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

Nagurne, James via llvm-dev
Tue, 08 Sep 2020 22:02:50 -0000

I greatly appreciate you going back to gather that intel, James. It actually
helps my understanding of the whole pipeliner puzzle quite a bit!

I did identify, like you, that the MachinePipeliner pass (more precisely,
SwingSchedulerDAG) was fairly rigid in that the target doesn’t get much of a say
in some heuristics or in the generation of the result loop. I’m definitely still
in a learning phase and am poking around to see where we can customize some
things. It might very well come down to the same decision to write our own pass
that implements our own scheduler, and then utilize ModuloSchedule and an
Expander to make changes. One interesting topic that came up was unrolling to
get more utilization of the FUs. Someone had already identified this
optimization in https://reviews.llvm.org/D53005, but it seems to have gone
stale. I’m also curious about customizing the expander, since we too have some
ways to make the prolog and epilog more efficient.

My current solution hides the PHI and IndVar update instructions added by
HardwareLoops inside the branch and rematerializes them after the stock
MachinePipeliner runs. Not having to do that would be great, but now that it’s
implemented, I think I can stop bothering you guys for historical data! ☺

Thanks again!

JB

From: James Molloy [mailto:jmolloy at google.com]
Sent: Monday, September 7, 2020 1:49 AM
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,

Having not worked on this for circa one year I've gone and refreshed my
memory.

We have a pretty capable implementation of swing modulo scheduling downstream,
distinct from the MachinePipeliner implementation. Historically,
MachinePipeliner had very tight coupling between the finding of a suitable
schedule and emitting the code that adheres to that schedule.

I spent quite a bit of time separating the two; this led to the
"ModuloSchedule" and "ModuloScheduleExpander" classes, which
we use downstream too (we actually have a minor variant, where we use
predication for the prolog and epilog). However, the code that analyzes loops
and determines a good schedule is completely custom. It is in this code that we
use "shouldIgnoreForPipelining".

All of this code is designed to go upstream. There is nothing stopping us
putting it upstream (modulo some non-upstreamable features that may have crept
in, but those can be removed). The major hurdle is that the target architecture
is NOT upstream, and no existing upstream architecture looks similar enough
apart from Hexagon (RISC, VLIW, predicate register file), and we don't have
enough knowledge of Hexagon to use it as a testing target.

We toyed with the idea of coming up with a toy architecture just so we could
have something to test against upstream, but the personpower required wasn't
available.

As always, it was with the best of intentions, and I do think that the cleanup
of MachinePipeliner was worth it (the PeelingScheduleExpander is much easier to
reason about, IMHO).

Cheers,

James

On Thu, 3 Sep 2020 at 20:41, Nagurne, James <j-nagurne at
ti.com<mailto:j-nagurne at ti.com>> wrote:
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<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<mailto: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>

Error! Filename not specified."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>

Error! Filename not specified.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

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