llvm dev - Sep 2020 - [EXTERNAL] Re: Machinepipeliner interface. shouldIgnoreForPipelining, actually not ignoring.

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

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



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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llvm-dev at lists.llvm.org
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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 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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