Intern Width

[jackkoenig]

API modification to improve performance. This could break code so I'm just marking this 7.0, fixing uses is easy and it's worth it.

Applying the same technique as used in Scala's BigInt (previously applied in SFC to great effect).

Using same benchmarking approach as in,

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
  val phase = new chisel3.stage.phases.Elaborate
  val annos = Seq(
    chisel3.stage.ChiselGeneratorAnnotation(() => new Foo(n))
  )
  println(phase.transform(annos).size)
}

Running:

./firrtl/benchmark/scripts/find_heap_bound.py -vvv --start-size 4G --min-step 10M --context 5 -- -cp assembly.jar Main 200000

Master:

Xmx	Max RSS (MiB)	Wall Clock (s)	User Time (s)
1G	1317	6.73	46.25
880M	1155	7.26	78.42
870M	1151	8.39	79.62
860M	1177	11.45	129.8
850M	-	-	-

This branch:

Xmx	Max RSS (MiB)	Wall Clock (s)	User Time (s)
1G	1321	6.4	60.91
830M	1126	6.87	59.81
820M	1117	9.38	96.9
810M	1154	20.24	308.01
800M	-	-	-

This benchmark isn't necessarily the best benchmark since all widths are shared (I only use 8.W), however, the full width cache Array is still allocated, so we sort of also pay the "worst cost". The only way that this wouldn't be a benefit is if a very high percentage of widths are outside of the memoized range.

Measured a 3% memory reduction (on this benchmark) and probably a very slight speedup too.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• API modification
• Performance improvement

Desired Merge Strategy

• Squash

Release Notes

• UnknownWidth becomes a case object (Drop () when using it).
• KnownWidths 0-1024 are interned

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.