[llvm-dev] [GSoC] Prospective student for Unify ways to move code or check if code is safe to be moved

Whitney T Tsang via llvm-dev llvm-dev at lists.llvm.org
Fri Mar 27 15:10:13 PDT 2020

Hi Rithik,

CodeMoverUtils is not aimed for only loop transformations, it can be used
by any transformation which want to move code or check if code is safe to
be moved.
It can currently do both hoist and sink but require control flow equivalent
between the original location and the intended to be moved location.
I would like to keep the API decoupled from loop, so other transformation
can use it.
In this project we want to limit the scope to loop transformations due to
the time constraint, but we should keep in mind that it should be usable
for any kind of transformations.

My expectations for the proposal should contains
1. the problem we want to solve
2. expected end result
     - how many percentage of the loop transformations do you think can be
changed to use the unified API at the end of the project?
3. roadmap + timeline
     a. identified all loop transformations which require code motion
     b. identified all existing code mover utilities
     c. understand the difference (some maybe more conservative, but does
it really need to be)
     d. improve CodeMoverUtils to accommodate different users
     e. change existing loop transformations to use the improved
CodeMoverUtils (should do with (d) at the same time).

Note: finding transformations that use code motion is part of the project,
so no need to include every opportunity in the proposal.

Hope this help.

Whitney Tsang

From:	RITHIK SHARMA <rithiksh02 at gmail.com>
To:	whitneyt at ca.ibm.com
Cc:	etiotto at gmail.com, Stefanos Baziotis
            <stefanos.baziotis at gmail.com>, llvm-dev
            <llvm-dev at lists.llvm.org>, bmahjour at ca.ibm.com
Date:	2020/03/27 03:08 PM
Subject:	[EXTERNAL] Re: [llvm-dev] [GSoC] Prospective student for Unify
            ways to move code or check if code is safe to be moved

Hello Whitney,

I spent some good time going through the exact implementation of a few loop
transformation passes in llvm and for others, I tried to quickly get an
idea of what is going inside them with respect to the implementation in

I also went through the CodeMoverUtils [1], I really appreciate the effort
of keeping it generic and decoupled from loop though being derived from
loop and aimed to be used for loop transformation. I also analyzed Loop
Fuse pass [2] as a reference as it already uses CodeMoverUtils which helped
me to concrete my understanding of the vision behind the unification of
code motion checks.

LICM [3] does hoisting [4] as well as sinking [5]. I thought about
extending the already available isSafeToMoveBefore [6] for hoisting the
code from loop body to the pre-header. In my opinion, hosting and sinking
are the basic code motions, exposing them in a robust and flexible way will
also help other transformations (maybe other non-loop transformations).

I also saw that Loop Sink pass [7] uses the same checks [8] from LICM and
also found another code hoisting opportunity in Loop Rotation pass where we
once hoist to the pre-header [9]. I also analyzed passes like Loop Unswitch
[10] in the hope for some code motion opportunities but it seems the passes
which are expected to run after LICM are void of them (may not be true for
all). I limited myself to the loop passes in Transform/Scalar and I'm
planning to take a look into other loop passes as well.

Having said that, my concern right now is the proposal whose deadline is
approaching in 4 days, I would like to get a small feedback before the
deadline. I am aiming to prepare a draft by Monday morning (PST) so then I
may have a short window to incorporate the feedback. I also have two
coursework assignment deadlines end of this week which is also demanding
some of my effort other than attending the lectures.

I'll be glad to hear about your expectations for the proposal. I'm planning
to somehow meet them and will also continue to look into loop passes for
more code motion checks after the proposal deadline. Does this look good to
you? In my proposal can I focus on the findings that I'm able to do before
the deadline and keep some buffer for things that I may find later as
identifying all the existing ways for code motion in loop transformations
is a big portion of the proposed project.












On Tue, 24 Mar 2020 at 23:30, Stefanos Baziotis <
stefanos.baziotis at gmail.com> wrote:
  Np, good luck! :)

  - Stefanos

  Στις Τρί, 24 Μαρ 2020 στις 8:55 μ.μ., ο/η RITHIK SHARMA <
  rithiksh02 at gmail.com> έγραψε:
   Many thanks, Stefanos! I really appreciate your help :) I heard from


   On Tue, 24 Mar 2020 at 23:08, Stefanos Baziotis <
   stefanos.baziotis at gmail.com> wrote:
     Hi Rithik,

     I CC'd the 2 other mentors and hopefully you'll get answer. Sorry, but
     I don't know of any other way to help. :/
     You may also try to contact them privately.


     Στις Τρί, 24 Μαρ 2020 στις 7:35 μ.μ., ο/η RITHIK SHARMA via llvm-dev <
     llvm-dev at lists.llvm.org> έγραψε:
      I'm bit concern about the approaching deadline next week, some
      pointers will be really appreciated and will be very helpful to me.

      Best regards,
      Rithik Sharma

      On Sun, 22 Mar 2020 at 01:24, RITHIK SHARMA <rithiksh02 at gmail.com>

        I am an undergrad from India. This summer I'm interested in working
        on unifying the code motion checks as a GSoC intern with LLVM. I
        started reading about classical data flow analysis since few
        months, I see this as a great opportunity to start contributing to
        LLVM. The idea of unifying all the code motion checks into one
        place seems very rational and interesting to me. As in the project
        description LICM was mentioned, I have a basic idea about it and I
        spent some time to go through its implementation in LLVM. It makes
        a lot of sense to me to keep all code motion checks at one place,
        say an analysis pass, I also looked into GVN hoist & sink passes as
        they also have some code motion logic. I'll start drafting the
        proposal after your initial feedback/suggestions. I'm really very
        excited to work on this.

        Best Regards,
        Rithik Sharma
      LLVM Developers mailing list
      llvm-dev at lists.llvm.org

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