llvm dev - Aug 2021 - llvm-reduce for MIR

Has anyone thought about extending llvm-reduce to also be able to do reduction
on MIR?

The current tool itself seems quite modular and nicely structured with the
delta-algorithm separated from the actual IR specific reductions.
When reducing IR instructions (ReduceInstructions.cpp) it just does RAUW undef
and then eraseFromParent so in theory it seems it would be possible to do a
similar thing for MIR. At least while on SSA form one could either RAUW $noreg
or perhaps a dominating definition of the same register class. So maybe it would
not be too much work to add a set of MIR specific reducers (at least for MBB and
MI) and generalize the remaining part of the tool (module import and
verification) to able to handle both IR and MIR.

At least for me manually reducing MIR is quite slow an painful so any automation
here would be appreciated.

Of course in general MIR is more constrained that IR so it may be "a
bit" more complicated than outlined here, but still seems worth a try.

Any thoughts on this?

-Markus
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Hi Markus,
> On Aug 17, 2021, at 2:04 AM, Markus Lavin via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Has anyone thought about extending llvm-reduce to also be able to do
reduction on MIR?
Thought about it, yes, doing anything about it, no :).
>  
> The current tool itself seems quite modular and nicely structured with the
delta-algorithm separated from the actual IR specific reductions.
> When reducing IR instructions (ReduceInstructions.cpp) it just does RAUW
undef and then eraseFromParent so in theory it seems it would be possible to do
a similar thing for MIR. At least while on SSA form one could either RAUW $noreg
or perhaps a dominating definition of the same register class. So maybe it would
not be too much work to add a set of MIR specific reducers (at least for MBB and
MI) and generalize the remaining part of the tool (module import and
verification) to able to handle both IR and MIR.
>  
> At least for me manually reducing MIR is quite slow an painful so any
automation here would be appreciated.
>  
> Of course in general MIR is more constrained that IR so it may be “a bit”
more complicated than outlined here, but still seems worth a try.
>  
> Any thoughts on this?
Having such tool would be super useful!

On a somewhat related note, I built a tool to extract a basic block from MIR
(i.e., like llvm-extract but on MIR). I’ve never bothered cleaning it up and
open sourcing it, but if there is interest for that I could certainly push that
out.

Cheers,
-Quentin
>  
> -Markus
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