[llvm-dev] Loop Opt WG Meeting Minutes for Sep 11, 2019

Bardia Mahjour via llvm-dev llvm-dev at lists.llvm.org
Wed Sep 11 09:51:21 PDT 2019

Wed, Sep 11, 2019:

- LICM vs Loop Sink Strategy (Whitney)
    - LICM and SCEV expander host code with no regards to increased
      live-ranges. This is a long standing issue where historically
      preference has been to keep LICM more aggressive.
    - Two questions from IBM side:
      a. This problem is not specific to the POWER platform, so we are
         wondering if other people are interested?
    - b. Where would be the best place to address this issue?
        - Since it's hard to come up with an accurate register pressure
          estimator in opt, it's probably better to be done fairly late,
          maybe after instruction scheduling.
        - A good place to start would be instruction re-materialization in
          the register allocator.
        - Problem is the logic in the register allocator can deal with a
          single instruction (instead of groups of instructions) at a time.
        - Start by handling one single-instruction at a time and apply the
          same logic to groups of instructions iteratively to see the
          impact on performance and compile-time.
        - live-range editor may have utilities to help with code motion.
    - lazy-code-motion may be a good long term solution, but no one seems
      to be actively working on it.

- Announcements:
    - flang call moved so we are no longer in conflict!

    - Philip is working on making loop vectorizer robust in the face of
      multiple exits. There are two subproblems
        1. vectorizer currently gives up because scev is not giving exit
           counts (due to a bug?). This is relatively easy to fix and
           Philip will have a patch for it soon.
        2. loop exit cannot be analyzed due to data dependent exit, which
           is currently handled via predication. There is a lot of room
           for improvement, specially for read-only loops.
           Please let him know if you are interested.

- Status Updates
  - Data Dependence Graph (https://reviews.llvm.org/D65350) (Bardia)
    - All review comments are addressed. Waiting for approval.
  - Bugzilla bugs update (Vivek)
    - Florian has a patch fixing loop bugs related to max trip count.

Tentative Agenda for Sept 25

Presentation from Marc Moreno Maza about his work on delinearization.

- Status Updates
  - Follow up on multi-dimensional array indexing RFC (Siddharth)
  - Impact of Loop Rotation on existing passes (Min-Yih)
  - Data Dependence Graph (https://reviews.llvm.org/D65350) (Bardia)
  - Bugzilla bugs update (Vivek)
  - Others?

Bardia Mahjour
Compiler Optimizations
IBM Toronto Software Lab
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