Share ChildBinding objects between siblings (backport #4238)

[mergify]

It's hard to measure the benefit from this, but I used the following little snippet of Chisel to make a bundle of Bundles:

import chisel3._
// _root_ disambiguates from package chisel3.util.circt if user imports chisel3.util._
import _root_.circt.stage.ChiselStage

class MyBundle extends Bundle {
  val a, b, c, d, e, f, g = UInt(8.W)
}

class Foo(n: Int) extends Module {
  val in = IO(Input(Vec(n, new MyBundle)))
  val out = IO(Output(Vec(n, new MyBundle)))

  out :#= in
}

object Main extends App {
  val n = args(0).toInt
  ChiselStage
    .emitCHIRRTLFile(
      gen = new Foo(n)
    )
    .toSeq
}

Then I built a fat assembly jar with Mill, and checked the heap size with find_heap_bound:

./firrtl/benchmark/scripts/find_heap_bound.py -vvv --start-size 4G --min-step 100M --context 2 -- -cp assembly.jar Main 200000

With this, I measured a ~2.5% memory reduction for this very Bundle-heavy code.

I had thought about a more aggressive change, where ChildBinding becomes a case object and we instead get the parent from ref (which does duplicate this information). However, using the above benchmark, I measured the exact same memory improvement. Since I cannot measure any benefit from the much more aggressive and hacky alternative approach, I think we should do the simpler thing which gets more-or-less the same benefit.

Contributor Checklist

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Type of Improvement

• Performance improvement

Desired Merge Strategy

• Squash

Release Notes

This reduces memory use by n - 1 times 16-bytes for an Aggregate with n elements.

Reviewer Checklist (only modified by reviewer)

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This is an automatic backport of pull request #4238 done by [Mergify](https://mergify.com).