llvm dev - May 2016 - Filter optimization remarks by the hotness of the code region

Hi Hal,
> On May 10, 2016, at 5:39 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> Hi Adam,
> 
> I think would be a really useful feature to have. I don't think that
the backend should be responsible for filtering, but should pass the relative
hotness information to the frontend. Given that these diagnostics are not just
going to be used for -Rpass and friends, but also for generating reports by
other tools (see the discussion around D19678, for example), I think it is
important to allow the frontend to filter.
I am not sure I follow, can you please elaborate.  Are you saying that for
example in the listing use case we don’t want the filtered diagnostics?  In
other words it should be up to the remark handler to decide whether it wants
filtered or unfiltered remarks?
> The frontend, or other tool, might also want to collect information from
different compilation jobs to provide the user with an overall ranking.
Strictly speaking about PGO, the different compilation jobs get the same PGO
with the aggregated profile, thus the hotness calculated should be global.  I am
not sure why an extra aggregation step is necessary.
> The default diagnostic, furthermore, does not provide enough information to
identify a code "region". I think that the pass generating the
diagnostic needs to provide the information, however, we could certainly create
some utility functions that take a pointer to the BFI analysis and a Value* that
can do the right thing in most simple cases.
Good point.

Thanks for your feedback.

Adam
> 
> Thanks again,
> Hal
> 
> ----- Original Message -----
>> From: "Adam Nemet" <anemet at apple.com>
>> To: "llvm-dev (llvm-dev at lists.llvm.org)" <llvm-dev at
lists.llvm.org>
>> Cc: "Hal Finkel" <hfinkel at anl.gov>
>> Sent: Wednesday, May 4, 2016 1:12:17 PM
>> Subject: Filter optimization remarks by the hotness of the code region
>> 
>> This idea came up a few times recently [1][2] so I’d like start
>> prototyping it.  To summarize, we can emit optimization remarks
>> using the -Rpass* options.  These are currently emitted by
>> optimizations like vectorization[3], unrolling, inlining and since
>> last week loop distribution.
>> 
>> For large programs however this can amount to a lot of diagnostics
>> output to sift through.  Filtering this by the hotness of the region
>> can help to focus the user on performance opportunities that are
>> likely to pay off.
>> 
>> The approach I am thinking of taking is to install a wrapper as the
>> diagnostics handler that will only forward to the original handler
>> if the region of code is considered hot.  This will be installed by
>> a new pass that will use BlockFrequencyInfo to determine the top N
>> hot regions.
>> 
>> This is at very early stage right now.  I would appreciate any
>> feedback.
>> 
>> Thanks,
>> Adam
>> 
>> [1] http://lists.llvm.org/pipermail/llvm-dev/2016-April/098492.html
>> [2] http://lists.llvm.org/pipermail/cfe-dev/2016-April/048526.html
>> [3]
>> http://blog.llvm.org/2014/11/loop-vectorization-diagnostics-and.html
> 
> -- 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory

----- Original Message -----
> From: "Adam Nemet" <anemet at apple.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "llvm-dev (llvm-dev at lists.llvm.org)" <llvm-dev at
lists.llvm.org>
> Sent: Wednesday, May 11, 2016 1:15:42 AM
> Subject: Re: Filter optimization remarks by the hotness of the code region
> 
> Hi Hal,
> 
> > On May 10, 2016, at 5:39 PM, Hal Finkel <hfinkel at anl.gov>
wrote:
> > 
> > Hi Adam,
> > 
> > I think would be a really useful feature to have. I don't think
> > that the backend should be responsible for filtering, but should
> > pass the relative hotness information to the frontend. Given that
> > these diagnostics are not just going to be used for -Rpass and
> > friends, but also for generating reports by other tools (see the
> > discussion around D19678, for example), I think it is important to
> > allow the frontend to filter.
> 
> I am not sure I follow, can you please elaborate.  Are you saying
> that for example in the listing use case we don’t want the filtered
> diagnostics?  In other words it should be up to the remark handler
> to decide whether it wants filtered or unfiltered remarks?
We might or might not want them. The user might want to select different ratios
and filters.
 
> > The frontend, or other tool, might also want to collect information
> > from different compilation jobs to provide the user with an
> > overall ranking.
> 
> Strictly speaking about PGO, the different compilation jobs get the
> same PGO with the aggregated profile, thus the hotness calculated
> should be global.  I am not sure why an extra aggregation step is
> necessary.
I agree. However, I think that the frontend might employ a combination of
factors in deciding what information to present. We might, for example, have
pick different hotness thresholds for different kinds of remarks.

Especially since we're likely going with a design for the optimization
reports where the frontend just creates some YAML files with the diagnostic
information, and then a separate tool processes the files to produce reports, I
think that we should give those tools the maximum about of practical
flexibility. Such a tool might provide the user with non-trivial filtering
options.

Thanks again,
Hal
> 
> > The default diagnostic, furthermore, does not provide enough
> > information to identify a code "region". I think that the
pass
> > generating the diagnostic needs to provide the information,
> > however, we could certainly create some utility functions that
> > take a pointer to the BFI analysis and a Value* that can do the
> > right thing in most simple cases.
> 
> Good point.
> 
> Thanks for your feedback.
> 
> Adam
> 
> > 
> > Thanks again,
> > Hal
> > 
> > ----- Original Message -----
> >> From: "Adam Nemet" <anemet at apple.com>
> >> To: "llvm-dev (llvm-dev at lists.llvm.org)" <llvm-dev
at lists.llvm.org>
> >> Cc: "Hal Finkel" <hfinkel at anl.gov>
> >> Sent: Wednesday, May 4, 2016 1:12:17 PM
> >> Subject: Filter optimization remarks by the hotness of the code
> >> region
> >> 
> >> This idea came up a few times recently [1][2] so I’d like start
> >> prototyping it.  To summarize, we can emit optimization remarks
> >> using the -Rpass* options.  These are currently emitted by
> >> optimizations like vectorization[3], unrolling, inlining and since
> >> last week loop distribution.
> >> 
> >> For large programs however this can amount to a lot of diagnostics
> >> output to sift through.  Filtering this by the hotness of the
> >> region
> >> can help to focus the user on performance opportunities that are
> >> likely to pay off.
> >> 
> >> The approach I am thinking of taking is to install a wrapper as
> >> the
> >> diagnostics handler that will only forward to the original handler
> >> if the region of code is considered hot.  This will be installed
> >> by
> >> a new pass that will use BlockFrequencyInfo to determine the top N
> >> hot regions.
> >> 
> >> This is at very early stage right now.  I would appreciate any
> >> feedback.
> >> 
> >> Thanks,
> >> Adam
> >> 
> >> [1]
> >> http://lists.llvm.org/pipermail/llvm-dev/2016-April/098492.html
> >> [2] http://lists.llvm.org/pipermail/cfe-dev/2016-April/048526.html
> >> [3]
> >>
http://blog.llvm.org/2014/11/loop-vectorization-diagnostics-and.html
> > 
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> 
> 
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

> On May 11, 2016, at 3:37 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> ----- Original Message -----
>> From: "Adam Nemet" <anemet at apple.com>
>> To: "Hal Finkel" <hfinkel at anl.gov>
>> Cc: "llvm-dev (llvm-dev at lists.llvm.org)" <llvm-dev at
lists.llvm.org>
>> Sent: Wednesday, May 11, 2016 1:15:42 AM
>> Subject: Re: Filter optimization remarks by the hotness of the code
region
>> 
>> Hi Hal,
>> 
>>> On May 10, 2016, at 5:39 PM, Hal Finkel <hfinkel at anl.gov>
wrote:
>>> 
>>> Hi Adam,
>>> 
>>> I think would be a really useful feature to have. I don't think
>>> that the backend should be responsible for filtering, but should
>>> pass the relative hotness information to the frontend. Given that
>>> these diagnostics are not just going to be used for -Rpass and
>>> friends, but also for generating reports by other tools (see the
>>> discussion around D19678, for example), I think it is important to
>>> allow the frontend to filter.
>> 
>> I am not sure I follow, can you please elaborate.  Are you saying
>> that for example in the listing use case we don’t want the filtered
>> diagnostics?  In other words it should be up to the remark handler
>> to decide whether it wants filtered or unfiltered remarks?
> 
> We might or might not want them. The user might want to select different
ratios and filters.
> 
>>> The frontend, or other tool, might also want to collect information
>>> from different compilation jobs to provide the user with an
>>> overall ranking.
>> 
>> Strictly speaking about PGO, the different compilation jobs get the
>> same PGO with the aggregated profile, thus the hotness calculated
>> should be global.  I am not sure why an extra aggregation step is
>> necessary.
> 
> I agree. However, I think that the frontend might employ a combination of
factors in deciding what information to present. We might, for example, have
pick different hotness thresholds for different kinds of remarks.
> 
> Especially since we're likely going with a design for the optimization
reports where the frontend just creates some YAML files with the diagnostic
information, and then a separate tool processes the files to produce reports, I
think that we should give those tools the maximum about of practical
flexibility. Such a tool might provide the user with non-trivial filtering
options.
I think this all makes sense.  So I guess the steps are:

1. YAML support for the existing remarks
2. Add optional relative hotness to the opt-remark API
3. Exposed relative hotness in the YAML output

Are you working on 1 or should I get started?

There was another issue that came up while discussing this with John McCall.  We
could have a pretty large number of such remarks.  I imagine that if the
post-processing is performed by the external tool, it may be useful to
effectively enable all optimization remarks (i.e.
-Rpass/-Rpass-missed/-Rpass-analysis=loop-vectorize/inline/etc.).  For large
programs this may not be feasible and we may need to locally filter the remarks
in LLVM.  This however seems like a somewhat orthogonal issue that we could
probably postpone for now.

Thanks for your input!
Adam
> 
> Thanks again,
> Hal
> 
>> 
>>> The default diagnostic, furthermore, does not provide enough
>>> information to identify a code "region". I think that the
pass
>>> generating the diagnostic needs to provide the information,
>>> however, we could certainly create some utility functions that
>>> take a pointer to the BFI analysis and a Value* that can do the
>>> right thing in most simple cases.
>> 
>> Good point.
>> 
>> Thanks for your feedback.
>> 
>> Adam
>> 
>>> 
>>> Thanks again,
>>> Hal
>>> 
>>> ----- Original Message -----
>>>> From: "Adam Nemet" <anemet at apple.com>
>>>> To: "llvm-dev (llvm-dev at lists.llvm.org)"
<llvm-dev at lists.llvm.org>
>>>> Cc: "Hal Finkel" <hfinkel at anl.gov>
>>>> Sent: Wednesday, May 4, 2016 1:12:17 PM
>>>> Subject: Filter optimization remarks by the hotness of the code
>>>> region
>>>> 
>>>> This idea came up a few times recently [1][2] so I’d like start
>>>> prototyping it.  To summarize, we can emit optimization remarks
>>>> using the -Rpass* options.  These are currently emitted by
>>>> optimizations like vectorization[3], unrolling, inlining and
since
>>>> last week loop distribution.
>>>> 
>>>> For large programs however this can amount to a lot of
diagnostics
>>>> output to sift through.  Filtering this by the hotness of the
>>>> region
>>>> can help to focus the user on performance opportunities that
are
>>>> likely to pay off.
>>>> 
>>>> The approach I am thinking of taking is to install a wrapper as
>>>> the
>>>> diagnostics handler that will only forward to the original
handler
>>>> if the region of code is considered hot.  This will be
installed
>>>> by
>>>> a new pass that will use BlockFrequencyInfo to determine the
top N
>>>> hot regions.
>>>> 
>>>> This is at very early stage right now.  I would appreciate any
>>>> feedback.
>>>> 
>>>> Thanks,
>>>> Adam
>>>> 
>>>> [1]
>>>> http://lists.llvm.org/pipermail/llvm-dev/2016-April/098492.html
>>>> [2]
http://lists.llvm.org/pipermail/cfe-dev/2016-April/048526.html
>>>> [3]
>>>>
http://blog.llvm.org/2014/11/loop-vectorization-diagnostics-and.html
>>> 
>>> --
>>> Hal Finkel
>>> Assistant Computational Scientist
>>> Leadership Computing Facility
>>> Argonne National Laboratory
>> 
>> 
> 
> -- 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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