[LLVMdev] Pointer "data direction"

[Sebastian Dreßler]

On 01/09/2013 04:20 PM, Dmitry Mikushin wrote:
> Are you analysing sizes in order to perform host<->accelerator memory
> synchronization?
> 

No. This is done for performance prediction of kernels with rather
complex data structures.

> 2013/1/9 Sebastian Dreßler <dressler at zib.de>
> 
>> Hi Dmitry,
>>
>> On 01/09/2013 03:48 PM, Dmitry Mikushin wrote:
>>> Hi Sebastian,
>>>
>>> This kind of analysis is a pretty complex problem in general case.
>>> Consider, for instance, function "f" has nested calls of other functions
>>> with "side effects", meaning they could potentially change the contents
>> of
>>> "in" or "out" indirectly. For this reason, even current state-of-art
>>> commercial APIs that imply strong data analysis (like OpenACC or HMPP)
>>> require functions to be free of side effects, because nobody could solve
>>> this problem well at compile-time.
>>>
>>
>> The functions I'm going to analyze are not having side effects (sorry
>> for not mentioning). Basically, they are enclosed kernels.
>>
>>> Depending on the purpose of your question, this may or may not help: in
>>> comparison to general analysis, LLVM community makes way better progress
>> in
>>> analysing data access patterns for vectorization and parallelization. In
>>> this case, particular code regions or loops are considered for matching
>>> suitable access patterns. For details on vectorization - you can look
>> into
>>> the work by Nadav Rotem, Hal Finkel et al, the work by Preston Briggs
>> [1],
>>> for details on polyhedral analysis - the Polly project [2]. These two
>> could
>>> be extended further with runtime-assisted data analysis, where knowing
>>> actual values of pointers and index ranges you can also make conclusions
>>> about read/write modes with respect to particular code regions, like in
>> [3].
>>>
>>> [1] https://sites.google.com/site/parallelizationforllvm/
>>> [2] http://polly.llvm.org/
>>> [3] http://kernelgen.org
>>>
>>
>> Thank you. At a glance [3] is really helpful, so I'll have a deeper look
>> onto it. I also know [2] but didn't used it yet, but now I'll "have to" ;)
>>
>> Maybe a few sentences regarding the purpose: the goal is to analyze the
>> sizes of the provided arguments to the function. What I've done so far
>> is, that I'm able to analyze data structures (C, C++) and mallocs at
>> run-time by injected LLVM IR code [4]. Currently I'm further extending
>> it to analyze C++ classes (most things work, include inheritance,
>> templates, also std::vector and std::map). However, my problem is the
>> described one, i.e. how to assign the directions in order to correctly
>> compute data volumes for in and out. One of the first ideas was to use a
>> kind of configuration file, since the kernel developer knows about the
>> data directions. But I think this can be done more elegant. Anyway,
>> based on your answer I'll maybe go back to this idea.
>>
>>
>> Thanks,
>> Sebastian
>>
>>
>> [4]
>>
>> http://opus4.kobv.de/opus4-zib/files/1556/kdv_dressler_steinke_zibreport.pdf
>>
>>> Hope it helps,
>>> - D.
>>>
>>> 2013/1/9 Sebastian Dreßler <dressler at zib.de>
>>>
>>>> Hi,
>>>>
>>>> suppose the following C function declaration:
>>>>
>>>>     void f(int *in, int *out);
>>>>
>>>> Now further suppose, that _in_ is an array only read from and _out_ is
>>>> an array that is only written to.
>>>>
>>>> Based on this, I was wondering whether there is some already existing
>>>> LLVM pass (or maybe a part of a pass) that detects those "data
>>>> directions" for pointers. I'm not quite sure whether e.g. Alias Analysis
>>>> can provide me this information (I suppose it *cannot*).
>>>>
>>>>
>>>> Best regards,
>>>> Sebastian
>>>>
>>>> --
>>>> Mit freundlichen Grüßen / Kind regards
>>>>
>>>> Sebastian Dreßler
>>>>
>>>> Zuse Institute Berlin (ZIB)
>>>> Takustraße 7
>>>> D-14195 Berlin-Dahlem
>>>> Germany
>>>>
>>>> dressler at zib.de
>>>> Phone: +49 30 84185-261
>>>>
>>>> http://www.zib.de/
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>>
>>>
>>
>>
>> --
>> Mit freundlichen Grüßen / Kind regards
>>
>> Sebastian Dreßler
>>
>> Zuse Institute Berlin (ZIB)
>> Takustraße 7
>> D-14195 Berlin-Dahlem
>> Germany
>>
>> dressler at zib.de
>> Phone: +49 30 84185-261
>>
>> http://www.zib.de/
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
> 


-- 
Mit freundlichen Grüßen / Kind regards

Sebastian Dreßler

Zuse Institute Berlin (ZIB)
Takustraße 7
D-14195 Berlin-Dahlem
Germany

dressler at zib.de
Phone: +49 30 84185-261

http://www.zib.de/