fix: improve StringBuilder elimination (#2148)

jadx-core/src/main/java/jadx/core/dex/instructions/args/InsnArg.java

@@ -132,6 +132,10 @@ public abstract class InsnArg extends Typed {
 		}
 		InsnArg arg = wrapInsnIntoArg(insn);
 		InsnArg oldArg = parent.getArg(i);
+		if (arg.getType() == ArgType.UNKNOWN) {
+			// restore arg type if wrapped insn missing result
+			arg.setType(oldArg.getType());
+		}
 		parent.setArg(i, arg);
 		InsnRemover.unbindArgUsage(mth, oldArg);
 		if (unbind) {

jadx-core/src/main/java/jadx/core/dex/instructions/args/InsnWrapArg.java

@@ -1,7 +1,5 @@
 package jadx.core.dex.instructions.args;
 
-import java.util.Objects;
-
 import org.jetbrains.annotations.NotNull;
 
 import jadx.core.dex.instructions.ConstStringNode;
@@ -75,10 +73,18 @@ public final class InsnWrapArg extends InsnArg {
 	}
 
 	@Override
-	public String toString() {
-		if (wrappedInsn.getType() == InsnType.CONST_STR && Objects.equals(type, ArgType.STRING)) {
+	public String toShortString() {
+		if (wrappedInsn.getType() == InsnType.CONST_STR) {
 			return "(\"" + ((ConstStringNode) wrappedInsn).getString() + "\")";
 		}
-		return "(wrap: " + type + " : " + wrappedInsn + ')';
+		return "(wrap:" + type + ":" + wrappedInsn.getType() + ')';
+	}
+
+	@Override
+	public String toString() {
+		if (wrappedInsn.getType() == InsnType.CONST_STR) {
+			return "(\"" + ((ConstStringNode) wrappedInsn).getString() + "\")";
+		}
+		return "(wrap:" + type + ":" + wrappedInsn + ')';
 	}
 }

jadx-core/src/main/java/jadx/core/dex/visitors/SimplifyVisitor.java

@@ -402,7 +402,8 @@ public class SimplifyVisitor extends AbstractVisitor {
 				}
 			}
 			if (!stringArgFound) {
-				mth.addDebugComment("TODO: convert one arg to string using `String.valueOf()`, args: " + args);
+				String argStr = Utils.listToString(args, InsnArg::toShortString);
+				mth.addDebugComment("TODO: convert one arg to string using `String.valueOf()`, args: " + argStr);
 				return null;
 			}
 
@@ -625,7 +626,9 @@ public class SimplifyVisitor extends AbstractVisitor {
 			for (int i = 1; i < argsCount; i++) {
 				concat.addArg(wrap.getArg(i));
 			}
-			return ArithNode.oneArgOp(ArithOp.ADD, fArg, InsnArg.wrapArg(concat));
+			InsnArg concatArg = InsnArg.wrapArg(concat);
+			concatArg.setType(ArgType.STRING);
+			return ArithNode.oneArgOp(ArithOp.ADD, fArg, concatArg);
 		} catch (Exception e) {
 			LOG.debug("Can't convert field arith insn: {}, mth: {}", insn, mth, e);
 		}

jadx-core/src/main/java/jadx/core/dex/visitors/shrink/CodeShrinkVisitor.java

@@ -3,7 +3,6 @@ package jadx.core.dex.visitors.shrink;
 import java.util.ArrayList;
 import java.util.BitSet;
 import java.util.List;
-import java.util.ListIterator;
 import java.util.Objects;
 import java.util.Set;
 
@@ -64,12 +63,9 @@ public class CodeShrinkVisitor extends AbstractVisitor {
 		List<WrapInfo> wrapList = new ArrayList<>();
 		for (ArgsInfo argsInfo : argsList) {
 			List<RegisterArg> args = argsInfo.getArgs();
-			if (!args.isEmpty()) {
-				ListIterator<RegisterArg> it = args.listIterator(args.size());
-				while (it.hasPrevious()) {
-					RegisterArg arg = it.previous();
-					checkInline(mth, block, insnList, wrapList, argsInfo, arg);
-				}
+			for (int i = args.size() - 1; i >= 0; i--) {
+				RegisterArg arg = args.get(i);
+				checkInline(mth, block, insnList, wrapList, argsInfo, arg);
 			}
 		}
 		if (!wrapList.isEmpty()) {

jadx-core/src/test/java/jadx/tests/integration/others/TestStringBuilderElimination5.java

@@ -0,0 +1,45 @@
+package jadx.tests.integration.others;
+
+import org.junit.jupiter.api.Test;
+
+import jadx.tests.api.IntegrationTest;
+
+import static jadx.tests.api.utils.assertj.JadxAssertions.assertThat;
+
+public class TestStringBuilderElimination5 extends IntegrationTest {
+
+	public static class TestCls {
+		@SuppressWarnings("StringConcatenationInLoop")
+		public static String test(long[] a) {
+			String s = "";
+			final char[] hexChars = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };
+			for (int i = a.length - 1; i >= 0; i--) {
+				s += hexChars[(int) (a[i] >>> 60) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 56) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 52) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 48) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 44) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 40) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 36) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 32) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 28) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 24) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 20) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 16) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 12) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 8) & 0x0f];
+				s += hexChars[(int) (a[i] >>> 4) & 0x0f];
+				s += hexChars[(int) (a[i]) & 0x0f];
+				s += " ";
+			}
+			return s;
+		}
+	}
+
+	@Test
+	public void test() {
+		assertThat(getClassNode(TestCls.class))
+				.code()
+				.doesNotContain(".append(");
+	}
+}