Debugging via high-level simulation is something my book spends a lot of time on. If you look at the sample chapters, you can see that the same Clash code can also be compiled into software Haskell, which you can then interface with non-synthesizable test benches, such as an interactive SDL frontend. So you can take your HDL logic and run it directly, interactively in real time. You can play Pong by compiling the code as software.
One level lower, you can use Clash's signal-level simulator. Basically it gives you a synchronous stream of signal values, either as a lazy list (for "offline" simulation), or as an automaton that you can turn the crank on by feeding it the next clock cycle's inputs (for "online" simulation, i.e. where you want to do IO to compute the next input from the previous outputs). So at this level, you'd take your Pong circuit and use the automaton interface of the simulator to feed the virtual "pushbutton" states computed from e.g. keypresses, and then consume the output to do the rendering. Or simulate the whole circuit end-to-end and feed its output into a VGA interpreter, which you also get to write in Haskell.
If you need to debug at the Verilog level, you can use Clashilator (https://github.com/gergoerdi/clashilator) to automate FFI-ing into a Verilator-generated simulation.
I've been using Clash for about 2 years now. I just debug the clash code. As in just use the normal Haskell test tooling. The only reason I ever read the generated HDL is when I make sure I have integrated another IP correctly.
qq: i know that for HLS sometimes (most?) the generated HDL is 10x the number of resources (flip-flops?) compared to hand-written HDL. how bad is clash in this respect?
Clash is not HLS. You have full control of register placement and pipelining just like VHDL and Verilog. Clash in that sense is not "higher level" then VHDL or Verilog. In some respects you could even say that Clash is "lower level" because you don't write things just right to be inferred correctly. You actually specify what hardware you want. E.g. you write I want a blockram with this size here, and not if I write this specific Verilog the tools will infer a blockram.
What Clash gives you is the power and tooling of Haskell.
Clash does generate Verilog or VHDL but the only reason it does this is to interface with vendor tooling.
HLS generally means you compile a very high level description of computation to VHDL/Verilog. This high level description doesn't contain hardware details like registers, ram usages, pipelining etc. During the process of HLS the synthesis tool will try to translate this description to a digital circuit. It will itself place registers, rams pipeline as necessary.
That is the reason HLS doesn't reach the performance of VHDL/Verilog, these HLS tools just aren't as good as a human making the digital circuit.
Clash is not itself coming up with a digital circuit like HLS is doing. The developer is specifying the digital circuit. Just like with VHDL or Verilog. It's just an alternative way of writing it.
Looking at your posting history I can be a little more concrete: Chisel is essentially a metaprogramming framework for VHDL/Verilog. Clash is a compiler closely based on GHC that compiles Haskell code, not a DSL defined within Haskell, to VHDL/Verilog.
you're saying seemingly contradictory things (that would best be resolved for me if i just dug into clash, so i will):
>HLS generally means you compile a very high level description of computation to VHDL/Verilog.
...
>Clash is not itself coming up with a digital circuit like HLS is doing. The developer is specifying the digital circuit.
...
>Clash is a compiler closely based on GHC that compiles Haskell code
what does it mean for clash to compile haskell code but not to come up with a digital circuit? haskell code (afaik) doesn't represent combinational or sequential logic. well maybe it does using clash (thanks to haskell crazy metaprogramming faciilities) but then what does it mean to "compile"? verilog isn't compiled, it's still synthesized to gates and luts and whatever right? what does clash compile to natively if not verilog/vhdl (which then gets synthesized)?
It's not contradictory. A Haskell function(modulo io and boundless recursion) gets compiled to combinational logic without registers. a digital circuit circuit in clash are normal Haskell function combined together with registers and other combinators. Clash can compile to an executable as well as to hardware. The executable is a cycle accurate simulation of the circuit.
For example, what is the design and debug flow of using Clash to target a Xilinx FPGA?
My guess is that you're going to tell me I will have to debug the generated Verilog code?