBuilder {
+        ensureExtraCapacity(11)
+        length += insertInt(array, length, it)
+        return this
+    }
     fun append(it: Long) = append(it.toString())
     fun append(it: Float) = append(it.toString())
     fun append(it: Double) = append(it.toString())