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

[Florian Hahn via llvm-dev]

Hi,


> On Sep 11, 2019, at 17:51, Bardia Mahjour via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> ---------------------------
> 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.
> 

This issue also motivated adding metadata to disable LICM (llvm.loop.licm.disable) recently. https://reviews.llvm.org/D64557 <https://reviews.llvm.org/D64557>
> - 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 <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 <https://reviews.llvm.org/D65350>) (Bardia)
> - Bugzilla bugs update (Vivek)
> - Others?
> 
> 
> Bardia Mahjour
> Compiler Optimizations
> IBM Toronto Software Lab
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> llvm-dev at lists.llvm.org
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