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

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

[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] 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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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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We have that behaves similarly to yours in this regard it seems. Specifically,
our target utilizes the HardwareLoop pass with CounterInReg true, and then
treats loops augmented by this pass as software pipeline candidates. It seems
PPC does this as well, but has CounterInReg false. Our loop body ends up looking
like this (in mildly simplified pseudocode):

%body:
                %indvar = PHI(%init, %preheader, %dec, %body)
                …
                %dec = subtract %indvar, count
                branch-compare (%dec > 0), %body
                branch %end
%end:
                …

The instructions I’ve bolded cause problems in the pipeliner as you are well
aware. They are the induction variable updates that shouldn’t be pipelined and
will be re-inserted or updated post-scheduling. When I saw “ignoreForPipelining”
I was excited that the problem was already solved, but alas! ☺

I’m in the process of trying to come up with a workaround pass before/after
pipelining that temporarily hides these instructions inside the branch, but I’m
always on the lookout for better alternatives.

Thanks for the response! I would appreciate anything that Hendrik could add to
the discussion as well. If it’s just that the implementation isn’t robust enough
to upstream, our team might be amenable to help out. I do, however, wonder if
the implementation was used in a custom scheduler rather than the default SMS
and expander. That would make generalizing quite a bit tougher.

JB

From: James Molloy [mailto:jmolloy at google.com]
Sent: Thursday, September 3, 2020 2:07 PM
To: Nagurne, James
Cc: Jinsong Ji; llvm-dev at lists.llvm.org; Sander; Hendrik Greving
Subject: Re: [EXTERNAL] RE: [llvm-dev] Machinepipeliner interface.
shouldIgnoreForPipelining, actually not ignoring.

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<mailto: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<mailto:jji at us.ibm.com>]
Sent: Thursday, September 3, 2020 10:03 AM
To: Nagurne, James; jmolloy at google.com<mailto:jmolloy at google.com>
Cc: llvm-dev at lists.llvm.org<mailto: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<mailto:jji at us.ibm.com>

[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<mailto:j-nagurne at
ti.com>>
To: Jinsong Ji <jji at us.ibm.com<mailto:jji at us.ibm.com>>, Sander
<sanderruben at gmail.com<mailto:sanderruben at gmail.com>>
Cc: "llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>" <llvm-dev at lists.llvm.org<mailto: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<mailto: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<mailto: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<mailto:llvm-dev at
lists.llvm.org>>
To: llvm-dev at lists.llvm.org<mailto: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<mailto: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<mailto:llvm-dev at lists.llvm.org>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev



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