Link PlusArg to emulator command line options

- adds a mutable singleton (PlusArgArtefacts) to store information
  about Rocket PlusArgs
- adds methods to PlusArgArtefacts to emit C snippets that are
  consumed by emulator.cc for correct argument parsing and help text
  generation
- emits snippets in $(CONFIG).plusArgs via BaseCoreplex-set
  ElaborationArtefacts
- modify emulator/Makefrag-verilator to include $(CONFIG).plusArgs
- cleanup help text (docstring) for existing PlusArgs

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@gmail.com>

src/main/scala/util/PlusArg.scala

@@ -4,6 +4,8 @@ package freechips.rocketchip.util
 
 import Chisel._
 
+case class PlusArgInfo(default: Int, docstring: String)
+
 class plusarg_reader(val format: String, val default: Int, val docstring: String) extends BlackBox(Map(
     "FORMAT"  -> chisel3.core.StringParam(format),
     "DEFAULT" -> chisel3.core.IntParam(default))) {
@@ -20,6 +22,42 @@ object PlusArg
   // Add a docstring to document the arg, which can be dumped in an elaboration
   // pass.
 
-  def apply(name: String, default: Int = 0, docstring: String = ""): UInt =
+  def apply(name: String, default: Int = 0, docstring: String = ""): UInt = {
+    PlusArgArtefacts.append(name, default, docstring)
     Module(new plusarg_reader(name + "=%d", default, docstring)).io.out
+  }
+}
+
+object PlusArgArtefacts {
+  private var artefacts: Map[String, PlusArgInfo] = Map.empty
+
+  /* Add a new PlusArg */
+  def append(name: String, default: Int, docstring: String): Unit =
+    artefacts = artefacts ++ Map(name -> PlusArgInfo(default, docstring))
+
+  /* From plus args, generate help text */
+  private def serializeHelp(tab: String = ""): String = artefacts
+    .map{ case(arg, PlusArgInfo(default, docstring)) =>
+      s"""|$tab+$arg=INT\\n\\
+          |$tab${" "*20}$docstring\\n\\
+          |$tab${" "*22}(default=$default)""".stripMargin }.toSeq
+    .mkString("\\n\\\n") ++ "\""
+
+  /* From plus args, generate a char array of their names */
+  private def serializeArray(tab: String = ""): String = {
+    val prettyTab = tab + " " * 44 // Length of 'static const ...'
+    s"${tab}static const char * verilog_plusargs [] = {\\\n" ++
+      artefacts
+        .map{ case(arg, _) => s"""$prettyTab"$arg",\\\n""" }
+        .mkString("")++
+    s"${prettyTab}0};"
+  }
+
+  /* Generate C code to be included in emulator.cc that helps with
+   * argument parsing based on available Verilog PlusArgs */
+  def serialize(): String =
+    s"""|#define PLUSARG_USAGE_OPTIONS \"EMULATOR VERILOG PLUSARGS\\n\\
+        |${serializeHelp(" "*7)}
+        |${serializeArray()}
+        |""".stripMargin
 }