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Mickleman's avatar
Mickleman
Icon for Occasional Contributor rankOccasional Contributor
4 years ago

Achieving parallel execution of loop on FPGA

I am having difficulty persuading the compiler to execute an outer loop in parallel in an FPGA kernel. I have constructed a simple example to illustrate the issue. Here is a simple array initialisa...
MGRAV's avatar
MGRAV
Icon for New Contributor rankNew Contributor
4 years ago

Hi,

Did you try with "#pragma unroll 2" ?

I am working on something similar and I would say that something like that should almost work

const int ITEM_LENGTH = 1000;
const int GROUP_SIZE = 10;
uint16_t a[2][GROUP_SIZE][ITEM_LENGTH];

#pragma unroll 2
[[intel::ivdep]]
for (int block = 0; block < 2; block++)

  [[intel::ivdep]]
  for (int j = 0; j < GROUP_SIZE; j++)
    for (int i = 0; i < ITEM_LENGTH; i++)
      if ( i == 0 )
        a[block][j][i] = j;
      else
        a[block][j][i] = a[block][j][i-1] + i;

good luck

HRZ's avatar
HRZ
Icon for Frequent Contributor rankFrequent Contributor
4 years ago

Solution mentioned by @MGRAV is the right way to run loop iterations in "parallel". However, the difference you are experiencing here is likely caused by the resource used to implement i[], rather than the loop-carried dependency. If i[] is implemented using registers (which can be quite inefficient in case ITEM_LENGTH is too large), then you should achieve an II of 1 in all of the cases you mentioned above. You can force implementation using registers using the respective pragma mentioned in Intel's documentation and see what happens. If, however, i[] is implemented using Block RAMs, then you will end up with a load/store dependency since it is not possible to perform single-cycle read/write from/to Block RAMs and that is where the II=6 comes from (latency of one Block RAM read + write). There is nothing you can do to improve the II in this case, since your the load/store dependency is real and any attempt to force the compiler to ignore it using ivdep will result in incorrect output.

P.S. You should probably move "if ( i == 0 ) a[i] = j; " outside of the "i" loop and start "i" from 1. This will allow you to completely eliminate the branch from the code and save some area.