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

[Chen, Yuanfang via llvm-dev]

Thanks, Craig. I will address this in the patch. It is unfortunate that MachineOutliner needs to keep all MachineFunction in memory and it is located just before emitting. So once it is used, all MachineFunctions are in memory almost for the whole codegen process. AFAIK, there is no good way to address this. Hopefully, we don't have many exceptions like this.

________________________________________
From: Craig Topper <craig.topper at gmail.com>
Sent: Tuesday, July 21, 2020 11:53 AM
To: Chen, Yuanfang
Cc: Nicolai Hähnle; llvm-dev at lists.llvm.org; Matt Arsenault
Subject: Re: [llvm-dev] [RFC] Introducing classes for the codegen driven by new pass manager

One thing I want to mention. I believe in the current legacy pass manager implementation only one MachineFunction ever exists at a time. It is deleted before the next MachineFunction is created. This is very important for memory usage. I think the MachineOutliner being in the pipeline may create an exception to this. I think the initial version of retpoline used a ModulePass and that had to be changed to avoid excessive memory usage.

~Craig


On Thu, Jul 16, 2020 at 5:01 PM Chen, Yuanfang via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
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<mailto:nhaehnle at gmail.com>>
Sent: Thursday, July 16, 2020 12:25 AM
To: Chen, Yuanfang
Cc: Matt Arsenault; llvm-dev at lists.llvm.org<mailto: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<mailto: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<mailto:whatmannerofburgeristhis at gmail.com>> on behalf of Matt Arsenault <arsenm2 at gmail.com<mailto:arsenm2 at gmail.com>>
> Sent: Wednesday, July 15, 2020 9:31 AM
> To: Chen, Yuanfang
> Cc: Robinson, Paul; llvm-dev at lists.llvm.org<mailto: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<mailto: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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev



--
Lerne, wie die Welt wirklich ist,
aber vergiss niemals, wie sie sein sollte.
_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev