fix: additional checks for 'if' blocks inside loops (#809)

jadx-core/src/main/java/jadx/core/codegen/InsnGen.java

@@ -467,7 +467,9 @@ public class InsnGen {
 			case MONITOR_EXIT:
 				if (isFallback()) {
 					code.add("monitor-exit(");
-					addArg(code, insn.getArg(0));
+					if (insn.getArgsCount() == 1) {
+						addArg(code, insn.getArg(0));
+					}
 					code.add(')');
 				}
 				break;

jadx-core/src/main/java/jadx/core/dex/visitors/regions/IfMakerHelper.java

@@ -155,6 +155,10 @@ public class IfMakerHelper {
 		if (curThen == curElse) {
 			return null;
 		}
+		if (BlockUtils.isFollowBackEdge(curThen)
+				|| BlockUtils.isFollowBackEdge(curElse)) {
+			return null;
+		}
 		boolean followThenBranch;
 		IfInfo nextIf = getNextIf(currentIf, curThen);
 		if (nextIf != null) {
@@ -380,6 +384,9 @@ public class IfMakerHelper {
 		if (next.getPredecessors().size() != 1) {
 			return null;
 		}
+		if (next.contains(AFlag.ADDED_TO_REGION)) {
+			return null;
+		}
 		List<InsnNode> insns = block.getInstructions();
 		boolean pass = true;
 		List<InsnNode> forceInlineInsns = new ArrayList<>();

jadx-core/src/main/java/jadx/core/utils/BlockUtils.java

@@ -14,6 +14,7 @@ import org.jetbrains.annotations.Nullable;
 import jadx.core.dex.attributes.AFlag;
 import jadx.core.dex.attributes.AType;
 import jadx.core.dex.attributes.nodes.IgnoreEdgeAttr;
+import jadx.core.dex.attributes.nodes.LoopInfo;
 import jadx.core.dex.attributes.nodes.PhiListAttr;
 import jadx.core.dex.instructions.IfNode;
 import jadx.core.dex.instructions.InsnType;
@@ -137,6 +138,27 @@ public class BlockUtils {
 		return from.getSuccessors().contains(to);
 	}
 
+	public static boolean isFollowBackEdge(BlockNode block) {
+		if (block == null) {
+			return false;
+		}
+		if (block.contains(AFlag.LOOP_START)) {
+			List<BlockNode> predecessors = block.getPredecessors();
+			if (predecessors.size() == 1) {
+				BlockNode loopEndBlock = predecessors.get(0);
+				if (loopEndBlock.contains(AFlag.LOOP_END)) {
+					List<LoopInfo> loops = loopEndBlock.getAll(AType.LOOP);
+					for (LoopInfo loop : loops) {
+						if (loop.getStart().equals(block) && loop.getEnd().equals(loopEndBlock)) {
+							return true;
+						}
+					}
+				}
+			}
+		}
+		return false;
+	}
+
 	/**
 	 * Check if instruction contains in block (use == for comparison, not equals)
 	 */

jadx-core/src/test/java/jadx/tests/integration/loops/TestDoWhileBreak2.java

+package jadx.tests.integration.loops;
+
+import java.util.Arrays;
+import java.util.Iterator;
+
+import org.junit.jupiter.api.Test;
+
+import jadx.tests.api.IntegrationTest;
+
+import static jadx.tests.api.utils.assertj.JadxAssertions.assertThat;
+
+public class TestDoWhileBreak2 extends IntegrationTest {
+
+	public static class TestCls {
+		Iterator<String> it;
+
+		@SuppressWarnings("ConstantConditions")
+		public Object test() {
+			String obj;
+			do {
+				obj = this.it.next();
+				if (obj == null) {
+					return obj; // 'return null' works
+				}
+			} while (this.it.hasNext());
+			return obj;
+		}
+
+		public void check() {
+			this.it = Arrays.asList("a", "b").iterator();
+			assertThat(test()).isEqualTo("b");
+
+			this.it = Arrays.asList("a", "b", null).iterator();
+			assertThat(test()).isEqualTo(null);
+		}
+	}
+
+	@Test
+	public void test() {
+		assertThat(getClassNode(TestCls.class))
+				.code()
+				.containsLine(2, "do {")
+				.containsLine(3, "obj = this.it.next();")
+				.containsLine(3, "if (obj == null) {")
+				.containsLine(2, "} while (this.it.hasNext());");
+	}
+}

jadx-core/src/test/java/jadx/tests/integration/loops/TestDoWhileBreak3.java

+package jadx.tests.integration.loops;
+
+import java.util.Iterator;
+
+import org.junit.jupiter.api.Test;
+
+import jadx.NotYetImplemented;
+import jadx.tests.api.IntegrationTest;
+
+import static jadx.tests.api.utils.assertj.JadxAssertions.assertThat;
+
+public class TestDoWhileBreak3 extends IntegrationTest {
+
+	public static class TestCls {
+		Iterator<String> it;
+
+		public void test() {
+			do {
+				if (!it.hasNext()) {
+					break;
+				}
+			} while (it.next() != null);
+		}
+	}
+
+	@NotYetImplemented
+	@Test
+	public void test() {
+		assertThat(getClassNode(TestCls.class))
+				.code()
+				.containsOnlyOnce("while");
+	}
+}