[llvm-dev] Q. about GlobalISel and order of instructions

[Quentin Colombet via llvm-dev]

Hi Paul,

Thanks for the heads-up!

> On Jan 28, 2021, at 8:12 PM, via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hello Global ISel fans,
> 
> I am trying to patch up some unittests that accidentally weren't
> actually running (https://reviews.llvm.org/D95257).  A number of
> these look like all the right instructions are coming out, but
> they aren't "in the right order."  That is, I see some tests
> expect something along the lines of
> 
>    %2:_(s16) = G_TRUNC
>    %3:_(s10) = G_TRUNC %2:_(s16)
>    %4:_(s10) = G_SEXT_INREG %3:_(s10)
>    %5:_(s16) = G_SEXT %4:_(s10)
> 
> That is, each instruction feeds nicely into the next one.
> But what I'm actually seeing is reordered:
> 
>    %5:_(s16) = G_SEXT %4:_(s10)
>    %2:_(s16) = G_TRUNC
>    %4:_(s10) = G_SEXT_INREG %3:_(s10)
>    %3:_(s10) = G_TRUNC %2:_(s16)
> 
> Now, I can certainly patch up the test to match the order I see,
> but...  Is that actually correct?  Naively it looks like we have
> uses preceding defs, which just looks weird.

That’s definitely broken!

> 
> If the order I'm seeing is wrong (and it's just a standard dump
> of a MachineFunction) then can somebody fix it?

Are these the tests you marked as FIXME in the PR you’ve linked?

Cheers,
-Quentin

>  It will sure
> make a lot less churn in the unittest.
> 
> Thanks,
> --paulr
> 
> 
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