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XLS: Elaboration

XLS IR has a notion of instantiation and elaboration similar to RTL.1 A hierarchy is a directed acyclic graph of elements connected via instantiation. An elaboration flattens the hierarchy into a tree by walking all paths in the hierarchy starting at a top proc/block where a path is a chain of instantiations. The elaboration creates an "instance" object for each path through the hierarchy from the top proc/block to each IR construct (channel or instantiation).

This outlines what elaboration looks like for each kind of FunctionBase.


Functions are "instantiated" via invoke operations. opt inlines all function invocations, so there is no separate elaboration of functions.


New-style procs can instantiate child procs. They have a notion of "channel references" that are a parent proc can bind to a child proc via instantiation, which is how procs communicate with the procs they instantiate. These channel references are bound to channel instances during elaboration.

Example proc hierarchy:

  proc leaf_proc<ch0: ... in, ch1: .... out>(...) { }

  proc other_proc<x: ... in, y: .... out>(...) {
    chan z(...)
    proc_instantiation other_inst0(x, z, proc=leaf_proc)
    proc_instantiation other_inst1(z, y, proc=leaf_proc)

  proc my_top<a: ... in, b: ... out>(...) {
    chan c(...)
    chan d(...)
    proc_instantiation my_inst0(a, b, proc=other_proc)
    proc_instantiation my_inst1(c, c, proc=other_proc)
    proc_instantiation my_inst2(d, d, proc=leaf_proc)

Elaborating this hierarchy from my_top yields the following elaboration tree. Each line is a instance of either a proc or a channel.

 <a, b>my_top
   chan c
   chan d
   other_proc<x=a, y=b> [my_inst0]
     chan z
     leaf_proc<ch0=x, ch1=z> [other_inst0]
     leaf_proc<ch0=z, ch1=y> [other_inst1]
   other_proc<x=c, y=c> [my_inst1]
     chan z
     leaf_proc<ch0=x, ch1=z> [other_inst0]
     leaf_proc<ch0=z, ch1=y> [other_inst1]
   leaf_proc<ch0=d, ch1=d> [my_inst2]

There are five instances of leaf_proc as there are five paths from top_proc to leaf_proc in the proc hierarchy.


Blocks can instantiate:

  • Other blocks
  • FIFOs
  • External modules

Blocks interact with their instantiations via InstantiationInput and InstantiationOutput operations, which bind a value to a named port on the instantiation.

A block elaboration has an instance for each instantiation and builds maps from parent→child and child→parent ports or instantiation input/output ops.

Topological Sorting

Elaborated blocks can be topo sorted via ElaboratedTopoSort(). Each element in the sort is a pair of Node* and BlockInstance*. This means that blocks instantiated multiple times will have their nodes appear in the sort multiple times (once for each instantiation).

InstantiationInput/InputPort and OutputPort/InstantiationOutput pairs are treated as edges in the DAG. This is in contrast to the "unelaborated" TopoSort() function which only produces a topo sort for an isolated Block. An example use-case of ElaboratedTopoSort() is evaluating IR with dependencies that span the hierarchy.

Note that our implementations of ElaboratedTopoSort() and TopoSort() produce the same order for blocks that have no instantiatiations.


In the same way that ElaboratedTopoSort() extends TopoSort() to elaborated blocks, ElaboratedBlockDfsVisitor extends DfsVisitor to elaborated blocks. The main difference is the signature for op handlers. For DfsVisitor, they take the form HandleNodeT(NodeT* node), but for ElaboratedBlockDfsVisitor they take the form HandleNodeT(NodeT* node, BlockInstance* instance).

Like ElaboratedTopoSort(), InstantiationInput/InputPort and OutputPort/InstantiationOutput pairs are treated as edges in the DAG.

  1. Note that elaboration is internal to the compiler, users generally don't need to be aware of or orchestrate an elaboration.