[llvm-dev] [RFC] Goals for VPlan-based cost modelling

[Anna Sophia Welker via llvm-dev]

Hi all,

I am looking into the benefits of a VPlan-based cost model, and into how 
such a cost model should be implemented to make the most out of these 
benefits.

Over the last year I have been working with LLVM, mainly focused on the 
ARM backend, in the course of a one-year internship at Arm Ltd. My main 
project from December 2019 to August 2020 was to introduce 
gather/scatters for MVE auto-vectorization. One of the recurring 
challenges during this work was to get things right in the cost model.
For example, gathers can extend the data they load, while scatters can 
truncate their input, meaning that an extend following the load, or a 
truncate preceding the store, is for free if it meets certain type 
conditions. As the current cost model is not designed for context-based 
analysis, this was a pain to model.

I have done some research and found out that one of the proposed 
benefits of VPlan is that a new cost model making use of it would be 
able to better support context-dependent decisions like this.
However, there does not exist much specification about what such a cost 
model should look like.

Also, I have read through the respective code to understand how loop 
vectorization is currently done and how far the introduction of VPlan 
has progressed and have realised that the only recipe that actually 
handles more than one instruction from the input IR is the one for 
interleaved groups. When the VPlan is generated on the VPlan-native 
path, every IR instruction is considered and transformed into a recipe 
separately, ignoring its context (to give a code location, I am looking 
at VPlanTransforms::VPInstructionsToVPRecipes).
And maybe there are architectures that for some cases do not have the 
same vector instructions, so a pattern that works great for one could be 
useless for others. So I am wondering: Is there any plan to have 
target-dependent flavours of recipes, or how will those things be handled?
Right now it makes sense that nothing like this has been implemented 
yet, as there is no cost model that could guide the transformation. But 
if recipes are a general thing, should the cost model be the component 
actually providing the target-specific pattern for a recipe, together 
with its cost?

I am considering choosing a topic related to VPlan, possibly cost 
modelling, for my Master thesis, with the goal to present a solution and 
implement a prototype.

What I would like to ask the community is

(1) what goals there are or should be for VPlan-based cost modelling,
(2) whether there have been any discussions on target-dependent patterns 
yet, and
(3) which examples of inefficiencies and shortcomings of the current 
cost model they have come across.

I am looking forward to your feedback.

Many thanks,
Anna Welker

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