Remove data dependency in C

Question

I have a code which has data dependency.

code example:

int data[1000*3];
int result[1000]={0,};

// initialize data[]

for(i=0; i<1000; i++)
{
   a = data[i*3 + 0];
   b = data[i*3 + 1];
   c = data[i*3 + 2];

   if( (a>b) && (a>c) )   // This line makes data dependency
      result[i]++;
}

I want to remove data dependency to parallelize this code. I know I have to edit my code, but don't know how to fix it.

How do I have to fix this code?

I don't see a data dependency, but sometimes the compiler gets confused when you do indexing math inside the square brackets. As long as you privatize a, b, and c, this should be parallelizable. Since you tagged this with openacc, I'd suggest using `acc parallel loop` instead of `acc kernels` if the compiler is telling you that it can't parallelize due to a data dependency. — jefflarkin, Oct 06 '15 at 22:01
Actually, I found data dependency from another code like that(used NVVP). But as this sample code, NVVP dosen't tell me that makes data dependency. I think my real code's dependency occured from other reasons. I'll try to found other reason. Thank you for help, jeff. :) — soongk, Oct 08 '15 at 06:13
Two things to look for: 1) If you're accessing multiple arrays, make sure the pointers are declared `restrict` so the compiler knows they're not aliased. 2) Indexing math inside the square brackets. — jefflarkin, Oct 08 '15 at 09:45

score 0 · Answer 1 · answered Oct 06 '15 at 11:01

0

Try:

int data[1000*3];
int result[1000]={0,};

// initialize data[]

for(i=0; i<1000; i++)
{
   if( (data[i*3 + 0] > data[i*3 + 1]) && (data[i*3 + 0] > data[i*3 + 2]) )   
      result[i]++;
}

answered Oct 06 '15 at 11:01

Miner_Glitch

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Remove data dependency in C

1 Answers1