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2337 :: Nim for embedded development, a few questions

• Bacterius (orginal) [2016-06-24T16:52:03+02:00] view original

  Hello everyone,

  Been playing around with the ESP8266 microcontroller lately and a couple days ago I decided to give Nim a try since it looks awesome and seems to be such a good match for embedded programming. I got it all mostly working (it's running Nim code right now!) however I have a couple questions:

  1. small one, when I pass --noMain it still seems to be generating a stub with cmdCount, cmdLine, gEnv and all. this isn't an issue since they get eliminated by the compiler, but it still feels like the intent of --noMain was to specifically not emit these and/or treat main as a normal function, or have I misunderstood? I am using --os:standalone.

  2. in embedded programming it is often important to select in which linker section functions get placed, and there doesn't seem to be a convenient way to do this in Nim, the only way I found was with codegenDecl by doing e.g. this:

  codegenDecl: "$# __attribute__((section(\".irom0.text\"))) $#$#"
  Run

  which is atrociously verbose (and GCC/Clang only I suppose). Is there a better way to do this, and if not, is there any chance of a section: "mysection" pragma coming to Nim? From what I see it seems a lot of the pragmas are meant as a 'bag of tricks' to help with various specific scenarios so this wouldn't be out of place I would say and seems (to me) essential for a systems language.

  I will probably have more questions in the future but these will be for a later thread. I look forward to your replies and to working with Nim.

  Thanks

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• jester (orginal) [2016-06-24T17:18:06+02:00] view original

  I think you can use the pragma pragma to simplify this case. Someone can probably improve my answer and create a pragma which takes an argument for the section instead of hard-coding it.

  {.pragma:irom, codegenDecl: "$# $# __attribute__((section (\".rom0.text\")))".}
  
  var a {.irom.} = 2
  
  Run

  My $#'s may be in the wrong spots.

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• Bacterius (orginal) [2016-06-25T02:03:48+02:00] view original

  Wow, I'm not sure how I missed the pragma pragma, seems like exactly what I need (besides the fact that it can't be imported by modules, which is a bit of a nuisance, but I assume it works through include statements). The way to pass arguments to it seems undocumented though, but I will keep searching and reading the compiler source code to figure it out :)

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• Bacterius (orginal) [2016-06-25T17:51:33+02:00] view original

  Figured out how to add a parameter if anyone is interested, it doesn't use a pragma pragma (couldn't figure out how to do it with a pragma pragma) but a macro:

  macro section(param: string, body: untyped): stmt =
      let sectionName = strVal(if param.kind == nnkSym: param.symbol.getImpl() else: param)
      let codegenAttribute = "$# __attribute__((section(\"" & sectionName & "\"))) $#$#"
      
      let pragma = body.findChild(it.kind == nnkPragma)
      
      pragma.add(newNimNode(nnkExprColonExpr).add(newIdentNode("codegenDecl"),
                                                  newStrLitNode(codegenAttribute)))
      
      return body
  Run

  This macro supports either a literal string:

  {. section: ".irom0.text" .}
  Run

  or a constant containing a string:

  const ROM = ".irom0.text"
  
  # ...
  
  {. section: ROM .}
  Run

  I suppose there is a way to generalise it to make it work with any compile-time expression that evaluates to a string too :)

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