[llvm-dev] [RFC] mir-canon: A new tool for canonicalizing MIR for cleaner diffing.

[Puyan Lotfi via llvm-dev]

Right now the mir-cannon Canonicalization pass is limited within a basic
block. I don't know the full details of how the outliner works, but I
suspect if outlining success rate is limited by factors like vreg names
and/or instruction ordering then it could be helpful. Of course, I think if
you used some canonicalization pass to reorder the schedule prior to
outlining, then you'd definitely want to run the outlined code through the
scheduler.

PL

On Thu, Aug 31, 2017 at 5:12 PM, Jessica Paquette <jpaquette at apple.com>
wrote:

> Hey Puyan,
>
> Do you think that mir-canon could be used with, say, the
> MachineOutliner/other outlining/code size technologies?
>
> If mir-canon is inter-procedural, I imagine that reducing the differences
> between semantically equivalent sequences of instructions might reveal more
> opportunities for outlining.
>
> - Jessica
>
> On Aug 15, 2017, at 12:06 PM, Puyan Lotfi via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi,
>
> My name is Puyan and I've been exploring ways to improve the state of
> instruction level diffing using llvm and MIR. Below is a proposal for a new
> llvm tool meant to address issues encountered when diffing at the machine
> level. I'm eager to hear the community's feedback.
>
> Thanks
>
> PL
>
> mir-canon: A new tool for canonicalizing MIR for cleaner diffing.
>
>
> Problem Statement and Context:
>
> Diff-tools are regularly used for comparing IR and assembly files. This
> often involves reasoning through differences that are semantically
> equivalent and it can be very time consuming for a person to do said
> reasoning.
>
> Specifically in the context of GlobalISel development there are problems
> of correctness verification. There is a need to compare two programs,
> compiled from identical IR by two different instruction selectors in a way
> where the true differences stand out.
>
>
> Proposal:
>
> We propose a new tool that we have tentatively named 'mir-canon' that
> performs canonical transformations on MIR. The goal is for MIR
> pre-processed with mir-canon to show fewer differences than if it were not
> pre-processed.
>
> At the time of this writing we have a prototype canonicalization tool.
> We’ve come up with some techniques that show promise and would like to open
> discussion with the community to get feedback and suggestions on refining
> said techniques. Currently we think of this as an open ended project.
>
>
> Techniques:
>
> Our prototype does the following for each basic block in a Reverse Post
> Ordering:
>
>
>    - Canonical instruction ordering is done by moving a given instruction
>    as close to the nearest use of its def as possible.
>
>
>
>    - Next, canonical VReg renaming is done by building a collection of
>    candidate instructions that can be thought of as sinks in the def-use
>    graph: they are typically instructions that write to physical registers or
>    store to memory. These candidates are used as the root of a breadth first
>    walk over the vreg operand def-use graph that determines a canonical vreg
>    ordering.
>
>
>
>    - Using said canonical vreg ordering we rename monotonically, but
>    before we do this we skip several vreg values in order to increase the
>    chance that we land on the same vreg number for two different input MIR
>    files. We also do this to reduce the chances that a difference in
>    previously def-use walks will affect the vreg renaming for subsequent
>    walks. This skipping step could be thought of as a kind of vreg number
>    reckoning: we skip modulo n vregs so that we are likely to land on the same
>    vreg for two different files.
>
>
>
> This approach is completely agnostic of ISA specific semantics so it
> should work for any target.
>
>
> Current status:
>
> At the moment we have a standalone llvm tool that uses a single pass to do
> the above described transformations. We have test inputs that show promise
> but we still need a wider set of tests as well as hard metrics.
>
> Our approach processes a single file at a time. The primary benefit to
> this approach is lower complexity in initial implementation and exploration
> of building such a tool. We are open to other approaches such as an
> llvm-diff like (two file at a time) approach, but we have not explored that
> avenue fully yet.
>
> We’re eager to hear community feedback and will be ready to share patches
> for review soon.
>
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>
>
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