llvm dev - Jun 2020 - FastIsel, SelectionDAGISel or GlobalISel?

Hi,
I am trying to add a new hardware target in LLVM backend. I am starting from
scratch based on LLVM 10.0 and wondering what the best strategy would be for my
own instruction selection, FastISel, SelectionDAG, or GlobalISel? What are the
factors I should consider and which existing target is a good reference for each
choice?
If I understand correctly, FastIsel and GlobalISel both may fail and fall back
to SelectionDAGISel. Therefore, it sounds like I cannot avoid SelectionDAG.
However, I read somewhere that GlobalISel will replace SelectionDAGISel
eventually. This makes me confused. If time to product is important, would
FastISel + some optimizaitons a reasonable choice?
Any suggestions would be highly appreciated.
Many thanks,PW
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Hi PW,

If you’re starting from scratch, GISel is probably the best forward looking
solution.
Given you will be implemented everything in GISel if something doesn’t work you
would just have to fix it within GISel.

What I am saying is the fallback path to SDISel is here for the exiting targets
that have put a lot of effort in SDISel in the past. You shouldn’t need to use
it when you start from scratch!

Hope this helps.

Cheers,
-Quentin
> On Jun 20, 2020, at 4:57 PM, realprincewall at yahoo.com via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> 
> Hi,
> 
> I am trying to add a new hardware target in LLVM backend. I am starting
from scratch based on LLVM 10.0 and wondering what the best strategy would be
for my own instruction selection, FastISel, SelectionDAG, or GlobalISel? What
are the factors I should consider and which existing target is a good reference
for each choice?
> 
> If I understand correctly, FastIsel and GlobalISel both may fail and fall
back to SelectionDAGISel. Therefore, it sounds like I cannot avoid SelectionDAG.
However, I read somewhere that GlobalISel will replace SelectionDAGISel
eventually. This makes me confused. If time to product is important, would
FastISel + some optimizaitons a reasonable choice?
> 
> Any suggestions would be highly appreciated.
> 
> Many thanks,
> PW
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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