llvm dev - Nov 2020 - Work on DAG Isel for TableGen and compiler

I have been working on improvements to TableGen's DAG Isel matcher backend.
This has led me to thinking about ways to speed up the compile-time interpreter
of the instruction selection matcher table.

Is this worth my time, given Fast Isel and the upcoming Global Isel selector?

Hi Paul,

I think this would be time well spent. At least in the WebAssembly backend,
the vast majority of our ISel work is still done with DAG ISel. I know this
is different from the performance work you have in mind, but one of my
biggest pain points working on LLVM so far has been the poor error messages
from the DAG ISel pattern type checker. If you could find time to improve
those error messages, I would be extremely grateful.

Thanks,

Thomas

On Wed, Nov 18, 2020 at 10:19 AM Paul C. Anagnostopoulos via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> I have been working on improvements to TableGen's DAG Isel matcher
> backend. This has led me to thinking about ways to speed up the
> compile-time interpreter of the instruction selection matcher table.
>
> Is this worth my time, given Fast Isel and the upcoming Global Isel
> selector?
>
> _______________________________________________
> 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 Nov 18, 2020, at 13:19, Paul C. Anagnostopoulos via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> 
> I have been working on improvements to TableGen's DAG Isel matcher
backend. This has led me to thinking about ways to speed up the compile-time
interpreter of the instruction selection matcher table.
> 
> Is this worth my time, given Fast Isel and the upcoming Global Isel
selector?
> 
> 
GlobalISel’s matcher table is in need of more optimization work. I think it’s
important to actually start shifting effort to working on GlobalISel. At the
current rate we’ll never reach a point where we can really switch. GlobalISel is
usable now, and I hate to see effort continuing on the legacy path.

-Matt

Are you talking about the type checking done in CodeGenDAGPatterns.cpp? Is it
easy to post an example?

At 11/18/2020 01:55 PM, Thomas Lively wrote:
>Hi Paul,
>
>I think this would be time well spent. At least in the WebAssembly backend,
the vast majority of our ISel work is still done with DAG ISel. I know this is
different from the performance work you have in mind, but one of my biggest pain
points working on LLVM so far has been the poor error messages from the DAG ISel
pattern type checker. If you could find time to improve those error messages, I
would be extremely grateful.
>
>Thanks,
>
>Thomas