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

[Jinsong Ji via llvm-dev]

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



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] 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

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 machinepipeSander 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

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