Dahlia Programmability

• There are cases that some loop is not unrollable but if we split part of it out, we have it.

  • This loop is not unrollable since it involves dynamic indexing of array b.

    for (let r = 0..64){
        a = b[c[r]];
        m[r] = a + c[r];
    }

  • However, it would be fine if we split it into two loops, we might get some kind of speed up since the second part is unrollable now.

    for (let r = 0..64){
        a[r] = b[c[r]];
    }
    view a_v = a[_: bank 8]
    for (let r = 0..64) unroll 8{
        m[r] = a[r] + c[r];
    }

• There are cases that the loop is not satisfying:

  • We can only unroll by 2 in this case:

    for (let r = 0..62)unroll 4{//wrong! Dahlia does not allow that
     ...
    }

  • But we can also increase loop number. But there could be a tradeoff since we introduce more hardware and need to recover original values.

  for (let r = 0..64) unroll 4{
   a[r] := b[r] + c[r];
  }
  for (let r = 63 ..64){
   ...
   //recover a[r] to its original value here
  }

• Trade of between inner loop and outer loop.

  • Examples can be found here - https://github.com/cucapra/fuse-benchmarks-sdaccel/blob/master/experiments/dahlia-dse/machsuite-stencil-stencil2d-inner/stencil.fuse and here - https://github.com/cucapra/fuse-benchmarks-sdaccel/tree/master/experiments/dahlia-dse/machsuite-stencil-stencil2d-outer/stencil.fuse

  • Notice HLS can do the second case automatically (completely unroll inner loops and then partially unroll outer loops) but in Dahlia we need to specify it.