Download - Gamedev-grade debugging

inequation.orginequation.org

SpreadIT 2013 · October 19th, 2013SpreadIT 2013 · October 19th, 2013

Gamedev-grade debugging

Leszek GodlewskiFreelance [email protected]

Code snippetsCode snippets

2 inequation.org2 inequation.org

All code used in the talk available online:github.com/inequation/ggd

http://github.com/inequation/ggd

AgendaAgenda


● Who is this guy?

● Why the talk?

● A taste of gamedev debugging

● Right tool for the job

● Noise filtering

● Memory corruption

● Questions

Who is this guy?Who is this guy?


Freelance Programmer (Sep 2013 – onwards)● inequation.org● Painkiller Hell & Damnation

Linux port finalization (2013)● Unannounced project

Generalist Programmer,The Farm 51 (Mar 2010 – Aug 2013)● thefarm51.com● Painkiller Hell & Damnation

(2012-2013; Win/Linux/X360/PS3)● Deadfall Adventures

(2011-2013; Win/X360)● A few unannounced projects

http://www.inequation.org/

http://www.thefarm51.com/

AgendaAgenda



● Why the talk?



● Noise filtering


● Questions

Why the talk?Why the talk?


Because THIS has happened to me:

http://imgur.com/yBa1OGm



Intern: I've read* at all the code andstill don't see the bug.

Me: So just debug it!

*read, as in „stare without actually running it”



Intern:

http://imgur.com/yBa1OGm© DreamWorks



The three uses of debugging

● Bug hunting (doh)●

●




● Bug hunting (doh)● Reverse engineering●




● Bug hunting (doh)● Reverse engineering● Testing a new feature

AgendaAgenda



● Why the talk?



● Noise filtering


● Questions

A taste of gamedev debuggingA taste of gamedev debugging


Code: Code: 01-taste01-taste

http://github.com/inequation/ggd/tree/master/01-taste

A taste of gamedev debuggingA taste of gamedev debugging


Can you spot the culprit?

// class declarationclass Crasher extends ActorComponent;var int DummyArray[1024];

// in ammo consumption codeCrash = new class'Crasher';Comp = new class'ActorComponent' (Crash);

AgendaAgenda



● Why the talk?



● Noise filtering


● Questions

Right tool for the jobRight tool for the job


„Fix a bug for an intern, they will get stuck on the next one.

Teach them debugging, they will fix most bugs they encounter on their own.”

– Paulo Coelho



Debugging tools are essential to this profession!

● When joining a new team or starting development for a new platform, demand debugging tools● Ask senior teammates● If they don't know, there must be documentation● Be proactive!● Don't give up until the debugger is fully working

● No tools? Roll your own!● You are a coder after all, right?



Not all bugs are in the code

● Animation graphs● Flow graphs/visual scripts● Post-process effects

Still need a way to debug them



Code: Code: 02-tools02-tools

http://github.com/inequation/ggd/tree/master/02-tools

AgendaAgenda



● Why the talk?



● Noise filtering


● Questions

Noise filteringNoise filtering


There are parts of code executed thousands of times each frame

● Object transformation● Collision detection

Also rarer, but still impractical to track

● Setting materials on objects● Attaching and detaching of components

Noise filteringNoise filtering


Code: Code: 03-material03-material04-ragdoll04-ragdoll05-assert05-assert

http://github.com/inequation/ggd/tree/master/03-material

http://github.com/inequation/ggd/tree/master/04-ragdoll

http://github.com/inequation/ggd/tree/master/05-assert

AgendaAgenda



● Why the talk?



● Noise filtering


● Questions

Memory corruptionMemory corruption


Look closely:





Look closely:





● UnrealScript object construction syntaxnew <class> [(<template object>)];

● But:sizeof(Crasher) > sizeof(ActorComponent)

● Verdict:



● UnrealScript object construction syntaxnew <class> [(<template object>)];

● But:sizeof(Crasher) > sizeof(ActorComponent)

● Verdict:BUFFER OVERFLOW!



But this can happen anywhere! How to find it?

● Use a memory fence● Many related techniques● Allocate additional space in front and behind actual allocations

● Then protect them from writing...● Or write a byte pattern and periodically assert its consistency

● Also it's useful to log stack traces● Memory and CPU overhead!

● Use a debug memory allocator (dmalloc)● Use a memory debugger (Valgrind)● Use a memory analysis tool (HeapInspector)



Memory fences

malloc

Regular allocation Fenced allocation

malloc

TakeawayTakeaway


● You can't be an effective programmer without debugging tools

● If there are no tools, make some● Noise filtering techniques save your time

● Time is not only money – nerves are just as important!● Know your machine (physical or virtual) down to the metal● Instruction opcodes, registers etc. come in handy● Tons of resources available

● Random crashes and/or content glitches may indicate memory corruption

● Memory corruption can be defeated!


Questions?

[email protected]



Thank you!

[email protected]@TheIneQuation


http://www.inequation.org/

http://twitter.com/TheIneQuation