From 7747fe457aef148f47c8432dbbd4813bc65e1f01 Mon Sep 17 00:00:00 2001
From: Graham <gpe@openrs2.dev>
Date: Tue, 27 Aug 2019 14:19:07 +0100
Subject: [PATCH] Remove dummy local variables

---
 .../java/dev/openrs2/asm/InsnNodeUtils.java   | 28 +++++++
 .../java/dev/openrs2/deob/Deobfuscator.java   |  4 +-
 .../deob/transform/DummyLocalTransformer.java | 83 +++++++++++++++++++
 3 files changed, 114 insertions(+), 1 deletion(-)
 create mode 100644 deob/src/main/java/dev/openrs2/deob/transform/DummyLocalTransformer.java

diff --git a/asm/src/main/java/dev/openrs2/asm/InsnNodeUtils.java b/asm/src/main/java/dev/openrs2/asm/InsnNodeUtils.java
index fdb8ea32..047726b2 100644
--- a/asm/src/main/java/dev/openrs2/asm/InsnNodeUtils.java
+++ b/asm/src/main/java/dev/openrs2/asm/InsnNodeUtils.java
@@ -1,7 +1,10 @@
 package dev.openrs2.asm;
 
+import java.util.ArrayList;
+
 import org.objectweb.asm.Opcodes;
 import org.objectweb.asm.tree.AbstractInsnNode;
+import org.objectweb.asm.tree.InsnList;
 import org.objectweb.asm.tree.InsnNode;
 import org.objectweb.asm.tree.IntInsnNode;
 import org.objectweb.asm.tree.LdcInsnNode;
@@ -305,6 +308,31 @@ public final class InsnNodeUtils {
 		throw new IllegalArgumentException();
 	}
 
+	public static boolean deleteSimpleExpression(InsnList list, AbstractInsnNode last) {
+		var deadInsns = new ArrayList<AbstractInsnNode>();
+
+		var height = 0;
+		var insn = last;
+		do {
+			deadInsns.add(insn);
+
+			var metadata = StackMetadata.get(insn);
+			if (insn != last) {
+				height -= metadata.getPushes();
+			}
+			height += metadata.getPops();
+
+			if (height == 0) {
+				deadInsns.forEach(list::remove);
+				return true;
+			}
+
+			insn = insn.getPrevious();
+		} while (insn != null && insn.getType() != AbstractInsnNode.LABEL && !hasSideEffects(insn));
+
+		return false;
+	}
+
 	private InsnNodeUtils() {
 		/* empty */
 	}
diff --git a/deob/src/main/java/dev/openrs2/deob/Deobfuscator.java b/deob/src/main/java/dev/openrs2/deob/Deobfuscator.java
index 6875d6fc..3d9c6fe3 100644
--- a/deob/src/main/java/dev/openrs2/deob/Deobfuscator.java
+++ b/deob/src/main/java/dev/openrs2/deob/Deobfuscator.java
@@ -18,6 +18,7 @@ import dev.openrs2.deob.transform.BitwiseOpTransformer;
 import dev.openrs2.deob.transform.CanvasTransformer;
 import dev.openrs2.deob.transform.ClassForNameTransformer;
 import dev.openrs2.deob.transform.CounterTransformer;
+import dev.openrs2.deob.transform.DummyLocalTransformer;
 import dev.openrs2.deob.transform.DummyTransformer;
 import dev.openrs2.deob.transform.ExceptionTracingTransformer;
 import dev.openrs2.deob.transform.FieldOrderTransformer;
@@ -40,7 +41,8 @@ public final class Deobfuscator {
 		new CanvasTransformer(),
 		new FieldOrderTransformer(),
 		new BitwiseOpTransformer(),
-		new DummyTransformer()
+		new DummyTransformer(),
+		new DummyLocalTransformer()
 	);
 
 	public static void main(String[] args) throws IOException, AnalyzerException {
diff --git a/deob/src/main/java/dev/openrs2/deob/transform/DummyLocalTransformer.java b/deob/src/main/java/dev/openrs2/deob/transform/DummyLocalTransformer.java
new file mode 100644
index 00000000..76030d67
--- /dev/null
+++ b/deob/src/main/java/dev/openrs2/deob/transform/DummyLocalTransformer.java
@@ -0,0 +1,83 @@
+package dev.openrs2.deob.transform;
+
+import com.google.common.collect.ImmutableSet;
+import dev.openrs2.asm.InsnNodeUtils;
+import dev.openrs2.asm.classpath.ClassPath;
+import dev.openrs2.asm.transform.Transformer;
+import org.objectweb.asm.Opcodes;
+import org.objectweb.asm.tree.ClassNode;
+import org.objectweb.asm.tree.MethodNode;
+import org.objectweb.asm.tree.VarInsnNode;
+import org.objectweb.asm.tree.analysis.AnalyzerException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public final class DummyLocalTransformer extends Transformer {
+	private static final Logger logger = LoggerFactory.getLogger(DummyLocalTransformer.class);
+
+	private static final ImmutableSet<Integer> LOAD_OPCODES = ImmutableSet.of(
+		Opcodes.ILOAD,
+		Opcodes.LLOAD,
+		Opcodes.FLOAD,
+		Opcodes.DLOAD,
+		Opcodes.ALOAD
+	);
+
+	private static final ImmutableSet<Integer> STORE_OPCODES = ImmutableSet.of(
+		Opcodes.ISTORE,
+		Opcodes.LSTORE,
+		Opcodes.FSTORE,
+		Opcodes.DSTORE,
+		Opcodes.ASTORE
+	);
+
+	private int localsRemoved;
+
+	@Override
+	protected void preTransform(ClassPath classPath) throws AnalyzerException {
+		localsRemoved = 0;
+	}
+
+	@Override
+	protected boolean transformCode(ClassNode clazz, MethodNode method) throws AnalyzerException {
+		/*
+		 * XXX(gpe): this is primitive (ideally we'd do a proper data flow
+		 * analysis, but we'd need to do it in reverse and ASM only supports
+		 * forward data flow), however, it seems to be good enough to catch
+		 * everything.
+		 */
+		var loads = new boolean[method.maxLocals];
+
+		for (var it = method.instructions.iterator(); it.hasNext(); ) {
+			var insn = it.next();
+			if (LOAD_OPCODES.contains(insn.getOpcode())) {
+				var load = (VarInsnNode) insn;
+				loads[load.var] = true;
+			}
+		}
+
+		for (var it = method.instructions.iterator(); it.hasNext(); ) {
+			var insn = it.next();
+			var opcode = insn.getOpcode();
+			if (!STORE_OPCODES.contains(opcode)) {
+				continue;
+			}
+
+			var store = (VarInsnNode) insn;
+			if (loads[store.var]) {
+				continue;
+			}
+
+			if (InsnNodeUtils.deleteSimpleExpression(method.instructions, insn)) {
+				localsRemoved++;
+			}
+		}
+
+		return false;
+	}
+
+	@Override
+	protected void postTransform(ClassPath classPath) throws AnalyzerException {
+		logger.info("Removed {} dummy local variables", localsRemoved);
+	}
+}