For loop is not unrolling in chisel

Question

I was trying to understand how we are generating verilog code out of "for" loop in chisel. Generally verilog code used to unroll body as many time as loop progress but here in chisel it's only unrolling it once.

val io = new Bundle {

    val a = UInt(INPUT, 2)

    val output = UInt(OUTPUT, 2)
  }

    io.output := UInt(0)

    for(j <- 0 to 4){

      io.output := io.a
    }

Corresponding verilog code for the above program is :

module LutSimpleALU(

    input [1:0] io_a,
    output[1:0] io_output
);

   assign io_output = io_a;

endmodule

it would be very helpful if someone can tell how for loop is working.

In for block, loop counter variable "j" is not used, so that for loop just executes "io.output := io_a" repeatedly. As the result, verilog extracts the first execution of for block ,i.e., "io.output := io_a" . — Fumu 7
That means i have to use "j" always in order for loop to work? — Vikas Chauhan
You execute same code " io.output := io.a" again and again. Verilog thought that repeated execution is meaningless and execute once is enough. I don't know the way to execute differently in each execution of for block in which the loop counter never be used. — Fumu 7
Can you share a small example in which loop unrolling will work ? — Vikas Chauhan
A part of code "for(j<-0 to 4){ a[j] := b[j]*j } " will be unrolled as "assign a[0] = 0; assing a[1]=b[1]; assign a[2]=b[2]*2; assign a[3]=b[3]*3; a[4]=b[4]*4; ". These five operations are independant each other, so could be executed in parallel. — Fumu 7

Chris Chris · Accepted Answer · 2014-09-30T20:19:14

Your for loop is doing the same thing for each iteration. You aren't using the "j" iterator variable anywhere, so this is what it expands to:

io.output := io.a
io.output := io.a
io.output := io.a 
io.output := io.a

The semantics here is that the last writer wins, so the last statement of "io.output = io.a" would be the final value. In fact, the previous three statements mean nothing, so they would be pruned from the graph.

For loop is not unrolling in chisel

1 Answers