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LDWG Meeting minutes: January 22, 2018

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Nate Foster edited this page Mar 2, 2018 · 1 revision

Github Pull Requests [20 minutes]

Issue: Proposes to make arithmetic shift left and logical shift left identical.

Mihai: Only possible objection could be hardware that cannot implement signed shift left.

There were no objections so we approved it.

Action Item: merge.

Issue: when is equality defined on two structs?

Andy F: only question is the phrase “exact same type”

Mihai: same type is non-trivial, e.g. with unions and typedefs.

Andy K: Is equality defined on varbits? This might cause trouble for some headers.

Mihai: We should define equality on varbits. Targets that do not support this can subset. Varbits would be required to have the same static and dynamic types (including length).

Action Item: Mihai to amend the PR with additional detail on when two types are equal.

Reviewers: Andy Keep, James Coole

Issue: P4_14 and P4_16 and BMv2 were all inconsistent wrt treatment of header stacks.

Main thrust of proposed change modifies the behavior of add_header and remove_header to simplify them — they only access element in place.

Action Item: Nate is to fix description of extract with next; Andy F to review.

GitHub Issues Discussion [30 minutes]

Issue: while working on a weird problem in a P4_14 program, discovered that operator precedence in P4_14 is not consistent with C. Since P4_16 has type checking, modifying precedence is not going to change any existing programs.

Chris: should we further change the precedence? For example, of arithmetic vs. bit-wise?

Action Item: ChrisDodd to make a pull request to make consistent with P4_14 (and not with C), and to explore whether further changes are appropriate.

Issue: there are four table properties specified in the P4_16 spec, but size and implementation are not. In the PSA WG, all new table properties have been prefixed by “psa_” to avoid collisions. This raises the question of whether it should be called “psa_size” or if we can rely on common use of “size”?

Action Item: Andy Fingerhut to write a proposal for making this a recommended property.

Review: Vladimir

Issue: cannot use both action hit and action run for the same table apply. And can’t save the value because there is not a type for that kind of data.

Proposal: add these types as first-class citizens in the language.

Action Item: James to consult with a small group to work on a concrete proposal, including discussing feasibility in p4c front-end and bmv2. (Or assume that this functionality is suitably lowered, assuming bmv2 is a faithful reflection of the capabilities of some targets.)

Other participants: Andy K, Mihai

Design Discussion [40 minutes]

  • Documentation (chrispsommers)

Want to have better language support for documentation, in order to have a precise description of things like the P4 Runtime APIs.

Three proposals: Doxygen-style comments Annotations Add new constructs?

Mihai: Should make a list of important use cases. This is a good idea.

Chris: playing with tools to extract information from P4 Runtime.

Chis: Annotations are easy because the compiler already supports them, but they are somewhat clunky — e.g, if you are referring to complex data.

Mihai: for example cross-referencing.

Andy F: does P4_16 spec allow multi-line strings?

Mihai: I think it does, but the C Preprocessor may complain about them.

Nate: having examples from PSA or from P4 Runtime would sharpen the discussion.

Calin: used comments with some conventions.

{ Mihai, Calin }: Please do the thing that’s right for the language in the long term.

Action Item: Chris to discuss with P4 Runtime group.

  • Generics (jnfoster)

Problem: can’t write generic parsers, controls, etc.

Outstanding issues: syntax for “type families”, structure of hierarchy.

Mihai: keep in mind that people who program networks may not be familiar with generics.

Proposal: bring back generics and add bounded types.

  • Serializable types (keep)

Issues: emit-able types are not serializable, since validity bit is discarded.

Mihai: main use-case seems to be control-plane.

Andy K: examples like converting between structs and bit vectors.

Mihai: another example seems to be table initializers (keys + actions, drawn from a union).

Action Item: smaller task force group to reconvene.

  • Value Sets (hanw) Action Item: Han to discuss at next meeting.
  • Virtual Functions (cc10512) Issue with variable capture? Action Item: Calin, Mihai and Antonin to have a sidebar.
  • Optional Arguments (mihai) Action Item: Chris to present on February 5th.
  • Named Parameters (TBD)

Action Item: Mihai to make a list of experimental features.

Next Meeting

Feb 5th, 2018 or fall back to 12th.

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