Skip to main content

Hierarchy Cookbook

How do I instantiate multiple instances with the same module parameterization?​

Prior to this package, Chisel users relied on deduplication in a FIRRTL compiler to combine structurally equivalent modules into one module (aka "deduplication"). This package introduces the following new APIs to enable multiply-instantiated modules directly in Chisel.

Definition(...) enables elaborating a module, but does not actually instantiate that module. Instead, it returns a Definition class which represents that module's definition.

Instance(...) takes a Definition and instantiates it, returning an Instance object.

Instantiate(...) provides an API similar to Module(...), except it uses Definition and Instance to only elaborate modules once for a given set of parameters. It returns an Instance object.

Modules (classes or traits) which will be used with the Definition/Instance api should be marked with the @instantiable annotation at the class/trait definition.

To make a Module's members variables accessible from an Instance object, they must be annotated with the @public annotation. Note that this is only accessible from a Scala sense—this is not in and of itself a mechanism for cross-module references.

Using Definition and Instance​

In the following example, use Definition, Instance, @instantiable and @public to create multiple instances of one specific parameterization of a module, AddOne.

import chisel3._
import chisel3.experimental.hierarchy.{Definition, Instance, instantiable, public}

@instantiable
class AddOne(width: Int) extends Module {
  @public val in  = IO(Input(UInt(width.W)))
  @public val out = IO(Output(UInt(width.W)))
  out := in + 1.U
}

class AddTwo(width: Int) extends Module {
  val in  = IO(Input(UInt(width.W)))
  val out = IO(Output(UInt(width.W)))
  val addOneDef = Definition(new AddOne(width))
  val i0 = Instance(addOneDef)
  val i1 = Instance(addOneDef)
  i0.in := in
  i1.in := i0.out
  out   := i1.out
}
// Generated by CIRCT firtool-1.96.0
module AddOne(
  input  [9:0] in,
  output [9:0] out
);

  assign out = in + 10'h1;
endmodule

module AddTwo(
  input        clock,
               reset,
  input  [9:0] in,
  output [9:0] out
);

  wire [9:0] _i0_out;
  AddOne i0 (
    .in  (in),
    .out (_i0_out)
  );
  AddOne i1 (
    .in  (_i0_out),
    .out (out)
  );
endmodule

Using Instantiate​

Similar to the above, the following example uses Instantiate to create multiple instances of AddOne.

import chisel3.experimental.hierarchy.Instantiate

class AddTwoInstantiate(width: Int) extends Module {
  val in  = IO(Input(UInt(width.W)))
  val out = IO(Output(UInt(width.W)))
  val i0 = Instantiate(new AddOne(width))
  val i1 = Instantiate(new AddOne(width))
  i0.in := in
  i1.in := i0.out
  out   := i1.out
}
// Generated by CIRCT firtool-1.96.0
module AddOne(
  input  [15:0] in,
  output [15:0] out
);

  assign out = in + 16'h1;
endmodule

module AddTwoInstantiate(
  input         clock,
                reset,
  input  [15:0] in,
  output [15:0] out
);

  wire [15:0] _i0_out;
  AddOne i0 (
    .in  (in),
    .out (_i0_out)
  );
  AddOne i1 (
    .in  (_i0_out),
    .out (out)
  );
endmodule

How do I access internal fields of an instance?​

You can mark internal members of a Module class or trait marked with @instantiable with the @public annotation. The requirements are that the field is publicly accessible, is a val or lazy val, and must have an implementation of Lookupable.

Types that are supported by default are:

  1. Data
  2. BaseModule
  3. MemBase
  4. IsLookupable
  5. Iterable/Option/Either containing a type that meets these requirements
  6. Basic type like String, Int, BigInt, Unit, etc.

To mark a superclass's member as @public, use the following pattern (shown with val clock).

import chisel3._
import chisel3.experimental.hierarchy.{instantiable, public}

@instantiable
class MyModule extends Module {
  @public val clock = clock
}

You'll get the following error message for improperly marking something as @public:

import chisel3._
import chisel3.experimental.hierarchy.{instantiable, public}

object NotValidType

@instantiable
class MyModule extends Module {
  @public val x = NotValidType
}
// error: @public is only legal within a class or trait marked @instantiable, and only on vals of types that have a Lookupable implementation. Chisel types like Data, BaseModule, and MemBase are supported, as are common Scala types like String, Int, Boolean, Iterable, Option, Either, and Tuples. Please implement Lookupable for MdocApp1.this.NotValidType.type.
//   @public val x = ParameterizedUserDefinedType("foo", wire)
//               ^

How do I make my fields accessible from an instance?​

If an instance's fields are simple (e.g. Int, String etc.) they can be marked directly with @public.

Often, fields are more complicated (e.g. a user-defined case class). If a case class is only made up of simple types (i.e. it does not contain any Data, BaseModules, memories, or Instances), it can extend the IsLookupable trait. This indicates to Chisel that instances of the IsLookupable class may be accessed from within instances. (If the class does contain things like Data or modules, the section below.)

However, ensure that these parameters are true for all instances of a definition. For example, if our parameters contained an id field which was instance-specific but defaulted to zero, then the definition's id would be returned for all instances. This change in behavior could lead to bugs if other code presumed the id field was correct.

Thus, it is important that when converting normal modules to use this package, you are careful about what you mark as IsLookupable.

In the following example, we added the trait IsLookupable to allow the member to be marked @public.

import chisel3._
import chisel3.experimental.hierarchy.{Definition, Instance, instantiable, IsLookupable, public}

case class MyCaseClass(width: Int) extends IsLookupable

@instantiable
class MyModule extends Module {
  @public val x = MyCaseClass(10)
}

class Top extends Module {
  val inst = Instance(Definition(new MyModule))
  println(s"Width is ${inst.x.width}")
}
Width is 10
Circuit(Top,List(DefModule(repl.MdocSession$MdocApp5$MyModule@6d54818f,MyModule,false,List(),List(Port(MyModule.clock: IO[Clock],Input,SourceLine(hierarchy.md,105,2)), Port(MyModule.reset: IO[Reset],Input,SourceLine(hierarchy.md,105,2))),chisel3.internal.firrtl.ir$Block@3358452e), DefModule(repl.MdocSession$MdocApp5$Top@183100c1,Top,true,List(),List(Port(Top.clock: IO[Clock],Input,SourceLine(hierarchy.md,111,7)), Port(Top.reset: IO[Bool],Input,SourceLine(hierarchy.md,111,7))),chisel3.internal.firrtl.ir$Block@3b35ece1)),List(),firrtl.renamemap.package$MutableRenameMap@72245af1,List(),List(),List(Layer(UnlocatableSourceInfo,Verification,Extract(Some(verification)),List(Layer(UnlocatableSourceInfo,Assert,Extract(Some(verification/assert)),List()), Layer(UnlocatableSourceInfo,Assume,Extract(Some(verification/assume)),List()), Layer(UnlocatableSourceInfo,Cover,Extract(Some(verification/cover)),List())))),List())

How do I make case classes containing Data or Modules accessible from an instance?​

For case classes containing Data, BaseModule, MemBase or Instance types, you can provide an implementation of the Lookupable typeclass.

Note that Lookupable for Modules is deprecated, please cast to Instance instead (with .toInstance).

Consider the following case class:

import chisel3._
import chisel3.experimental.hierarchy.{Definition, Instance, instantiable, public}

@instantiable
class MyModule extends Module {
  @public val wire = Wire(UInt(8.W))
}
case class UserDefinedType(name: String, data: UInt, inst: Instance[MyModule])

By default, instances of UserDefinedType will not be accessible from instances:

@instantiable
class HasUserDefinedType extends Module {
  val inst = Module(new MyModule)
  val wire = Wire(UInt(8.W))
  @public val x = UserDefinedType("foo", wire, inst.toInstance)
}
// error: @public is only legal within a class or trait marked @instantiable, and only on vals of types that have a Lookupable implementation. Chisel types like Data, BaseModule, and MemBase are supported, as are common Scala types like String, Int, Boolean, Iterable, Option, Either, and Tuples. Please implement Lookupable for MdocApp3.this.UserDefinedType.
//   @public val x = UserDefinedType("foo", wire, inst.toInstance)
//               ^

We can implement the Lookupable type class for UserDefinedType in order to make it accessible. This involves defining an implicit val in the companion object for UserDefinedType. Because UserDefinedType has three fields, we use the Lookupable.product3 factory. It takes 4 type parameters: the type of the case class, and the types of each of its fields.

If any fields are BaseModules, you must change them to be Instance[_] in order to define the Lookupable typeclass.

For more information about typeclasses, see the DataView section on Type Classes.

import chisel3.experimental.hierarchy.Lookupable
object UserDefinedType {
  // Use Lookupable.Simple type alias as return type.
  implicit val lookupable: Lookupable.Simple[UserDefinedType] =
    Lookupable.product3[UserDefinedType, String, UInt, Instance[MyModule]](
      // Provide the recipe for converting the UserDefinedType to a Tuple.
      x => (x.name, x.data, x.inst),
      // Provide the recipe for converting a Tuple to a user defined type.
      // For case classes, you can use the built-in factory method.
      UserDefinedType.apply
    )
}

Now, we can access instances of UserDefinedType from instances:

@instantiable
class HasUserDefinedType extends Module {
  val inst = Module(new MyModule)
  val wire = Wire(UInt(8.W))
  @public val x = UserDefinedType("foo", wire, inst.toInstance)
}
class Top extends Module {
  val inst = Instance(Definition(new HasUserDefinedType))
  println(s"Name is: ${inst.x.name}")
}

How do I make type parameterized case classes accessible from an instance?​

Consider the following type-parameterized case class:

import chisel3._
import chisel3.experimental.hierarchy.{Definition, Instance, instantiable, public}

case class ParameterizedUserDefinedType[A, T <: Data](value: A, data: T)

Similarly to HasUserDefinedType we need to define an implicit to provide the Lookupable typeclass. Unlike the simpler example above, however, we use an implicit def to handle the type parameters:

import chisel3.experimental.hierarchy.Lookupable
object ParameterizedUserDefinedType {
  // Type class materialization is recursive, so both A and T must have Lookupable instances.
  // We required this for A via the context bound `: Lookupable`.
  // Data is a Chisel built-in so is known to have a Lookupable instance.
  implicit def lookupable[A : Lookupable, T <: Data]: Lookupable.Simple[ParameterizedUserDefinedType[A, T]] =
    Lookupable.product2[ParameterizedUserDefinedType[A, T], A, T](
      x => (x.value, x.data),
      ParameterizedUserDefinedType.apply
    )
}

Now, we can access instances of ParameterizedUserDefinedType from instances:

class ChildModule extends Module {
  @public val wire = Wire(UInt(8.W))
}
@instantiable
class HasUserDefinedType extends Module {
  val wire = Wire(UInt(8.W))
  @public val x = ParameterizedUserDefinedType("foo", wire)
  @public val y = ParameterizedUserDefinedType(List(1, 2, 3), wire)
}
class Top extends Module {
  val inst = Instance(Definition(new HasUserDefinedType))
  println(s"x.value is: ${inst.x.value}")
  println(s"y.value.head is: ${inst.y.value.head}")
}

How do I make case classes with lots of fields accessible from an instance?​

Lookupable provides factories for product1 to product5. If your class has more than 5 fields, you can use nested tuples as "pseduo-fields" in the mapping.

case class LotsOfFields(a: Data, b: Data, c: Data, d: Data, e: Data, f: Data)
object LotsOfFields {
  implicit val lookupable: Lookupable.Simple[LotsOfFields] =
    Lookupable.product5[LotsOfFields, Data, Data, Data, Data, (Data, Data)](
      x => (x.a, x.b, x.c, x.d, (x.e, x.f)),
      // Cannot use factory method directly this time since we have to unpack the tuple.
      { case (a, b, c, d, (e, f)) => LotsOfFields(a, b, c, d, e, f) },
    )
}

How do I look up fields from a Definition, if I don't want to instantiate it?​

Just like Instances, Definition's also contain accessors for @public members. As such, you can directly access them:

import chisel3._
import chisel3.experimental.hierarchy.{Definition, instantiable, public}

@instantiable
class AddOne(val width: Int) extends RawModule {
  @public val width = width
  @public val in  = IO(Input(UInt(width.W)))
  @public val out = IO(Output(UInt(width.W)))
  out := in + 1.U
}

class Top extends Module {
  val definition = Definition(new AddOne(10))
  println(s"Width is: ${definition.width}")
}
// Generated by CIRCT firtool-1.96.0
module Top(
  input clock,
        reset
);

endmodule

How do I parameterize a module by its children instances?​

Prior to the introduction of this package, a parent module would have to pass all necessary parameters when instantiating a child module. This had the unfortunate consequence of requiring a parent's parameters to always contain the child's parameters, which was an unnecessary coupling which lead to some anti-patterns.

Now, a parent can take a child Definition as an argument, and instantiate it directly. In addition, it can analyze the parameters used in the definition to parameterize itself. In a sense, now the child can actually parameterize the parent.

In the following example, we create a definition of AddOne, and pass the definition to AddTwo. The width of the AddTwo ports are now derived from the parameterization of the AddOne instance.

import chisel3._
import chisel3.experimental.hierarchy.{Definition, Instance, instantiable, public}

@instantiable
class AddOne(val width: Int) extends Module {
  @public val width = width
  @public val in  = IO(Input(UInt(width.W)))
  @public val out = IO(Output(UInt(width.W)))
  out := in + 1.U
}

class AddTwo(addOneDef: Definition[AddOne]) extends Module {
  val i0 = Instance(addOneDef)
  val i1 = Instance(addOneDef)
  val in  = IO(Input(UInt(addOneDef.width.W)))
  val out = IO(Output(UInt(addOneDef.width.W)))
  i0.in := in
  i1.in := i0.out
  out   := i1.out
}
// Generated by CIRCT firtool-1.96.0
module AddOne(
  input  [9:0] in,
  output [9:0] out
);

  assign out = in + 10'h1;
endmodule

module AddTwo(
  input        clock,
               reset,
  input  [9:0] in,
  output [9:0] out
);

  wire [9:0] _i0_out;
  AddOne i0 (
    .in  (in),
    .out (_i0_out)
  );
  AddOne i1 (
    .in  (_i0_out),
    .out (out)
  );
endmodule

How do I use the new hierarchy-specific Select functions?​

Select functions can be applied after a module has been elaborated, either in a Chisel Aspect or in a parent module applied to a child module.

There are seven hierarchy-specific functions, which (with the exception of ios) either return Instance's or Definition's:

  • instancesIn(parent): Return all instances directly instantiated locally within parent
  • instancesOf[type](parent): Return all instances of provided type directly instantiated locally within parent
  • allInstancesOf[type](root): Return all instances of provided type directly and indirectly instantiated, locally and deeply, starting from root
  • definitionsIn: Return definitions of all instances directly instantiated locally within parent
  • definitionsOf[type]: Return definitions of all instances of provided type directly instantiated locally within parent
  • allDefinitionsOf[type]: Return all definitions of instances of provided type directly and indirectly instantiated, locally and deeply, starting from root
  • ios: Returns all the I/Os of the provided definition or instance.

To demonstrate this, consider the following. We mock up an example where we are using the Select.allInstancesOf and Select.allDefinitionsOf to annotate instances and the definition of EmptyModule. When converting the ChiselAnnotation to firrtl's Annotation, we print out the resulting Target. As shown, despite EmptyModule actually only being elaborated once, we still provide different targets depending on how the instance or definition is selected.

import chisel3._
import chisel3.experimental.hierarchy.{Definition, Instance, Hierarchy, instantiable, public}
import firrtl.annotations.{IsModule, NoTargetAnnotation}
case object EmptyAnnotation extends NoTargetAnnotation
case class MyChiselAnnotation(m: Hierarchy[RawModule], tag: String) extends experimental.ChiselAnnotation {
  def toFirrtl = {
    println(tag + ": " + m.toTarget)
    EmptyAnnotation
  }
}

@instantiable
class EmptyModule extends Module {
  println("Elaborating EmptyModule!")
}

@instantiable
class TwoEmptyModules extends Module {
  val definition = Definition(new EmptyModule)
  val i0         = Instance(definition)
  val i1         = Instance(definition)
}

class Top extends Module {
  val definition = Definition(new TwoEmptyModules)
  val instance   = Instance(definition)
  aop.Select.allInstancesOf[EmptyModule](instance).foreach { i =>
    experimental.annotate(MyChiselAnnotation(i, "instance"))
  }
  aop.Select.allDefinitionsOf[EmptyModule](instance).foreach { d =>
    experimental.annotate(MyChiselAnnotation(d, "definition"))
  }
}
Elaborating EmptyModule!
instance: ~Top|Top/instance:TwoEmptyModules/i0:EmptyModule
instance: ~Top|Top/instance:TwoEmptyModules/i1:EmptyModule
definition: ~Top|EmptyModule

You can also use Select.ios on either a Definition or an Instance to annotate the I/Os appropriately:

case class MyIOAnnotation(m: Data, tag: String) extends experimental.ChiselAnnotation {
  def toFirrtl = {
    println(tag + ": " + m.toTarget)
    EmptyAnnotation
  }
}

@instantiable
class InOutModule extends Module {
  @public val in = IO(Input(Bool()))
  @public val out = IO(Output(Bool()))
  out := in
}

@instantiable
class TwoInOutModules extends Module {
  val in = IO(Input(Bool()))
  val out = IO(Output(Bool()))
  val definition = Definition(new InOutModule)
  val i0         = Instance(definition)
  val i1         = Instance(definition)
  i0.in := in
  i1.in := i0.out
  out := i1.out
}

class InOutTop extends Module {
  val definition = Definition(new TwoInOutModules)
  val instance   = Instance(definition)
  aop.Select.allInstancesOf[InOutModule](instance).foreach { i =>
    aop.Select.ios(i).foreach { io =>
      experimental.annotate(MyIOAnnotation(io, "instance io"))
  }}
  aop.Select.allDefinitionsOf[InOutModule](instance).foreach { d =>
    aop.Select.ios(d).foreach {io =>
      experimental.annotate(MyIOAnnotation(io, "definition io"))
  }}
}
instance io: ~InOutTop|InOutTop/instance:TwoInOutModules/i0:InOutModule>clock
instance io: ~InOutTop|InOutTop/instance:TwoInOutModules/i0:InOutModule>reset
instance io: ~InOutTop|InOutTop/instance:TwoInOutModules/i0:InOutModule>in
instance io: ~InOutTop|InOutTop/instance:TwoInOutModules/i0:InOutModule>out
instance io: ~InOutTop|InOutTop/instance:TwoInOutModules/i1:InOutModule>clock
instance io: ~InOutTop|InOutTop/instance:TwoInOutModules/i1:InOutModule>reset
instance io: ~InOutTop|InOutTop/instance:TwoInOutModules/i1:InOutModule>in
instance io: ~InOutTop|InOutTop/instance:TwoInOutModules/i1:InOutModule>out
definition io: ~InOutTop|InOutModule>clock
definition io: ~InOutTop|InOutModule>reset
definition io: ~InOutTop|InOutModule>in
definition io: ~InOutTop|InOutModule>out