llvm dev - Dec 2012 - [LLVMdev] Predictive Commoning / Scalar Replacement

Hello,

 

It seems there is the no redundancy elimination over loop iterations, such

as predictive communing or scalar replacement in LLVM. They are quite
usefully

for computation code. I am wondering if any party is working or has plan to 

implement those optimizations?

 

Thanks,

 

                                       Yin 

 

 

 

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Yin Ma wrote:
> Hello,
>
> It seems there is the no redundancy elimination over loop iterations, such
>
> as predictive communing or scalar replacement in LLVM. They are quite
> usefully
>
> for computation code. I am wondering if any party is working or has plan to
>
> implement those optimizations?
I don't know of any, but I was wondering if you could point me to the 
paper that describes predictive commoning? I could only find the 
second-order predictive commoning paper, which if I understand correctly 
is a much newer and different algorithm.

Nick

On 12/1/2012 7:10 PM, Nick Lewycky wrote:
>
> I don't know of any, but I was wondering if you could point me to the
> paper that describes predictive commoning? I could only find the
> second-order predictive commoning paper, which if I understand correctly
> is a much newer and different algorithm.
I think the original paper was some internal IBM publication.

The idea is basically to reuse values loaded in previous loop iterations.

For example
   for (i) {
     x += 2*t[i-1] - t[i+1];
   }
would become
   for (i) {
     tm1 = tz;     // t-minus-1 = t-zero
     tz  = tp1;    // t-zero    = t-plus-1
     tp1 = t[i+1]; // t-plus-1  = load
     x += 2*tm1 - tp1;
   }

-Krzysztof


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