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Comment by cillian64 | Hacker News Reader

Is anyone around who has implemented a substantial project in nMigen? Looking at the syntax it looks very unintuitive and awkward, mainly due to being shoehorned into Python syntax. Do the advantages of being able to metaprogram in Python outweigh the disadvantages of the syntax? (I’m comparing to a hypothetical HDL which is at a similar abstraction level but has a dedicated syntax and some other way of embedding metaprogramming)

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I'm about 20kloc in to a (not public) nMigen project, including a UDP/IP stack, various peripheral drivers, a DSP pipeline, and some custom networking code. So far I think "do the advantages of being able to metaprogram outweigh..."is a resounding "yes", but I came to it with extensive Python experience so it feels like a very natural fit to me, and given how other attempts at embedded metaprogramming go (Tcl...) I'm not sure Python is easily beat. Being able to integrate with numpy for DSP work, pytest for simulation and testing, Python packaging (such as it is) for distributing modules and managing dependencies and versioning, and simulating inside Python too are all additional compelling advantages.

nMigen is a pretty fledgling project so there aren't a ton of big projects in it yet, but for example Luna[0] (from the makers of the HackRF) implements a full USB stack including USB3 support, with enough abstraction that you can create a USB-serial converter inside your FPGA using two I/Os for full-speed USB and wire it into the rest of your project in about ten lines of Python[1].

[0]: https://github.com/greatscottgadgets/luna [1]: https://github.com/greatscottgadgets/luna/blob/master/exampl...

Migen, the Python-based project nMigen is based off, has been around for longer and has some large projects, such as LiteX[2] which uses Migen to glue together entire SoCs, including peripheral cores such as GigE, DDR3/4, SATA, PCIe, etc, all written in Migen, and is pretty widely used. It also pulls in Verilog/VHDL designs (such as many of its CPU core choices) since it's easy to pull in those from the Python side.

[2]: https://github.com/enjoy-digital/litex/

I'd be interested in this too, I'm all for improving on the standard HDLs, but from this example I see more downsides than upsides, mostly due to the fact the synatx looks very verbose. Part of the is because its layered on top of python, so even a simple switch must be written in a more elaborate way. The code shown in the example would be very readable in SystemVerilog and quite a bit shorter. Also how does something like this handle carry for integer addition, sign extension, etc.

Carries are documented. Two n-bit numbers result in an (n+1)-bit number. The “n+1th” bit is obviously the carry of the result. Sign extension is handled by declaring a `Signal` as signed or unsigned during creation.

For example, both:

    x = Signal(4, true)
    y = signed(4)

will create a 4 bit signed number. Use `false` or `unsigned` for a signed. Setting either x or y to a Python integer will handle the sign extension behind the scenes.

I will agree with you on the syntax, however. It’s caused by the fact that you’re not synthesizing your program, but writing code that generates code.

nmigen is a library, not a language. So it has to use what’s available to it. The upside is you don’t have to write tokenizers, parsers, etc, but the downside is it looks “hackish”.

For example:

    m.d.comb += x.eq(y + 1)

...means: in the combinatorial domain (clockless) of the m `Module`, set x equal to y+1. If you come from a VHDL/Verilog background, nmigen’s “syntax” is pretty off putting, but if you come from a programming background (like me), the Python syntax is easier to grok IMO.

Robert Baruch has a nice tutorial on nmigen: https://github.com/RobertBaruch/nmigen-tutorial

Thanks that helps clarify things, also nice to see Robert Baruch's tutorial.

Shameless plug: I had a similar impression when looking at nMigen, but wasn't super happy with Verilog either, so I wrote a new HDL called Wyre [0]. No metaprogramming, but a Verilog-like language with a focus on ergonomics instead. I'm currently making a basic Minecraft clone for the Lattice iCE40 with it.

[0] https://github.com/nickmqb/wyre

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