Tutorial: How To Write An LLVM Register Allocator

[Sean Silva via llvm-commits]

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. 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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