llvm dev - Feb 2020 - Use Global ISel or SelectionDAG

Hi llvm-dev,

I am currently writing a backend for an architecture I am designing in my
bachelor's thesis and recently came across the GlobalISel Selection
Framework.
Should already start using this over the more common Selection DAG?
Being new llvm backend development, I am not sure if GlobalISel would be
"easier" to implement than ISelDAG and if there is any decent
documentation
on how to do so.

Cheers,
Floris
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Hi Floris,

For a bachelor thesis I would probably recommend going with the safest
choice of SelectionDAG. When I started doing LLVM development I was
surprised by how common it is to have to solve LLVM issues that other
architectures didn't happen to run into, despite me working on a very
conventional target arch (RISC-V). You don't want to spend all your time
budget on incidental problems that aren't core to your thesis. That said,
GlobalISel is cool, so if you can make it work for you... ¯\_(ツ)_/¯

Alex Bradbury did some great work on creating a series of patches that show
how to create the RISC-V target in a piecemeal and systematic way, so you
might want to check that out:

https://github.com/lowRISC/riscv-llvm
https://github.com/lowRISC/riscv-llvm/tree/master/docs

Cheers,
Luis

On Thu, Feb 13, 2020 at 10:08 PM floris westermann via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi llvm-dev,
>
> I am currently writing a backend for an architecture I am designing in my
> bachelor's thesis and recently came across the GlobalISel Selection
> Framework.
> Should already start using this over the more common Selection DAG?
> Being new llvm backend development, I am not sure if GlobalISel would be
> "easier" to implement than ISelDAG and if there is any decent
documentation
> on how to do so.
>
> Cheers,
> Floris
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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> On Feb 13, 2020, at 17:08, floris westermann via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi llvm-dev,
> 
> I am currently writing a backend for an architecture I am designing in my
bachelor's thesis and recently came across the GlobalISel Selection
Framework.
> Should already start using this over the more common Selection DAG?
> Being new llvm backend development, I am not sure if GlobalISel would be
"easier" to implement than ISelDAG and if there is any decent
documentation on how to do so.
> 
> Cheers,
> Floris
I think it depends on how much you care about optimization, how weird your
target is, and how long you plan to maintain it. If you don’t care about
optimization, and have a very strange target, you would probably be better off
with GlobalISel. There will be fewer examples to work with however.

-Matt

Hi matt,
Thanks for your advice, ill stick to SelectionDAG then, since my target
isn't that strange and I'll have more examples to look at.

Cheers,
Floris


On Thu, Feb 13, 2020, 11:59 PM Matt Arsenault <arsenm2 at gmail.com>
wrote:
>
>
> > On Feb 13, 2020, at 17:08, floris westermann via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Hi llvm-dev,
> >
> > I am currently writing a backend for an architecture I am designing in
> my bachelor's thesis and recently came across the GlobalISel Selection
> Framework.
> > Should already start using this over the more common Selection DAG?
> > Being new llvm backend development, I am not sure if GlobalISel would
be
> "easier" to implement than ISelDAG and if there is any decent
documentation
> on how to do so.
> >
> > Cheers,
> > Floris
>
> I think it depends on how much you care about optimization, how weird your
> target is, and how long you plan to maintain it. If you don’t care about
> optimization, and have a very strange target, you would probably be better
> off with GlobalISel. There will be fewer examples to work with however.
>
> -Matt
>
>
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Another thing to consider is that GlobalISel will report an error if your
target is big-endian.

On Thu, Feb 13, 2020 at 5:59 PM Matt Arsenault via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
>
> > On Feb 13, 2020, at 17:08, floris westermann via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Hi llvm-dev,
> >
> > I am currently writing a backend for an architecture I am designing in
> my bachelor's thesis and recently came across the GlobalISel Selection
> Framework.
> > Should already start using this over the more common Selection DAG?
> > Being new llvm backend development, I am not sure if GlobalISel would
be
> "easier" to implement than ISelDAG and if there is any decent
documentation
> on how to do so.
> >
> > Cheers,
> > Floris
>
> I think it depends on how much you care about optimization, how weird your
> target is, and how long you plan to maintain it. If you don’t care about
> optimization, and have a very strange target, you would probably be better
> off with GlobalISel. There will be fewer examples to work with however.
>
> -Matt
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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