[LLVMdev] Pointer "data direction"

[Sebastian Dreßler]

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
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>>
> 


-- 
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/