Simulation with Modelsim about forsyde-deep HOT 1 OPEN

Comment by HWoidt
Thursday Jan 21, 2016 at 15:32 GMT

This issue is a bit more involved than initially thought:

There is a simulation mismatch between Haskell and VHDL concerning the Integer types:

• Haskell: Native Int8,Int16,... have traditional wrap-around semantics
• VHDL: Integers will not wrap around (-128 -1 = -129 which is not a valid Integer value)

For this specific example, the bug originates in the fact, that all processes are executed once initially. That way the nextState function is called once with the initial value of the delayOut signal, and not with the one assigned by the reset statement (if reset then delay = 0...).

This nextState function is not formally correct VHDL as it produces an invalid output for the valid (although not normally attainable) input of -128. This specific input value is the default initial value for the Integer type.

There are some options for solving this:

1. Fix the simulation mismatch: Implement the Int8 type using numeric_std.unsigned instead of Integer in VHDL. This has the correct semantics regarding the Haskell-Int8 type and would make this simulate correctly in this context.
2. Fix the initial values of all signals connected to the delay element. This would need to have some way of tracing the netlist and catch all the important signals.
3. fix the example to behave correctly for out-of range values less then 0 (which doesn't solve the underlying issue):

nextStateFun = $(newProcFun
   [d| nextState state dir
          = if dir == H then
              if state < 9 then state + 1
              else 0
            else
              if state <= 0 then 9
              else state - 1
     |])

I currently have no estimate of the amount of work needed for either 1. or 2.

from forsyde-deep.