[LLVMdev] Using LLVM to compile Objective-C on an Xbox 360

[Kevin Wooten]

Thanks for the articles! There is definitely some stuff in there I hadn't thought of; also, it quite obviously points out that it's actually not "toll free" at all but "very low cost"!

This won't affect us too much though since we will be creating our own objective-c classes from scratch and we are not trying to maintain backward compatibility.  A simple solution would just be to have a method that returns our underlying implementation object (e.g. a std::basic_string for GString) and/or make the ivar implementation object public and let anybody access it.

On May 27, 2010, at 1:18 AM, Jean-Daniel Dupas wrote:

> 
> Le 27 mai 2010 à 09:15, Kevin Wooten a écrit :
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>> Implementing the backend (or editing the current PPC backend as needed) is a definite option.  This seems to be the real question... which is easier... maintaining the PPC backend or maintaining the rewriter.  Currently (in admittedly trivial tests) I have gotten the rewriter to work and output C code.  There are some outstanding issues to do with linking and accessing the reflection information objective-c provides (and the rewriter properly outputs) but I am sure the person/people who wrote the rewriter can answer those fairly simply (or else the rewriter would be fairly useless).
>> 
>> As far as the runtime goes there is an implementation for windows, linux and most likely the PS3 (since it is already a linux derivative using GCC as mentioned).  If we have to create a runtime for PS3 and XBox that seems trivial as the functions are very basic in nature.
>> 
>> My comments on OpenStep were more meant to point to the fact that we would write our own library; really just throwing all notion of OpenStep away.  Instead of NSObject we would create our own base DObject or something along with a DString, DAarry, DSet, DMap, etc.  Truthfully this would be our plan anyway because we want to follow the lead of OSX and provide these objects "toll free bridged"... meaning we would implement them using a std c++ library object (e.g. std::basic_string or std::vector) as the first member of the object which will allow us to cast a DString to std::string or DArray to std::vector without any manual or automatic marshaling. As mentioned this is how OSX implements its NS classes, by using the equivalent CF version which again allows casting between the objects. 
> 
> 
> This is not how the CFBridge works. Actually, the first member of an object MUST be a pointer to its Class (the isa pointer).  If you want to be able to put something else, you will have to rewrite the runtime to support it. 
> 
> You can read this great article about the bridge here:
> 
> http://ridiculousfish.com/blog/archives/2006/09/09/bridge/#fish_made_a_mess
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> and an other one here:
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> http://www.mikeash.com/pyblog/friday-qa-2010-01-22-toll-free-bridging-internals.html
> 
> And you can also have a look at the stripped down version of the CoreFoundation sources to get some other clues about how it works now.
> 
> http://www.opensource.apple.com/source/CF/CF-550.13/
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> -- Jean-Daniel
> 
>> On May 26, 2010, at 6:19 PM, Dale Johannesen wrote:
>> 
>>> llvm can output C code, but that target has bitrotted severely over the last few months and nobody seems to be interested in fixing it.  You may need to do some work there.  Alternatively you could implement the PPC ABI that you need.  There are several examples of supporting multiple ABIs on the same hardware, x86 being the most obvious.  A lot of simple stuff will probably Just Work with the existing PPC ABI, more complicated stuff may not.  (Only 32-bit PPC is really maintained, though, there are probably lots of problems with 64-bit.)
>>> 
>>> Objective C in llvm, AFAIK, is only used on the MacOSX targets and only tested there.   There are sufficient secret handshakes between the compiler and the ObjC runtime that it is unlikely to Just Work in an untested environment.  OpenStep has a familial relationship to MacOSX ObjC runtime, but they aren't the same and are unlikely to still be binary compatible at this point.  You may need to do some work there also.
>>> 
>>> Summary, you can probably get this approach to work, but it's not as easy as you're hoping.
>>> 
>>> On May 26, 2010, at 5:20 PMPDT, kdubb wrote:
>>> 
>>>> We are looking at using Objective-C/C++ in a new game engine.  Objective C's duality of being both very dynamic and very "C" gives us exactly what we need to make the SDK and engineering of games simpler.
>>>> 
>>>> This means that we will need a way to compile it on all platforms our games will target.  Currently the major platforms we are concerned with include... PC, Mac, XBox 360, PS3, iPhone.  Now the PC, Mac, iPhone and PS3 are fairly simple.  If we build our own OpenStep libraries we can simply use LLVM to compile directly to these platforms; the PS3 although proprietary uses GCC as a C/C++ compiler so I am assuming Objective-C can be used fairly simply.  This leaves us with the XBox 360.
>>>> 
>>>> The 360 is a special chip (PowerPC based) with, as far as I have researched, a special ABI (Windows derivative).  I haven't the faintest clue of whether code from the LLVM PPC backend would even work on the 360, much less interoperate with the system libraries. So my formulated solution has become this: use an LLVM backend to output C code and then compile that code with using MS's XBox 360 compiler. I believe I have read that LLVM has a C backend already but I don't know how to select it.
>>>> 
>>>> If I can get a proof of concept showing Objective-C code running on the 360 we are off to the races.  Any help is appreciated just not sure if all the pieces/parts exist and/or what I am missing.  So... is this feasible?  If so... how do I get LLVM to output C code?
>>>> 
>>>> Thanks,
>>>> Kevin
>>>> 
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