llvm dev - May 2019 - RFC: Extending optimization reporting

Hi Adam,

Thanks for your input.

If I understand correctly, you’re saying that we can handle the loop versioning
issue by explicitly identifying new loops as they are created. So, the
unswitching optimization, for example, would report that it unswitched loop-0 at
source location X, creating loop-1 and loop-2, and then later the vectorizer
would report that it was unable to vectorize loop-1 at source location X, and
later still that it was able to vectorize loop-2 at source location X. And at
each stage the optimization pass emitting the remark would get the version
information from loop metadata. Is that right?

If so, I think that is in basic agreement with what I wanted to do. I’ll need to
talk about it with some of my co-workers who have been thinking about ways to
solve this problem, but I like this direction.

As you say, unrolled loops are still a problem. There’s basically no place to
reliably hang metadata so that we can report that an optimization is working
with an unrolled portion of a loop.

I’m not sure about the inlining issue. At first glance it does sound as simple
as you suggest. However, my colleagues who have done a lot of work with inlining
tell me there are some complications that make recovering all of the necessary
data to form a coherent description of what has happened difficult. They’ve
explained it to me a couple of times, but I haven’t internalized the subtleties
yet. I’ll have to get back to you about this part.

-Andy

From: anemet at apple.com <anemet at apple.com>
Sent: Friday, May 03, 2019 2:27 PM
To: Kaylor, Andrew <andrew.kaylor at intel.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; Francis Visoiu Mistrih
<francisvm at apple.com>
Subject: Re: [llvm-dev] RFC: Extending optimization reporting

Hi Andrew,


On Apr 30, 2019, at 2:47 PM, Kaylor, Andrew via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

I would like to begin a discussion about updating LLVM's opt-report
infrastructure. There are some things I'd like to be able to do with
optimization reports that I don't think can be done, or at least aren't
natural to do, with the current implementation.

I understand that there is a lot of code in place already to produce
optimization remarks, and one of my explicit goals is to minimize the amount of
updating existing code while still enabling the new features I would like to
support. I have some ideas in mind for how to achieve what I'm proposing,
but I want to start out by just describing the desired results and if we can
reach a consensus on these being nice things to have then we can move on to
talking about the best way to get there.

I think the extensions I have in mind can broadly be organized into two
categories: (1) ways to support different sorts of output, and

As you may have seen Francis’ recent work, we have some plans in this area as
well.  Francis is going to cover that part ...


(2) ways to create connections between different events represented in the
report.

… let me cover this part.



Near as I can tell, the only support we have in the code base is for YAML
output. I think I could implement a new RemarkStreamer to get other formats, but
nothing in the LLVM code base does that. Is that correct?

I'd like to be able to:
- Embed some subset of optimizations remarks as annotations in the generated
assembly output
- Embed the remarks in the generated executable in a binary format consumed by
the Intel Advisor tool
- Produce text output in a format recognized by Microsoft Visual Studio or other
IDEs

The last of these is probably straightfoward since it's basically a
streaming format such as the current infrastructure expects. The other two seem
like they might be more complicated, since they involve keeping the information
around, potentially across LTO, and correlated with the evolving IR until the
final machine code or assembly is produced.

This leads me back to my second category of extensions, creating connections
between different events in the opt report. My goal here is to be able to
produce some kind of coherent report after compilation is complete that lets the
user make some sense of how the IR evolved over the course of compilation and
what effects that may have had on optimizations. This mostly has to do with the
handling of loops, vectorization, and inlining.

Let's say, for example, I've got code like this:

for (...)
    A
    if (lic)
        B
    C

And the loop-unswitch pass turns it into this:

if (lic)
    for (...)
        A; B; C
else
    for (...)
        A; C

Now let's say the vectorizer for some reason is able to vectorize the loop
in the else-clause but not the if-clause. (I don't know if this kind of
thing is possible with the current phase ordering, but I think this theoretical
example illustrates the idea anyway.)

I want some way to produce a report that tells the user about the existence of
the two loops that were created when we unswitched the loop so that we can then
tell the user in some sensible way that we couldn't vectorize one loop but
that we could vectorize the second.

I'm not sure what the opt-viewer would currently do with a case like this,
but what I want to avoid is getting stuck where the report we can emit
essentially conveys the following not very helpful information.

for (...)
  // Loop was unswitched.
  // Loop could not be vectorized because...
  // Loop was vectorized.
    A
    if (lic)
        B
    C

Instead I'd like to have a way to produce something like this:

for (...)
  // Loop was unswitched for condition (srcloc)
  //   Unswitched loop version #1
  //     Unswitched for IF condition (srcloc)
  //     Loop was not vectorized:
  //   Unswitched loop version #2
  //     Loop was vectorized

The primary thing missing, I think, is a way for the vectorizer to give some
indication of which version of the loop it is talking about in its optimization
remarks and maybe a way for the opt-viewer to be able to make sense of that.

Likewise, there are things I want to be able to track with inlining. Let's
say we go through the inlining pass pre-LTO and we make some decisions and
report them. Then during LTO we go through another round of inlining and
possibly make different decisions. I'd like to be able to either produce a
report that shows just the inlining from the LTO pass or produce a report that
shows a composite of all the inlining decisions that we made.

I think the general problem is that we’re currently missing the ability to
distinguish between code versions generated for the same source code, i.e. as
functions get inlined multiple times or as loops get versioned.  (There is also
loop-unrolling where we duplicate iterations with potentially removing the loop
altogether.)

I think that for the inlined case we can walk the inlinedAt metadata to identify
the code version in question.  For loops we could attach a version number to the
loop id.  Then as long as the versioning transformation properly declares the
new version, subsequent transformations can refer to code version they modify. 
Then on the client side we should have all the information to reconstruct what
happened.

I see this as essentially extending the DebugLoc field with code version
information in the remark.

This still wouldn’t handle unrolling which would require some new metadata but
it would be a good start.

What do you think?

Adam



We tried something like this with an inlining report before
(https://reviews.llvm.org/D19397), but it had the misfortune of being proposed
at about the same time that the current opt-viewer mechanism was being developed
and we didn’t manage to get aligned with that. I’m hoping that we can correct
that now.
_______________________________________________
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

-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20190508/acb3f7d2/attachment-0001.html>

> On May 8, 2019, at 11:11 AM, Kaylor, Andrew <andrew.kaylor at intel.com
<mailto:andrew.kaylor at intel.com>> wrote:
> 
> Hi Adam,
>  
> Thanks for your input.
>  
> If I understand correctly, you’re saying that we can handle the loop
versioning issue by explicitly identifying new loops as they are created. So,
the unswitching optimization, for example, would report that it unswitched
loop-0 at source location X, creating loop-1 and loop-2, and then later the
vectorizer would report that it was unable to vectorize loop-1 at source
location X, and later still that it was able to vectorize loop-2 at source
location X. And at each stage the optimization pass emitting the remark would
get the version information from loop metadata. Is that right?
Exactly!
>  
> If so, I think that is in basic agreement with what I wanted to do. I’ll
need to talk about it with some of my co-workers who have been thinking about
ways to solve this problem, but I like this direction.
>  
> As you say, unrolled loops are still a problem. There’s basically no place
to reliably hang metadata so that we can report that an optimization is working
with an unrolled portion of a loop.
>  
> I’m not sure about the inlining issue. At first glance it does sound as
simple as you suggest. However, my colleagues who have done a lot of work with
inlining tell me there are some complications that make recovering all of the
necessary data to form a coherent description of what has happened difficult.
They’ve explained it to me a couple of times, but I haven’t internalized the
subtleties yet. I’ll have to get back to you about this part.
Please keep me posted about this.

Thanks,
Adam
>  
> -Andy
>  
>  <>From: anemet at apple.com <mailto:anemet at apple.com>
<anemet at apple.com <mailto:anemet at apple.com>>
> Sent: Friday, May 03, 2019 2:27 PM
> To: Kaylor, Andrew <andrew.kaylor at intel.com <mailto:andrew.kaylor
at intel.com>>
> Cc: llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>>; Francis Visoiu Mistrih <francisvm at apple.com
<mailto:francisvm at apple.com>>
> Subject: Re: [llvm-dev] RFC: Extending optimization reporting
>  
> Hi Andrew,
> 
> 
> On Apr 30, 2019, at 2:47 PM, Kaylor, Andrew via llvm-dev <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>  
> I would like to begin a discussion about updating LLVM's opt-report
infrastructure. There are some things I'd like to be able to do with
optimization reports that I don't think can be done, or at least aren't
natural to do, with the current implementation.
>  
> I understand that there is a lot of code in place already to produce
optimization remarks, and one of my explicit goals is to minimize the amount of
updating existing code while still enabling the new features I would like to
support. I have some ideas in mind for how to achieve what I'm proposing,
but I want to start out by just describing the desired results and if we can
reach a consensus on these being nice things to have then we can move on to
talking about the best way to get there.
>  
> I think the extensions I have in mind can broadly be organized into two
categories: (1) ways to support different sorts of output, and
>  
> As you may have seen Francis’ recent work, we have some plans in this area
as well.  Francis is going to cover that part ...
> 
> 
> (2) ways to create connections between different events represented in the
report.
>  
> … let me cover this part.
> 
> 
>  
> Near as I can tell, the only support we have in the code base is for YAML
output. I think I could implement a new RemarkStreamer to get other formats, but
nothing in the LLVM code base does that. Is that correct?
>  
> I'd like to be able to:
> - Embed some subset of optimizations remarks as annotations in the
generated assembly output
> - Embed the remarks in the generated executable in a binary format consumed
by the Intel Advisor tool
> - Produce text output in a format recognized by Microsoft Visual Studio or
other IDEs
>  
> The last of these is probably straightfoward since it's basically a
streaming format such as the current infrastructure expects. The other two seem
like they might be more complicated, since they involve keeping the information
around, potentially across LTO, and correlated with the evolving IR until the
final machine code or assembly is produced.
>  
> This leads me back to my second category of extensions, creating
connections between different events in the opt report. My goal here is to be
able to produce some kind of coherent report after compilation is complete that
lets the user make some sense of how the IR evolved over the course of
compilation and what effects that may have had on optimizations. This mostly has
to do with the handling of loops, vectorization, and inlining.
>  
> Let's say, for example, I've got code like this:
>  
> for (...)
>     A
>     if (lic)
>         B
>     C
>  
> And the loop-unswitch pass turns it into this:
>  
> if (lic)
>     for (...)
>         A; B; C
> else
>     for (...)
>         A; C
>  
> Now let's say the vectorizer for some reason is able to vectorize the
loop in the else-clause but not the if-clause. (I don't know if this kind of
thing is possible with the current phase ordering, but I think this theoretical
example illustrates the idea anyway.)
>  
> I want some way to produce a report that tells the user about the existence
of the two loops that were created when we unswitched the loop so that we can
then tell the user in some sensible way that we couldn't vectorize one loop
but that we could vectorize the second.
>  
> I'm not sure what the opt-viewer would currently do with a case like
this, but what I want to avoid is getting stuck where the report we can emit
essentially conveys the following not very helpful information.
>  
> for (...)
>   // Loop was unswitched.
>   // Loop could not be vectorized because...
>   // Loop was vectorized.
>     A
>     if (lic)
>         B
>     C
>  
> Instead I'd like to have a way to produce something like this:
>  
> for (...)
>   // Loop was unswitched for condition (srcloc)
>   //   Unswitched loop version #1
>   //     Unswitched for IF condition (srcloc)
>   //     Loop was not vectorized:
>   //   Unswitched loop version #2
>   //     Loop was vectorized
>  
> The primary thing missing, I think, is a way for the vectorizer to give
some indication of which version of the loop it is talking about in its
optimization remarks and maybe a way for the opt-viewer to be able to make sense
of that.
>  
> Likewise, there are things I want to be able to track with inlining.
Let's say we go through the inlining pass pre-LTO and we make some decisions
and report them. Then during LTO we go through another round of inlining and
possibly make different decisions. I'd like to be able to either produce a
report that shows just the inlining from the LTO pass or produce a report that
shows a composite of all the inlining decisions that we made.
>  
> I think the general problem is that we’re currently missing the ability to
distinguish between code versions generated for the same source code, i.e. as
functions get inlined multiple times or as loops get versioned.  (There is also
loop-unrolling where we duplicate iterations with potentially removing the loop
altogether.)
>  
> I think that for the inlined case we can walk the inlinedAt metadata to
identify the code version in question.  For loops we could attach a version
number to the loop id.  Then as long as the versioning transformation properly
declares the new version, subsequent transformations can refer to code version
they modify.  Then on the client side we should have all the information to
reconstruct what happened.
>  
> I see this as essentially extending the DebugLoc field with code version
information in the remark.
>  
> This still wouldn’t handle unrolling which would require some new metadata
but it would be a good start.
>  
> What do you think?
>  
> Adam
> 
> 
>  
> We tried something like this with an inlining report before
(https://reviews.llvm.org/D19397 <https://reviews.llvm.org/D19397>), but
it had the misfortune of being proposed at about the same time that the current
opt-viewer mechanism was being developed and we didn’t manage to get aligned
with that. I’m hoping that we can correct that now.
> _______________________________________________
> 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
<https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20190514/97f2a016/attachment-0001.html>

Hi Adam,

I don’t have much to report here, but I wanted to let you know that I haven’t
forgotten about this completely.

I talked to Robert Cox about the inlining part of things, and he agreed that at
a high level the problem we want to solve is essentially like the loop
versioning problem in that we mostly need some kind of ID and a place to hang
it. He was a little nervous about depending on debug information because he’s
seen some things in the past where that changed how things were optimized. I
guess with the way debug info is implemented in LLVM there’s a more or less
continuous spectrum from just keep a couple of pieces of metadata to track
source location through to full DWARF support, so any attempt to solve this with
metadata that isn’t really debug info would be pointless and redundant. Speaking
for myself I’m also a little concerned about the tendency of debug info to get
dropped during optimization, but I guess that doesn’t happen as regularly as it
used to?

I hope to be able to do more with this soon.

-Andy

From: anemet at apple.com <anemet at apple.com>
Sent: Tuesday, May 14, 2019 9:49 AM
To: Kaylor, Andrew <andrew.kaylor at intel.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; Francis Visoiu Mistrih
<francisvm at apple.com>
Subject: Re: [llvm-dev] RFC: Extending optimization reporting




On May 8, 2019, at 11:11 AM, Kaylor, Andrew <andrew.kaylor at
intel.com<mailto:andrew.kaylor at intel.com>> wrote:

Hi Adam,

Thanks for your input.

If I understand correctly, you’re saying that we can handle the loop versioning
issue by explicitly identifying new loops as they are created. So, the
unswitching optimization, for example, would report that it unswitched loop-0 at
source location X, creating loop-1 and loop-2, and then later the vectorizer
would report that it was unable to vectorize loop-1 at source location X, and
later still that it was able to vectorize loop-2 at source location X. And at
each stage the optimization pass emitting the remark would get the version
information from loop metadata. Is that right?

Exactly!



If so, I think that is in basic agreement with what I wanted to do. I’ll need to
talk about it with some of my co-workers who have been thinking about ways to
solve this problem, but I like this direction.

As you say, unrolled loops are still a problem. There’s basically no place to
reliably hang metadata so that we can report that an optimization is working
with an unrolled portion of a loop.

I’m not sure about the inlining issue. At first glance it does sound as simple
as you suggest. However, my colleagues who have done a lot of work with inlining
tell me there are some complications that make recovering all of the necessary
data to form a coherent description of what has happened difficult. They’ve
explained it to me a couple of times, but I haven’t internalized the subtleties
yet. I’ll have to get back to you about this part.

Please keep me posted about this.

Thanks,
Adam



-Andy

From: anemet at apple.com<mailto:anemet at apple.com> <anemet at
apple.com<mailto:anemet at apple.com>>
Sent: Friday, May 03, 2019 2:27 PM
To: Kaylor, Andrew <andrew.kaylor at intel.com<mailto:andrew.kaylor at
intel.com>>
Cc: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>>; Francis Visoiu Mistrih <francisvm at
apple.com<mailto:francisvm at apple.com>>
Subject: Re: [llvm-dev] RFC: Extending optimization reporting

Hi Andrew,



On Apr 30, 2019, at 2:47 PM, Kaylor, Andrew via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

I would like to begin a discussion about updating LLVM's opt-report
infrastructure. There are some things I'd like to be able to do with
optimization reports that I don't think can be done, or at least aren't
natural to do, with the current implementation.

I understand that there is a lot of code in place already to produce
optimization remarks, and one of my explicit goals is to minimize the amount of
updating existing code while still enabling the new features I would like to
support. I have some ideas in mind for how to achieve what I'm proposing,
but I want to start out by just describing the desired results and if we can
reach a consensus on these being nice things to have then we can move on to
talking about the best way to get there.

I think the extensions I have in mind can broadly be organized into two
categories: (1) ways to support different sorts of output, and

As you may have seen Francis’ recent work, we have some plans in this area as
well.  Francis is going to cover that part ...



(2) ways to create connections between different events represented in the
report.

… let me cover this part.




Near as I can tell, the only support we have in the code base is for YAML
output. I think I could implement a new RemarkStreamer to get other formats, but
nothing in the LLVM code base does that. Is that correct?

I'd like to be able to:
- Embed some subset of optimizations remarks as annotations in the generated
assembly output
- Embed the remarks in the generated executable in a binary format consumed by
the Intel Advisor tool
- Produce text output in a format recognized by Microsoft Visual Studio or other
IDEs

The last of these is probably straightfoward since it's basically a
streaming format such as the current infrastructure expects. The other two seem
like they might be more complicated, since they involve keeping the information
around, potentially across LTO, and correlated with the evolving IR until the
final machine code or assembly is produced.

This leads me back to my second category of extensions, creating connections
between different events in the opt report. My goal here is to be able to
produce some kind of coherent report after compilation is complete that lets the
user make some sense of how the IR evolved over the course of compilation and
what effects that may have had on optimizations. This mostly has to do with the
handling of loops, vectorization, and inlining.

Let's say, for example, I've got code like this:

for (...)
    A
    if (lic)
        B
    C

And the loop-unswitch pass turns it into this:

if (lic)
    for (...)
        A; B; C
else
    for (...)
        A; C

Now let's say the vectorizer for some reason is able to vectorize the loop
in the else-clause but not the if-clause. (I don't know if this kind of
thing is possible with the current phase ordering, but I think this theoretical
example illustrates the idea anyway.)

I want some way to produce a report that tells the user about the existence of
the two loops that were created when we unswitched the loop so that we can then
tell the user in some sensible way that we couldn't vectorize one loop but
that we could vectorize the second.

I'm not sure what the opt-viewer would currently do with a case like this,
but what I want to avoid is getting stuck where the report we can emit
essentially conveys the following not very helpful information.

for (...)
  // Loop was unswitched.
  // Loop could not be vectorized because...
  // Loop was vectorized.
    A
    if (lic)
        B
    C

Instead I'd like to have a way to produce something like this:

for (...)
  // Loop was unswitched for condition (srcloc)
  //   Unswitched loop version #1
  //     Unswitched for IF condition (srcloc)
  //     Loop was not vectorized:
  //   Unswitched loop version #2
  //     Loop was vectorized

The primary thing missing, I think, is a way for the vectorizer to give some
indication of which version of the loop it is talking about in its optimization
remarks and maybe a way for the opt-viewer to be able to make sense of that.

Likewise, there are things I want to be able to track with inlining. Let's
say we go through the inlining pass pre-LTO and we make some decisions and
report them. Then during LTO we go through another round of inlining and
possibly make different decisions. I'd like to be able to either produce a
report that shows just the inlining from the LTO pass or produce a report that
shows a composite of all the inlining decisions that we made.

I think the general problem is that we’re currently missing the ability to
distinguish between code versions generated for the same source code, i.e. as
functions get inlined multiple times or as loops get versioned.  (There is also
loop-unrolling where we duplicate iterations with potentially removing the loop
altogether.)

I think that for the inlined case we can walk the inlinedAt metadata to identify
the code version in question.  For loops we could attach a version number to the
loop id.  Then as long as the versioning transformation properly declares the
new version, subsequent transformations can refer to code version they modify. 
Then on the client side we should have all the information to reconstruct what
happened.

I see this as essentially extending the DebugLoc field with code version
information in the remark.

This still wouldn’t handle unrolling which would require some new metadata but
it would be a good start.

What do you think?

Adam




We tried something like this with an inlining report before
(https://reviews.llvm.org/D19397), but it had the misfortune of being proposed
at about the same time that the current opt-viewer mechanism was being developed
and we didn’t manage to get aligned with that. I’m hoping that we can correct
that now.
_______________________________________________
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

-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20190522/6e5600cf/attachment.html>