llvm dev - Mar 2012 - [LLVMdev] Problem with LoopDependenceAnalysis

Hi,
I am using LLVM for implementing LoopFission pass.
I am using LoopPass.
I know that for checking circular dependency in loop I have to use
LoopDependenceAnalysis

This is what i want to do.
        for(int i = 0; i< n ; i++){

s1 :                 a[i] = a[i] + x[i];
s2 :                 x[i] = x[i+1] + i*2 ;
        }

/**there is no dependence from s2 to s1/
so after distribution(it should be) :

        for(int i = 0; i< n ; i++)
s1:                a[i] = a[i] + x[i];

        for(int i = 0; i< n ; i++)
s2:                x[i] = x[i+1] + i*2 ;


but in llvm i couldn't able to find there is no dependency from s2 to s1.

LoopDependenceAnalyis always gives there is a dependency from every
load instructions to every store instructions.


is there any other alternative to LoopDependencyAnalysis ?
thank you

......
Regards,
Shanmukha Rao
Compilers lab,
Indian Institute of Science, Bangalore.
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20120315/94716630/attachment.html>

Hi, did you do this on optimized IR?  These kind of analyses only do a decent
job if at least a basic set of optimizations has been applied.

Ciao, Duncan.
> I am using LLVM for implementing LoopFission pass.
> I am using LoopPass.
> I know that for checking circular dependency in loop I have to use
> LoopDependenceAnalysis
>
> This is what i want to do.
>          for(int i = 0; i< n ; i++){
>
> s1 :                 a[i] = a[i] + x[i];
> s2 :                 x[i] = x[i+1] + i*2 ;
>          }
>
> /**there is no dependence from s2 to s1/
> so after distribution(it should be) :
>
>          for(int i = 0; i<  n ; i++)
> s1:                a[i] = a[i] + x[i];
>
>          for(int i = 0; i<  n ; i++)
> s2:                x[i] = x[i+1] + i*2 ;
>
>
> but in llvm i couldn't able to find there is no dependency from s2 to
s1.
>
> LoopDependenceAnalyis always gives there is a dependency from every load
instructions to every store instructions.
>
>
> is there any other alternative to LoopDependencyAnalysis ?
> thank you
>
> ......
> Regards,
> Shanmukha Rao
> Compilers lab,
> Indian Institute of Science, Bangalore.
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

thank you all for ur replies..

I looked at the sanjoys patch for SIV Test. And i figured out that this is
exactly what i need.
as the comments said, check if subscript A can possibly have the same value
as B in *analyseSIV(A,B)*
but i didn't get How to use this information ?

lets just say in the above program
When i use *depends* function it shows the dependency from load of x to
store in x. similarily for array a.
but how can i be sure that there is no dependence from store of x to load
of x in the next iteration ?

On Thu, Mar 15, 2012 at 11:28 AM, shanmuk rao <shanmuk.rao008 at
gmail.com>wrote:
> Hi,
> I am using LLVM for implementing LoopFission pass.
> I am using LoopPass.
> I know that for checking circular dependency in loop I have to use
> LoopDependenceAnalysis
>
> This is what i want to do.
>         for(int i = 0; i< n ; i++){
>
> s1 :                 a[i] = a[i] + x[i];
> s2 :                 x[i] = x[i+1] + i*2 ;
>         }
>
> /**there is no dependence from s2 to s1/
> so after distribution(it should be) :
>
>         for(int i = 0; i< n ; i++)
> s1:                a[i] = a[i] + x[i];
>
>         for(int i = 0; i< n ; i++)
> s2:                x[i] = x[i+1] + i*2 ;
>
>
> but in llvm i couldn't able to find there is no dependency from s2 to
s1.
>
> LoopDependenceAnalyis always gives there is a dependency from every load
instructions to every store instructions.
>
>
> is there any other alternative to LoopDependencyAnalysis ?
> thank you
>
> ......
> Regards,
> Shanmukha Rao
> Compilers lab,
> Indian Institute of Science, Bangalore.
>


-- 


......
Regards,
Shanmukha Rao
Compilers lab,
Indian Institute of Science, Bangalore.
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20120320/c66d5c41/attachment.html>