Add NoChiselNamePrefix to ignore instances in @chiselName (#1383)

Add trait chisel3.experimental.NoChiselNamePrefix which causes
@chiselName to skip naming of the instance effectively preventing it
from prefixing any vals inside the instance. It can be applied to
classes such that all instances of that class have this property, or to
individual instances (via creating an anonymous class inline).

Also add basic ScalaDoc for NoChiselNamePrefix and chiselName.

chiselFrontend/src/main/scala/chisel3/experimental/package.scala

@@ -71,7 +71,63 @@ package object experimental {  // scalastyle:ignore object.name
 
   class dump extends chisel3.internal.naming.dump  // scalastyle:ignore class.name
   class treedump extends chisel3.internal.naming.treedump  // scalastyle:ignore class.name
+  /** Experimental macro for naming Chisel hardware values
+    *
+    * By default, Chisel uses reflection for naming which only works for public fields of `Bundle`
+    * and `Module` classes. Applying this macro annotation to a `class` or `object` enables Chisel
+    * to name any hardware values within the annotated `class` or `object.
+    *
+    * @example {{{
+    * import chisel3._
+    * import chisel3.experimental.chiselName
+    *
+    * @chiselName
+    * class MyModule extends Module {
+    *   val io = IO(new Bundle {
+    *     val in = Input(UInt(8.W))
+    *     val out = Output(UInt(8.W))
+    *   })
+    *   def createReg(): Unit = {
+    *     // @chiselName allows Chisel to name this Reg
+    *     val myReg = RegInit(io.in)
+    *     io.out := myReg
+    *   }
+    *   createReg()
+    * }
+    * }}}
+    */
   class chiselName extends chisel3.internal.naming.chiselName  // scalastyle:ignore class.name
+  /** Do not name instances of this type in [[chiselName]]
+    *
+    * By default, `chiselName` will include `val` names of instances of annotated classes as a
+    * prefix in final naming. Mixing in this trait to a `class`, `object`, or anonymous `class`
+    * instances will exclude the `val` name from `chiselName` naming.
+    *
+    * @example {{{
+    * import chisel3._
+    * import chisel3.experimental.{chiselName, NoChiselNamePrefix}
+    *
+    * // Note that this is not a Module
+    * @chiselName
+    * class Counter(w: Int) {
+    *   val myReg = RegInit(0.U(w.W))
+    *   myReg := myReg + 1.U
+    * }
+    *
+    * @chiselName
+    * class MyModule extends Module {
+    *   val io = IO(new Bundle {
+    *     val out = Output(UInt(8.W))
+    *   })
+    *   // Name of myReg will be "counter0_myReg"
+    *   val counter0 = new Counter(8)
+    *   // Name of myReg will be "myReg"
+    *   val counter1 = new Counter(8) with NoChiselNamePrefix
+    *   io.out := counter0.myReg + counter1.myReg
+    * }
+    * }}}
+    */
+  trait NoChiselNamePrefix
 
   object BundleLiterals {
     implicit class AddBundleLiteralConstructor[T <: Bundle](x: T) {

chiselFrontend/src/main/scala/chisel3/internal/Namer.scala

@@ -3,6 +3,7 @@
 // This file contains part of the implementation of the naming static annotation system.
 
 package chisel3.internal.naming
+import chisel3.experimental.NoChiselNamePrefix
 
 import scala.collection.mutable.Stack
 import scala.collection.mutable.ListBuffer
@@ -88,6 +89,7 @@ class NamingContext extends NamingContextInterface {
   def name[T](obj: T, name: String): T = {
     assert(!closed, "Can't name elements after name_prefix called")
     obj match {
+      case _: NoChiselNamePrefix => // Don't name things with NoChiselNamePrefix
       case ref: AnyRef => items += ((ref, name))
       case _ =>
     }

src/test/scala/chiselTests/NamingAnnotationTest.scala

@@ -203,6 +203,32 @@ class PartialNamedModule extends NamedModuleTester {
   val test = innerNamedFunction()
 }
 
+@chiselName
+class NoChiselNamePrefixTester extends NamedModuleTester {
+  @chiselName
+  class NoChiselNamePrefixClass extends chisel3.experimental.NoChiselNamePrefix {
+    val a = expectName(1.U +& 2.U, "a")
+  }
+  val inst = new NoChiselNamePrefixClass
+  @chiselName
+  class NormalClass {
+    val b = 1.U +& 2.U
+  }
+  val foo = new NormalClass
+  expectName(foo.b, "foo_b")
+  val bar = new NormalClass with chisel3.experimental.NoChiselNamePrefix
+  expectName(bar.b, "b")
+
+  // Check that we're not matching by name but actual type
+  trait NoChiselNamePrefix
+  @chiselName
+  class FakeNoChiselNamePrefix extends NoChiselNamePrefix {
+    val c = 1.U +& 2.U
+  }
+  val fizz = new FakeNoChiselNamePrefix
+  expectName(fizz.c, "fizz_c")
+}
+
 
 /** A simple test that checks the recursive function val naming annotation both compiles and
   * generates the expected names.
@@ -240,4 +266,10 @@ class NamingAnnotationSpec extends ChiselPropSpec {
   property("NonBuilderFunction should run outside a Builder context") {
     NonNamedHelper.NonBuilderFunction() should be (2)
   }
+
+  property("NoChiselNamePrefix should prevent prefixing when using @chiselName") {
+    var module: NoChiselNamePrefixTester = null
+    elaborate { module = new NoChiselNamePrefixTester; module }
+    assert(module.getNameFailures().isEmpty)
+  }
 }