Tutorial: How To Write An LLVM Register Allocator

Wed Feb 10 11:42:55 PST 2016

Thank you Madhur! My intentions are to help the community, publishing the
knowledge about this process that I have learned during my project.

Actually, I'm keeping this document on GitHub (
https://github.com/nael8r/How-To-Write-An-LLVM-Register-Allocator). If at
some point this document gets merged in the LLVM documentation, I think
I'll remove it from the repository.

2016-02-10 3:26 GMT-02:00 Madhur Amilkanthwar <madhur13490 at gmail.com>:

> +1 for such efforts!
>  It is of utter importance to have such things documented to avoid
> reinventing the wheel every time someone wants to work with LLVM.
> I don't know if LLVM community archives such docs, but if not these should
> be kept somewhere safe.
>
> On Wed, Feb 10, 2016 at 10:28 AM, Hal Finkel via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> ----- Original Message -----
>>
>> > From: "Sean Silva via llvm-commits" <llvm-commits at lists.llvm.org>
>> > To: "Matthias Braun" <matze at braunis.de>
>> > Cc: "Natanael Ramos" <naelr8 at gmail.com>, "llvm-commits"
>> > <llvm-commits at lists.llvm.org>
>> > Sent: Tuesday, February 9, 2016 10:40:56 PM
>> > Subject: Re: Tutorial: How To Write An LLVM Register Allocator
>>
>> > On Tue, Feb 9, 2016 at 6:56 PM, Matthias Braun < matze at braunis.de >
>> > wrote:
>>
>> > > Interesting,
>> >
>>
>> > > I don't think the document is in a shape for documentation that
>> > > ships
>> > > with llvm. Just some points:
>> >
>>
>> > > - The document keeps jumping between general register allocation
>> > > concepts, general llvm concepts and code snippets of llvms register
>> > > allocators. It's hard to follow which is which.
>> >
>> > I agree.
>>
>> > > - The basic concepts would need better and more in-depth
>> > > descriptions, just to give some topics:
>> >
>> > > - How does the basic program representation work, MachineInstr,
>> > > MachineOperand (esp. the register kind with the undef, dead and
>> > > kill
>> > > flags), subregisters, subregisterindexes, register classes, how we
>> > > express most constraints with register classes, virtual registers
>> > > how register are encoded in unsigneds, a proper description of
>> > > register units, what't does it mean if a register is reserved or
>> > > allocatable, pristine registers, lanemasks, ...
>> >
>> > Yeah, there needs to be some acknowledgement of the MI
>> > representation.
>>
>> > > - How does the liveness representation work: What is a Segment, a
>> > > VNInfo, a SlotIndex, how can we maintain SSA form in the liveness
>> > > info (and why do we do that)
>> >
>> > > - How do all the regalloc related passes in llvm play together:
>> > > PhiElimination, TwoAddress, RegisterCoalescer, RegAllocXXX,
>> > > VirtRegRewriter
>> >
>> > Do you think all this needs to be covered in a HowTo? That seems like
>> > an awful lot.
>>
>> I agree. This one document is not going to solve our systemic problem of
>> lack of documentation for the backend infrastructure. Nor should we make
>> solving that problem a prerequisite for accepting this contribution.
>>
>>  -Hal
>>
>> > Which do you think are the essential content for a
>> > HowTo? The goal isn't becoming an expert, but just wiring up the
>> > boilerplate and getting an end-to-end experience writing something
>> > simple (here's an example of another howto for reference:
>> > http://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html ).
>>
>> > We might be able to punt on a lot of the MI stuff for this document
>> > (just state it as a prerequisite) and eventually link to a "How to
>> > write a MachineFunctionPass" or something which can explain the MI
>> > basics.
>>
>> > -- Sean Silva
>>
>> > > On the other hand I applaud going through the work and actually
>> > > writing something down, this will surely help other people who just
>> > > want to get started with the matter. Maybe there is a way to get
>> > > this up onto the llvm blog (who maintains that)?
>> >
>>
>> > > - Matthias
>> >
>>
>> > > > On Feb 9, 2016, at 6:18 PM, Sean Silva via llvm-commits <
>> > > > llvm-commits at lists.llvm.org > wrote:
>> > >
>> >
>>
>> > > > Matthias, it's rare that we get a substantial piece of
>> > > > documentation
>> > > > like this. Can you take a look at the content?
>> > >
>> >
>>
>> > > > On Fri, Feb 5, 2016 at 3:50 PM, Natanael Ramos via llvm-commits <
>> > > > llvm-commits at lists.llvm.org > wrote:
>> > >
>> >
>>
>> > > > > Hello
>> > > >
>> > >
>> >
>>
>> > > > > My name is Natanael Ramos, I'm a student of bachelor degree on
>> > > > > Computer Science Course, here on Brazil.
>> > > >
>> > >
>> >
>>
>> > > > > Recently I have worked with LLVM for my undergraduate thesis (I
>> > > > > don't
>> > > > > really know how is called in other countries, here is called
>> > > > > Completion of course work ), in my work I have implemented an
>> > > > > register allocator using LLVM and have tested him with the
>> > > > > built-in
>> > > > > allocators in LLVM (Probably I'll publish a paper soon, if all
>> > > > > goes
>> > > > > as expected).
>> > > >
>> > >
>> >
>>
>> > > > > As another product of my work, I have created a tutorial of how
>> > > > > to
>> > > > > write an LLVM register allocator, extending the RegAllocBase
>> > > > > interface, this tutorial is based on my understanding of the
>> > > > > LLVM
>> > > > > framework for working with the register allocation pass.
>> > > >
>> > >
>> >
>>
>> > > > > The tutorial have been written in reStructuredText following
>> > > > > the
>> > > > > LLVM
>> > > > > instructions and orientations (
>> > > > > http://www.llvm.org/docs/SphinxQuickstartTemplate.html ). As
>> > > > > it's
>> > > > > suggested, I'm sending the tutorial to this mailing list in
>> > > > > order
>> > > > > to
>> > > > > contribute to the community of developers, which use LLLVM.
>> > > >
>> > >
>> >
>>
>> > > > > Any suggestions, please let me know.
>> > > >
>> > >
>> >
>>
>> > > > > --
>> > > >
>> > >
>> >
>>
>> > > > > Natanael Ramos
>> > > >
>> > >
>> >
>> > > > > Membro do corpo discente de Ciência da Computação pelo
>> > > > > Instituto
>> > > > > Federal de
>> > > >
>> > >
>> >
>> > > > > Minas Gerais - Campus Formiga
>> > > >
>> > >
>> >
>>
>> > > > > _______________________________________________
>> > > >
>> > >
>> >
>> > > > > llvm-commits mailing list
>> > > >
>> > >
>> >
>> > > > > llvm-commits at lists.llvm.org
>> > > >
>> > >
>> >
>> > > > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>> > > >
>> > >
>> >
>>
>> > > > _______________________________________________
>> > >
>> >
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>> > >
>> >
>> > > > llvm-commits at lists.llvm.org
>> > >
>> >
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>> > >
>> >
>>
>> > _______________________________________________
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>>
>> --
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>
>
>
> --
> Thank You.
> Madhur D. Amilkanthwar
> RISE lab,
> IIT Madras.
>

-- 
Natanael Ramos
Membro do corpo discente de Ciência da Computação pelo Instituto Federal de
Minas Gerais - Campus Formiga
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