llvm dev - Nov 2019 - Loop Opt WG Meeting Minutes for Nov 6, 2019

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Wed, Nov 6, 2019:
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- LLVM dev conference update
  - Tutorial on how to write loop passes and a panel on Loop Opt Working
    Group went well and were well attended.

- Loop Nest Analysis (Ettore Tiotto)
  - What instructions should be allowed/disallowed in between perfect
    loop nests?
    - Take the usual approach of being restrictive and gradually allow
      more instructions.
    - Using isSafeToSpeculativelyExecute() may be a good starting point,
      but need to consider other instructions commonly occurring in loop
      nests such as branches and phis.

- `#pragma clang transform` and `#pragma clang loop ivdep` (Michael Kruse)
  - Should it be a hint or an order?
    - Current proposal is to have it dictate and require the specified
      transformations in that order.
  - ivdep is defined in a way that makes it difficult to use because it
    depends on the specific limitations of dependence analysis in a
    given compiler, as such it won't be pursued in llvm.

- The "freeze" instruction has landed
(https://reviews.llvm.org/D29011).
  Watch out for potential impact to loop transformations (Philip Reames).

- Status Updates
  - DomTreeUpdater investigation (Kit)
    - No update
  - Data Dependence Graph (Bardia)
    - https://reviews.llvm.org/D68827 is ready for further comments
      or approval.
  - Follow up on multi-dimensional array indexing RFC (Siddharth)
    - No update.
  - Impact of Loop Rotation on existing passes (Min-Yih)
    - No update.
  - Bugzilla bugs update
    - No update.


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Tentative Agenda for November 20th
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Loop Trip Count Value Profiling - Wael Yehia

<Please let me know if you have other topics to discuss>

- Status Updates
  - Loop Nest Analysis (Ettore)
  - Data Dependence Graph (Bardia)
  - DomTreeUpdater investigation (Kit)
  - Follow up on multi-dimensional array indexing RFC (Siddharth)
  - Impact of Loop Rotation on existing passes (Min-Yih)
  - Bugzilla bugs update


Bardia Mahjour
Compiler Optimizations
IBM Toronto Software Lab
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