Chisel, Generate Blocks and Large Intermediate/Output Files

Question

Is there a construct in Chisel to generate Verilog generate blocks instead of unrolling Scala for-loops into very large (>100k line) output Verilog and FIRRTL files.

For example, I have the following code, that constructs a 2D lattice of MatrixElement modules and connects their inputs and outputs.

private val mat_elems = Seq.tabulate(rows, cols) { (i, j) => {
    Module(new MatrixElement(n=i, m=j))
}}

for (i <- 0 until rows; j <- 0 until cols) {
    // Wavefront propagation
    if (i == 0 && j != 0) {
      // First row
      mat_elems(i)(j).io.in <> (false.B, false.B, mat_elems(i)(j - 1).io.out)
    } else if (i != 0 && j == 0) {
      // First col
      mat_elems(i)(j).io.in <> (false.B, mat_elems(i - 1)(j).io.out, false.B)
    } else if (i >= 1 && j >= 1) {
      // Internal matrix
      mat_elems(i)(j).io.in <> (mat_elems(i - 1)(j - 1).io.out, mat_elems(i - 1)(j).io.out,
        mat_elems(i)(j - 1).io.out)
    }
  }

I am looking to compile this code for values of rows and cols >= 256. So this matrix gets very large in size.

If I were writing this as Verilog module, I would make use of generate blocks. However, in Chisel, since I am using the Scala loops, the entire lattice/matrix gets unrolled in the FIRRTL/Verilog outputs. Often producing >100k lines with all the _T* wires for 512x512 lattices. This causes a whole bunch of JVM out of memory errors in the Chisel compilation and makes the VCS simulation (just parsing the files takes forever) of the output files VERY slow.

Is there any way around this? Maybe get Chisel to generate Verilog generate blocks?

Jack Koenig Jack Koenig · Accepted Answer · 2018-03-27T23:26:26

There is no support in Chisel nor FIRRTL for compressing this. Such a feature would probably be pretty useful but we have no plan or timeline for it. You can always use a blackbox and write the Verilog to do it yourself if you find the compilation time saved to be worth it.

Chisel, Generate Blocks and Large Intermediate/Output Files

1 Answers