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LEGO Island Decompilation Tools

Accuracy to the game's original code is the main goal of the decompilation project. To facilitate the decompilation effort and maintain overall quality, we have devised a set of annotations, to be embedded in the source code, which allow us to automatically verify the accuracy of re-compiled functions' assembly, virtual tables, variable offsets and more.

In order for contributions to be accepted, the annotations must be used in accordance to the rules outlined here. Proper use is enforced by GitHub Actions which run the Python tools found in this folder. It is recommended to integrate these tools into your local development workflow as well.

Overview

We are continually working on extending the capabilities of our "decompilation language" and the toolset around it. Some of the following annotations have not made it into formal verification and thus are not technically enforced on the source code level yet (marked as WIP). Nevertheless, it is recommended to use them since it is highly likely they will eventually be fully integrated.

Functions

All non-inlined functions in the code base with the exception of 3rd party code must be annotated with one of the following markers, which include the module name and address of the function as found in the original binaries. This information is then used to compare the recompiled assembly with the original assembly, resulting in an accuracy score. Functions in a given compilation unit must be ordered by their address in ascending order.

The annotations can be attached to the function implementation, which is the most common case, or use the "comment" syntax (see examples below) for functions that cannot be referred to directly (such as templated, synthetic or non-inlined inline functions). The latter should only ever appear in .h files.

FUNCTION

Functions with a reasonably complete implementation which are not templated or synthetic (see below) should be annotated with FUNCTION.

// FUNCTION: LEGO1 0x100b12c0
MxCore* MxObjectFactory::Create(const char* p_name)
{
  // implementation
}

// FUNCTION: LEGO1 0x100140d0
// MxCore::IsA

STUB

Functions with no or a very incomplete implementation should be annotated with STUB. These will not be compared to the original assembly.

// STUB: LEGO1 0x10011d50
LegoCameraController::LegoCameraController()
{
  // TODO
}

TEMPLATE

Templated functions should be annotated with TEMPLATE. Since the goal is to eventually have a full accounting of all the functions present in the binaries, please make an effort to find and annotate every function of a templated class.

// TEMPLATE: LEGO1 0x100c0ee0
// list<MxNextActionDataStart *,allocator<MxNextActionDataStart *> >::_Buynode

// TEMPLATE: LEGO1 0x100c0fc0
// MxStreamListMxDSSubscriber::~MxStreamListMxDSSubscriber

// TEMPLATE: LEGO1 0x100c1010
// MxStreamListMxDSAction::~MxStreamListMxDSAction

SYNTHETIC

Synthetic functions should be annotated with SYNTHETIC. A synthetic function is generated by the compiler; most common is the "scalar deleting destructor" found in virtual tables. Other cases include default destructors and assignment operators. Note: SYNTHETIC takes precedence over TEMPLATE.

// SYNTHETIC: LEGO1 0x10003210
// Helicopter::`scalar deleting destructor'

// SYNTHETIC: LEGO1 0x100c4f50
// MxCollection<MxRegionLeftRight *>::`scalar deleting destructor'

// SYNTHETIC: LEGO1 0x100c4fc0
// MxList<MxRegionLeftRight *>::`scalar deleting destructor'

LIBRARY

Functions located in 3rd party libraries should be annotated with LIBRARY. Since the goal is to eventually have a full accounting of all the functions present in the binaries, please make an effort to find and annotate every function of every statically linked library, including the MSVC standard libraries.

// LIBRARY: ISLE 0x4061b0
// _MemPoolInit@4

// LIBRARY: ISLE 0x406520
// _MemPoolSetPageSize@8

// LIBRARY: ISLE 0x406630
// _MemPoolSetBlockSizeFS@8

Virtual tables

Classes with a virtual table should be annotated using the VTABLE marker, which includes the module name and address of the virtual table. Additionally, virtual function declarations should be annotated with a comment indicating their relative offset. Please use the following example as a reference.

// VTABLE: LEGO1 0x100dc900
class MxEventManager : public MxMediaManager {
public:
	MxEventManager();
	virtual ~MxEventManager() override;

	virtual void Destroy() override;                                     // vtable+0x18
	virtual MxResult Create(MxU32 p_frequencyMS, MxBool p_createThread); // vtable+0x28

Class size

Classes should be annotated using the SIZE marker to indicate their size. If you are unsure about the class size in the original binary, please use the currently available information (known member variables) and detail the circumstances in an extra comment if necessary.

// SIZE 0x1c
class MxCriticalSection {
public:
	MxCriticalSection();
	~MxCriticalSection();
	static void SetDoMutex();

Furthermore, add DECOMP_SIZE_ASSERT(MxCriticalSection, 0x1c) to the respective .cpp file (if the class has no dedicated .cpp file, use any appropriate .cpp file where the class is used).

Member variables

Member variables should be annotated with their relative offsets.

class MxDSObject : public MxCore {
private:
	MxU32 m_sizeOnDisk;   // 0x8
	MxU16 m_type;         // 0xc
	char* m_sourceName;   // 0x10
	undefined4 m_unk0x14; // 0x14

Global variables

Global variables should be annotated using the GLOBAL marker, which includes the module name and address of the variable.

// GLOBAL: LEGO1 0x100f456c
MxAtomId* g_jukeboxScript = NULL;

// GLOBAL: LEGO1 0x100f4570
MxAtomId* g_pz5Script = NULL;

// GLOBAL: LEGO1 0x100f4574
MxAtomId* g_introScript = NULL;

Strings

String values should be annotated using the STRING marker, which includes the module name and address of the string.

inline virtual const char* ClassName() const override // vtable+0x0c
{
	// STRING: LEGO1 0x100f03fc
	return "Act2PoliceStation";
}

Tooling

Use pip to install the required packages to be able to use the Python tools found in this folder:

pip install -e .

All scripts will become available to use in your terminal with the reccmp- prefix. The example usages below assume that the retail binaries have been copied to ./legobin.

  • decomplint: Checks the decompilation annotations (see above)
    • e.g. reccmp-decomplint --module LEGO1 LEGO1
  • isledecomp: A library that implements a parser to identify the decompilation annotations (see above)
  • reccmp: Compares an original binary with a recompiled binary, provided a PDB file. For example:
    • Display the diff for a single function: reccmp-reccmp --verbose 0x100ae1a0 legobin/LEGO1.DLL build/LEGO1.DLL build/LEGO1.PDB .
    • Generate an HTML report: reccmp-reccmp --html output.html legobin/LEGO1.DLL build/LEGO1.DLL build/LEGO1.PDB .
    • Create a base file for diffs: reccmp-reccmp --json base.json --silent legobin/LEGO1.DLL build/LEGO1.DLL build/LEGO1.PDB .
    • Diff against a base file: reccmp-reccmp --diff base.json legobin/LEGO1.DLL build/LEGO1.DLL build/LEGO1.PDB .
  • stackcmp: Compares the stack layout for a given function that almost matches.
    • e.g. reccmp-stackcmp legobin/BETA10.DLL build_debug/LEGO1.DLL build_debug/LEGO1.pdb . 0x1007165d
  • roadmap: Compares symbol locations in an original binary with the same symbol locations of a recompiled binary
  • verexp: Verifies exports by comparing the exports of the original DLL and the recompiled DLL
  • vtable: Asserts virtual table correctness by comparing a recompiled binary with the original
    • e.g. reccmp-vtable legobin/LEGO1.DLL build/LEGO1.DLL build/LEGO1.PDB .
  • datacmp: Compares global data found in the original with the recompiled version
    • e.g. reccmp-datacmp legobin/LEGO1.DLL build/LEGO1.DLL build/LEGO1.PDB .

Testing

isledecomp comes with a suite of tests. Install requirements-tests.txt and run it like this:

pip install -r requirements-tests.txt
pytest .

Tool Development

In order to keep the Python code clean and consistent, we use pylint and black:

pip install -r requirements-tests.txt

Run pylint (ignores build and virtualenv)

pylint reccmp

Check Python code formatting without rewriting files

black --check reccmp

Apply Python code formatting

black reccmp

Modules

The following is a list of all the modules found in the annotations (e.g. // FUNCTION: [module] [address]) and which binaries they refer to. See this list of all known versions of the game.

Retail v1.1.0.0 (v1.1)

  • LEGO1 -> LEGO1.DLL
  • CONFIG-> CONFIG.EXE
  • ISLE -> ISLE.EXE

These modules are the most important ones and refer to the English retail version 1.1.0.0 (often shortened to v1.1), which is the most widely released one. These are the ones we attempt to decompile and match as best as possible.

BETA v1.0

  • BETA10 -> LEGO1D.DLL

The Beta 1.0 version contains a debug build of the game. While it does not have debug symbols, it still has a number of benefits:

  • It is built with less or no optimisation, leading to better decompilations in Ghidra
  • Far fewer functions are inlined by the compiler, so it can be used to recognise inlined functions
  • It contains assertions that tell us original variable names and code file paths

It is therefore advisable to search for the corresponding function in BETA10 when decompiling a function in LEGO1. Finding the correct function can be tricky, but is usually worth it, especially for longer functions.

Unfortunately, some code has been changed after this beta version was created. Therefore, we are not aiming for a perfect binary match of BETA10. In case of discrepancies, LEGO1 (as defined above) is our "gold standard" for matching.

Re-compiling a beta build (WIP)

If you want to match the code against BETA10, use the following cmake setup to create a debug build:

cmake <path-to-source> -G "NMake Makefiles" -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_BUILD_TYPE=Debug -DISLE_USE_SMARTHEAP=OFF

TODO: If you can figure out how to make a debug build with SmartHeap enabled, please add it here.

If you want to run scripts to compare your debug build to BETA10 (e.g. reccmp), it is advisable to add a copy of LEGO1D.DLL to /legobin and rename it to BETA10.DLL.

Finding matching functions

This is not a recipe, but rather a list of things you can try.

  • If you are working on a virtual function in a class, try to find the class' vtable. Many (but not all) classes implement ClassName(). These functions are usually easy to find by searching the memory for the string consisting of the class name. Keep in mind that not all child classes overwrite this function, so if the function you found is used in multiple vtables (or if you found multiple ClassName()-like functions), make sure you actually have the parent's vtable.
  • If that does not help, you can try to walk up the call tree and try to locate a function that calls the function you are interested in.
  • Assertions can also help you - most .cpp file names have already been matched based on BETA10, so you can search for the name of your .cpp file and check all the assertions in that file. While that does not find all functions in a given source file, it usually finds the more complex ones.
  • If you have found any other strategies, please add them here.

Others (WIP)

  • ALPHA (only used twice)