DevMeeting - 2021-08-23/24

[jackkoenig]

2021-08-23/24

Outreach

• [jack] Outreachy intern (sort of like GSoC for underrepresented groups)
  • https://www.outreachy.org/docs/community/
  • https://www.outreachy.org/communities/cfp/
  • Sept. 10th deadline for community signup
  • Would need to be via Chips Alliance - looping in Henry
  • We could potentially provide mentors, sponsorships come from elsewhere (eg. Chips Alliance, SiFive)

Updates

Pinned Items

• Priority PRs:
  *

Discussion 1

• [kevin] RTLIL backend
  • what tests do we want to ask for?
  • can we get this into the 3.5-RC1?
  • [jack] I think it’s fine, the tests are good enough for experimental backend
    • Only concern is version of Yosys, in the past I had to go back a version because something was wrong with the formal tests and Yosys doesn’t do releases very often. We may have to build yosys from source which is a bit of a pain (but we do for chisel3)
    • [kevin] Maybe we can pick one of the many binary releases
  • [jack] It would be nice to have a formal checking flow between RTLIL and Verilog, main issue is it takes several minutes per test so this might be nice in a post merge CI rather than in standard PR checking
  • [chick] is it strictly necessary for this sort of thing to be part of the repo?
    • [schuyler] No, it is possible to build libraries for this sort of thing, can publish them yourself
    • [kevin] it is nice to just get the next version and it has it
• [kevin] make chiseltest available for tests in chisel3 repo
  • PR
  • short-term goals:
    • make it easier to test the chisel standard library components
    • identify missing features in chiseltest
    • provide users some examples of how to use chiseltest
  • long-term goal:
    • deprecate all simulator bindings outside of chiseltest
  • problem:
    • chiseltest depends on chisel3
    • if chisel3 breaks binary compatibility of a feature that chiseltest relies on, then the tests would fail with a runtime linker error
  • solution:
    • keep tests in a separate project called integration-tests
    • make it possible to skip these tests if there is an issue
    • [jiuyang] unit test from chuse “build-from-source” chiseltest which linked to “THIS” chisel3.
    • [jack] More nuclear option is to just merge chiseltest into the chisel3 repo
      • [jiuyang] Maybe can decouple chiseltest into frontend and backend, keep frontend in chisel3 and backends in separate repo
      • [kevin] Main concern about merging in chiseltest is it raises the bar of backwards compatibility maintenance, it’s just too much overhead as a grad student
      • [jack] If we merge repos, we should keep chiseltest in pre-1.0 so it has the lower bar
      • Make sure to merge histories with git tree-merge or whatever
    • [kevin] Why don’t we just YOLO it -- don’t merge repos, have chisel-integration-tests depend on chiseltest and see how long it takes something breaks
• [jiuyang] Dependencyapi phase4 ucb-bar/chiseltest#256
  • A legacy PR which used to try to switch chiseltest to Stage and Phase.
    • Should I reopen it?
      • If we do this, it needs to not be an entire 1000s of lines architectural overhaul
  • [kevin] chiseltest is not really a compiler, not sure why it needs a Stage or Phase
    • What’s the use case that currently doesn’t work?
      • Switch different backend with annotations -- this can be done now
      • [kevin] we can add annotations for the various use cases wanted
    • User facing API is a test function
    • [adam] Our use case inside SiFive is that we have custom phases (like emitting Artefacts, see rocket-chip), we need these stages to run and are having a tough time getting them to run
      • [kevin]** if you can open a PR that has a test for the API that you want, we can discuss there**
      • [adam] RunFirrtlTransformAnnotation won’t work because it’s not a transform, it’s a phase
      • [schuyler] it’s sort of like needing a RunPhaseAnnotation
      • [kevin/adam] want to use the same compiler flow [down to low-firrtl] that one uses for tapeouts
    • [jiuyang] can we have test function in an annotation, then one could provide their own CLI
    • [jiuyang] Used to provide custom Phases that would enable building one mega-Verilog (full design) yet providing multiple test functions to test different APIs in that single design
    • [kevin] could possibly provide alternative API that works with Stage/Phase
    • [shuyler] Chisel Stage should not be the top, it’s just 1 tool for doing 1 thing
      • Could encapsulate chiseltest’s need for typesafe DUT in a Stage
      • Sometimes we need something that does elaboration, not necessary the “ElaboratePhase”
  • Resolution: SiFive to show mockup of API they need, also with a high level design document

Discussion 2

• [jiuyang] Pending PR from Jiuyang
  • add new APIs to BitPat #2076
    • Good to merge now.
  • Speed up CI #2043
    • Why did this become a slowdown?
      • Sat, 21 Aug 2021 18:41:19 GMT [info] ImplicitModule directly in a RawModule in an ImplicitModule
      • Sat, 21 Aug 2021 18:41:19 GMT [info] - should fail
      • Sat, 21 Aug 2021 19:24:37 GMT [info] - Infering widths on huge Vecs should not cause a stack overflow
    • Should this be suppressed by replace custom model checker with chiseltest formal verify command #2075 ?
      • No, these are independently useful
  • Single thread ucb-bar/chiseltest#371
    • Add UseSingleThreadingAnnotation to speed up.
  • Signal Trace API #2077
    • WIP this week, I was too busy these days :(
• [jack] lazy FIR emission
  • Lazy .fir Emission #2083
  • Overcomes 2 GiB JVM String size limit
  • The Legacy Emitter in chisel3 is now gone!
• [adam] What is our backporting policy?
  • Proposal:
    • Only backport 1 major version and only farther upon request
    • Default bug fixes to 2 major versions
  • [chick] Is this more of a CWG (Chisel Working Group) question?
    • Yes, but we haven’t really been meeting
  • We should write this somewhere, contributor guidelines? PR template?
• [jack] 3.4.4
  • Jack and Chick to touch base on Wednesday
• [jack] 3.5-RC1 or 3.5-M1
  • Adam and Jack working on Definition/Instance: Instance/Definition (formerly Template) #2045