[llvm-dev] [RFC] Introducing classes for the codegen driven by new pass manager

[Chen, Yuanfang via llvm-dev]

TBH, I don't know the history of this choice. Although it makes sense from the debugging/testing point of view. We have IR / MIR difference so it makes sense to have their own pipeline (yeah, MIR pipeline is prefixed with an IR pipeline to prepare the module, but that is just implementation detail). And then we have opt/llc to test the pipeline separately. Combining them in a single pipeline is possible, but AFAIK it does not have many benefits in practice.

________________________________________
From: Nicolai Hähnle <nhaehnle at gmail.com>
Sent: Thursday, July 16, 2020 12:25 AM
To: Chen, Yuanfang
Cc: Matt Arsenault; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] [RFC] Introducing classes for the codegen driven by new pass manager

On Wed, Jul 15, 2020 at 6:39 PM Chen, Yuanfang via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Indeed, but there is a distinction about their position in the pipeline. We run opt & codegen pipeline separately,

Why, though? Is there a reason why this inherently makes sense, or is
it just a historical accident? At least to me it seems that it would
make more sense to run all passes within a single pipeline.

Cheers,
Nicolai



>
> Do you run “codegen” IR passes with regular IR passes? If so, do you mind sharing the use cases? I might have missed this use case.
>
> ________________________________________
> From: Matt Arsenault <whatmannerofburgeristhis at gmail.com> on behalf of Matt Arsenault <arsenm2 at gmail.com>
> Sent: Wednesday, July 15, 2020 9:31 AM
> To: Chen, Yuanfang
> Cc: Robinson, Paul; llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] [RFC] Introducing classes for the codegen driven by new pass manager
>
>
>
> > On Jul 15, 2020, at 12:28, Chen, Yuanfang <Yuanfang.Chen at sony.com> wrote:
> >
> > In codegen with NPM, I've made all codegen passes (IR or MIR pass) to be only driven by `llc`. Both due to the way NPM registering pass (on-demand&dynamic instead of static initialization in Legacy PM), and reduce the confusion about which tool (`llc` or `opt`) to test codegen IR passes.
> >
>
>
> I think there’s no real distinction between “codegen” IR passes and noncodegen IR passes. I routinely run “codegen only” passes with opt in conjunction with other passes when experimenting. I think losing the ability to run any IR pass with opt would be a functionality regression.
>
> -Matt
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