llvm dev - Dec 2019 - Loop Opt WG Meeting Minutes for Dec 18, 2019

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Wed, Dec 18, 2019:
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- Next Meeting will be on Jan 15th, 2020.

  ****Please consider the meeting on Jan 1st CANCELED.****

- Dependence Analysis - Delinearization of constant size arrays. (Bardia)
  - Support for delinearization of fixed size arrays were removed in
    https://reviews.llvm.org/D35430.
  - Some lost coverage in DA and interchange, also makes writing useful
    tests harder.
  - Bardia to post a patch soon to use GEPs for computing subscripts for
    fixed-size arrays similar to how polly does it (and reuse/refactor
    function in polly and SCEV).
  - The patch will initially make use of
    `-da-disable-delinearization-checks` but eventually can be replaced
     with checking for the `inrange` keyword in GEP.
    - `inrange` improvements needed:
      1. remove restriction of `constant` GEPs
      2. allow inrange on more than one index.

- Implementation of Unroll-and-Jam (Whitney)
  - Currently does not handle loops with more than one inner subloop.
  - It is implemented as loop pass, thus traversing loops from inner to
    outer and making transformation along the way. Unrolling often creates
    residual subloops making subsequent UAJ of the outer loops impossible.
  - Proposal:
    - make it a function pass and walk the nest from outer to inner, which
      should also result in cleaner code after the transform.
    - Whitney to write a small RFC and send it to llvm.dev mailing list.

- Status Updates
  - Data Dependence Graph (Bardia)
     - DDG ordinals https://reviews.llvm.org/D70986 ready for
review/approval
  - Loop Nest Analysis (Ettore)
    - https://reviews.llvm.org/D68789
    - Ettore to consider making it a utility instead of an analysis because
      invalidation and update of the cached analysis results is problematic
      and the compile-time benefit of caching it may not be realized with
the
      currently intended use cases.
  - Loop Pragma RFC on llvm.dev (Michael Kruse)
    - http://lists.llvm.org/pipermail/cfe-dev/2019-December/064122.html
  - Hal and Michael Kruse are preparing another RFC for loop opt framework
    (to be also discussed in one of the upcoming calls as well).

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Tentative Agenda for Jan 15, 2020
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<Looking for topics>

- Status Updates
  - Data Dependence Graph (Bardia)
  - Loop Nest Analysis (Ettore)


Bardia Mahjour
Compiler Optimizations
IBM Toronto Software Lab
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