Tutorial: How To Write An LLVM Register Allocator

[Hal Finkel via llvm-commits]

----- 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
> > > 
> > 
> 

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-- 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory