LDWG meeting minutes, July 1, 2019

Attendees

• Mario Baldi
• Gordon Brebner (Xilinx)
• Mihai Budiu (VMware)
• Chris Dodd (Barefoot)
• Andy Fingerhut (Cisco)
• Nate Foster (Cornell)
• Jed Liu (Barefoot)
• Chris Sommers (Keysight)

Status

• NPL
  • Broadcom's NPL was released!
  • Should we reinitialize a harmonization effort?
  • Chris suggested perhaps we could invite someone from NPLang.org to present to us as a first step.

Structure Initializers

• References:

  • https://github.com/p4lang/p4c/pull/1498
  • https://github.com/p4lang/p4c/pull/1951
  • https://github.com/p4lang/p4-spec/pull/717

• Status: Structure initializers are in p4c and more or less ready to be added to the spec. However, as we fleshed out the design, we discovered that structure expressions would also be useful. However, there are some complexities related to type inference.

• Action Item: We'll move to merge structure initializers soon, and split off structure expressions into a separate issue.

Signed Slices

• References:

  • https://github.com/p4lang/p4-spec/issues/668
  • https://github.com/p4lang/p4-spec/pull/701

• Status: no progress since last time.

• Action Item: Jed to take a look so we can merge this change.

Recirculate in V1Model

• References:

  • https://github.com/p4lang/p4c/pull/1698
  • https://github.com/p4lang/p4c/issues/1669

• Status:

  • V1model does not preserve (non-empty) metadata on recirculate.
  • We have a proposed implementation based on annotations.
  • Barefoot has implemented a P4_14
  • Action Items:
    • Han will work on P4 from P4_14 to PSA
    • Mihai to add warning to p4c
    • Nate to investigate PSA-style externs as annotations

Structure Overlays

• Reference:

  • https://github.com/p4lang/p4-spec/pull/656

• Status:

  • This feature has been in design for a while
  • We might look at NPL's overlays for inspiration
  • Our proposal is different because it generates a new structure (motivation: can import structure from library and declare overlay)
  • Questions:
    • What happens if bits are referenced twice?
    • If bits are referenced twice, can an overlay be used as an l-value?

• Action Items:

  • Mihai to simplify PR to forbid non-injective mappings, then others will review

Logical Operations

• Reference:

• https://github.com/p4lang/p4c/issues/1548
• https://github.com/p4lang/p4c/pull/1859

• Status:

• assert and assume have been merged into p4c!
• However, they are not in the spec
• Current behavior in Bmv2 is that the process aborts if assertion fails
• We could imagine putting these constructs into the P4 ecosystem at three levels:
  • Part of architectures
  • Part of libraries
  • Part of language

• Action Item: Add description to PSA and V1Model documentation for now.

Conditionals in Actions

• Reference:

• https://github.com/p4lang/p4c/issues/644

• Action Item: Andy to double check that the spec is accurate

Abstract Methods

• Reference:

• https://github.com/p4lang/p4-spec/issues/561

• Action Item: Mihai to have a pass and we'll revisit.

Open Discussion

• Andy: When does language allow multiple objects to have the same name (e.g., controls, actions, etc.)?
• Han: Overloading would be nice. Implementation is rather restricted right now -- e.g., need to disambiguate using named parameters.

Next Meeting

• August 5th