[llvm-dev] [EXTERNAL] RE: Machinepipeliner interface. shouldIgnoreForPipelining, actually not ignoring.

Thu Sep 3 12:07:15 PDT 2020

Hi James,

Adding Hendrik, who has taken over ownership of the downstream code
involved.

I can also add background about the rationale, of that helps? It was added
to ignore induction variable update code (scalar code) that is rewritten
when we unroll / peel the prolog epilog anyway.

Targets like Hexagon or PPC with dedicated loop control instructions for
pipelined loops don't need this, but our target was simple RISC. It was for
that reason that I felt the feature would be useful for other targets.

James

On Thu, 3 Sep 2020, 17:19 Nagurne, James, <j-nagurne at ti.com> wrote:

> Ah, I apologize for not seeing the meaning of your first email. I had not
> considered that he was working on an out-of-tree target that utilizes the
> ignore capability. You’ve made things very clear, thank you!
>
>
>
> Since he’s on the email thread now:
>
>
>
> James, do you plan on upstreaming any portion of the ignore capability? If
> not, do you have any pointers for a target that may want to implement it?
> The one issue I see that might exist is that the scheduler wants a region
> begin and end as iterators, meaning contiguous instructions. It doesn’t
> really support the removal of instructions between those two points, so
> you’d have to synthesize a region, create a meta-iterator, or some other
> intrusive modification.
>
>
>
> JB
>
>
>
> *From:* Jinsong Ji [mailto:jji at us.ibm.com]
> *Sent:* Thursday, September 3, 2020 10:03 AM
> *To:* Nagurne, James; jmolloy at google.com
> *Cc:* llvm-dev at lists.llvm.org; Sander
> *Subject:* [EXTERNAL] RE: [llvm-dev] Machinepipeliner interface.
> shouldIgnoreForPipelining, actually not ignoring.
>
>
>
> As I mentioned before,
> this API was introduced by James, mostly for his out-of-tree
> implementations,
> the in tree implementation NEVER implement it.
>
> If it is causing confusion, I think we have two choices:
> 1. See whether James would like to upstream part of his code? Or any other
> target that would like to use this can post a patch to use it.
> 2. Remove this confusing API in tree.
>
>
> Best,
>
> Jinsong Ji (纪金松), PhD.
>
> XL/LLVM on Power Compiler Development
> E-mail: jji at us.ibm.com
>
> [image: Inactive hide details for "Nagurne, James" ---09/02/2020 01:43:29
> PM---Sorry to bring this thread from 3 months ago back, but I]"Nagurne,
> James" ---09/02/2020 01:43:29 PM---Sorry to bring this thread from 3 months
> ago back, but I’m running into this issue too. I do see tha
>
> From: "Nagurne, James" <j-nagurne at ti.com>
> To: Jinsong Ji <jji at us.ibm.com>, Sander <sanderruben at gmail.com>
> Cc: "llvm-dev at lists.llvm.org" <llvm-dev at lists.llvm.org>
> Date: 09/02/2020 01:43 PM
> Subject: RE: [EXTERNAL] Re: [llvm-dev] Machinepipeliner interface.
> shouldIgnoreForPipelining, actually not ignoring.
> ------------------------------
>
>
>
>
> Sorry to bring this thread from 3 months ago back, but I’m running into
> this issue too.
> I do see that shouldIgnore is not called in the MachinePipeliner, however,
> James’ comment doesn’t really resolve the issue or make the story any
> clearer.
>
> My summary of the comment is: “Hexagon and PPC9 do not need to ignore any
> instructions. However, in the case that you do, such as when the indvar
> update is explicit, this function is provided to allow the target to strip
> those instructions from the pipelined kernel.”
>
> However, the reality is that the implementation seems incomplete and
> there’s no instructions on how to achieve the desired result. Is it left as
> an exercise to the reader/implementer? Is there something I’m missing?
>
> J.B. Nagurne
> Code Generation
> Texas Instruments
>
> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org
> <llvm-dev-bounces at lists.llvm.org>] *On Behalf Of *Jinsong Ji via llvm-dev
> * Sent:* Tuesday, June 2, 2020 2:58 PM
> * To:* Sander
> * Cc:* llvm-dev at lists.llvm.org
> * Subject:* [EXTERNAL] Re: [llvm-dev] Machinepipeliner interface.
> shouldIgnoreForPipelining, actually not ignoring.
>
> Hi Sander:
>
> It may be confusing, but it is not a bug.
> And you are right, if you are using the in-tree machinepipeliner, it won't
> have any effects,
> because there is NO In tree reference of this API at all.
>
> You can see the comments when James introduced this API in
> https://reviews.llvm.org/D67167
>
>
>
> Best,
>
> Jinsong Ji (纪金松), PhD.
>
> XL/LLVM on Power Compiler Development
> E-mail: jji at us.ibm.com
>
> [image: Inactive hide details for Sander via llvm-dev ---06/02/2020
> 03:47:53 PM---Hi all, I think there is a mistake in the machinepipe]Sander
> via llvm-dev ---06/02/2020 03:47:53 PM---Hi all, I think there is a mistake
> in the machinepipeliner interface. In the
>
> From: Sander via llvm-dev <llvm-dev at lists.llvm.org>
> To: llvm-dev at lists.llvm.org
> Date: 06/02/2020 03:47 PM
> Subject: [EXTERNAL] [llvm-dev] Machinepipeliner interface.
> shouldIgnoreForPipelining, actually not ignoring.
> Sent by: "llvm-dev" <llvm-dev-bounces at lists.llvm.org>
> ------------------------------
>
>
>
>
>
> Hi all,
>
> I think there is a mistake in the machinepipeliner interface. In the
> TargetInstrInfo.h in the class PipelinerLoopInfo there is a function
> "bool shouldIgnoreForPipelining(const MachineInstr *MI)". The
> description says that if this function returns true for a given
> MachineInstr it will not be pipelined.
>
> However in reality it is not ignored and is being considered for
> pipelining. I implemented this function in my own backend, and put an
> instruction there that I want to be ignored, but still this instruction
> end up trying to be pipelined. I implemented the same way as in
> PPCInstrInfo.cpp, and I think it has the same bug. Is this a bug, or am
> I forgetting something?
>
> Kind regards,
>
> Sander Ruben
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
>
>
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