Add MultipleAssignmentTransformer

This improves deobfuscation of a static field initializer in the 667
client that looks like:

  a = b = new X()

This transformer translates it to two separate expressions:

  b = new X(); a = b

This allows the StaticFieldUnscrambler to move both fields
independently, rather than them both being forced to remain in their
scrambled class.

Signed-off-by: Graham <gpe@openrs2.org>

deob-bytecode/src/main/kotlin/org/openrs2/deob/bytecode/BytecodeDeobfuscatorModule.kt

@@ -22,6 +22,7 @@ import org.openrs2.deob.bytecode.transform.FinalMethodTransformer
 import org.openrs2.deob.bytecode.transform.InvokeSpecialTransformer
 import org.openrs2.deob.bytecode.transform.MethodOrderTransformer
 import org.openrs2.deob.bytecode.transform.MonitorTransformer
+import org.openrs2.deob.bytecode.transform.MultipleAssignmentTransformer
 import org.openrs2.deob.bytecode.transform.OpaquePredicateTransformer
 import org.openrs2.deob.bytecode.transform.OriginalNameTransformer
 import org.openrs2.deob.bytecode.transform.OriginalPcRestoreTransformer
@@ -53,6 +54,7 @@ public object BytecodeDeobfuscatorModule : AbstractModule() {
         binder.addBinding().to(OriginalNameTransformer::class.java)
         binder.addBinding().to(ClassLiteralTransformer::class.java)
         binder.addBinding().to(InvokeSpecialTransformer::class.java)
+        binder.addBinding().to(MultipleAssignmentTransformer::class.java)
         binder.addBinding().to(RemapTransformer::class.java)
         binder.addBinding().to(PatcherTransformer::class.java)
         binder.addBinding().to(ResourceTransformer::class.java)

deob-bytecode/src/main/kotlin/org/openrs2/deob/bytecode/transform/MultipleAssignmentTransformer.kt

@@ -0,0 +1,57 @@
+package org.openrs2.deob.bytecode.transform
+
+import com.github.michaelbull.logging.InlineLogger
+import jakarta.inject.Singleton
+import org.objectweb.asm.Opcodes
+import org.objectweb.asm.tree.ClassNode
+import org.objectweb.asm.tree.FieldInsnNode
+import org.objectweb.asm.tree.MethodNode
+import org.openrs2.asm.InsnMatcher
+import org.openrs2.asm.classpath.ClassPath
+import org.openrs2.asm.classpath.Library
+import org.openrs2.asm.transform.Transformer
+import org.openrs2.deob.bytecode.remap.StaticFieldUnscrambler
+
+/**
+ * A [Transformer] that splits multiple assignments to static fields in a
+ * single expression in `<clinit>` methods. For example, `a = b = new X()` is
+ * translated to `b = new X(); a = b`. This allows [StaticFieldUnscrambler] to
+ * move the fields independently.
+ */
+@Singleton
+public class MultipleAssignmentTransformer : Transformer() {
+    private var assignments = 0
+
+    override fun preTransform(classPath: ClassPath) {
+        assignments = 0
+    }
+
+    override fun transformCode(classPath: ClassPath, library: Library, clazz: ClassNode, method: MethodNode): Boolean {
+        if (method.name != "<clinit>") {
+            return false
+        }
+
+        for (match in MATCHER.match(method)) {
+            for (i in 0 until match.size - 1 step 2) {
+                val dup = match[i]
+                val putstatic = match[i + 1] as FieldInsnNode
+
+                method.instructions.remove(dup)
+                method.instructions.insert(putstatic, FieldInsnNode(Opcodes.GETSTATIC, putstatic.owner, putstatic.name, putstatic.desc))
+
+                assignments++
+            }
+        }
+
+        return false
+    }
+
+    override fun postTransform(classPath: ClassPath) {
+        logger.info { "Split $assignments multiple assignment expressions into separate expressions" }
+    }
+
+    private companion object {
+        private val logger = InlineLogger()
+        private val MATCHER = InsnMatcher.compile("(DUP PUTSTATIC)+ PUTSTATIC")
+    }
+}