[LLVMdev] Generating target dependent function calls

[Tobias Grosser]

On 12/26/2010 01:31 AM, Eric Christopher wrote:
>>>>
>>>>
>>>> The reason for the difference is that e.g "long" in
>>>>
>>>>> bool GOMP_loop_runtime_next(long, long)
>>>>
>>>> has a different size on different architectures.
>>>>
>>>> Currently we generate the prototypes and functions ourselves:
>>>>> declare i8 @GOMP_loop_runtime_next(i64*, i64*) nounwind
>>>>
>>>> To support a 32bit architecture we would need to generate:
>>>>> declare i8 @GOMP_loop_runtime_next(i32*, i32*) nounwind
>>>>
>>>> Has anybody an idea what's a conceptually clean way to generate OpenMP
>>>> function calls for different architectures and best even for different
>>>> OpenMP implementations (e.g. mpc.sf.net)
>>>>
>>>> Would overloaded intrinsics be a possible approach? How could we
>>>> model/derive the right signature for our target architecture. TargetData
>>>> does not seem to be enough for this. Is there a better approach than
>>>> passing this information using a command line switch?
>>>>
>>>> Cheers and thanks for your help
>>>
>>> Hi Tobias,
>>>
>>> I'm facing a similar problem generating CUDA runtime code for
>>> different architectures.  One solution I'm currently considering
>>> is to use the TargetInfo class from Clang's Basic library which can
>>> be used to obtain target specific parameters (such as sizeof(long))
>>> given a target triple.
>>
>> Interesting. Maybe I can get enough information from the target triple
>> itself. Module::&getTargetTriple should be enough. I did not think about
>> this, but that might work. Thanks for the pointer.
>
> You could also just use a small function to define the API function you
> wish to use and then use TargetData to get some of the final parts:
>
> i.e. if you know you've got something like
>
> char GOMP_foo(pointer, pointer)
>
> you could declare in a function that would then inquire pointer size
> from TargetData like:
>
> FunctionType *geti8LongLong() {
>    const PointerType *LongTy = Type::getIntNPtrTy(Context, TD.getPointerSizeInBits());
>    std::vector<const Type*>  Params = { LongTy, LongTy };
>    const Type *ResultTy = Type::getInt8Ty(Context);
>    return FunctionType::get(ResultTy, ArgTys, 0);
> }
>
> and then just use the return from geti8LongLong() as the function type when you declare the function. Or you can abstract out away the LongTy and use it in general.

Hey Eric,

thanks for this hint. This seems to be a reasonable approach.

However, is TD.getPointerSizeInBits() always equivalent to the number of 
bits a long has on a platform? It might be correct on Linux, however I 
do not think this is true on Windows.

I believe together with Peter's ideas I can create something that will 
be correct on Linux and that will warn if used on any other platform.

Thanks for your help
Tobi