[llvm-dev] Filter optimization remarks by the hotness of the code region

Thu May 12 19:30:27 PDT 2016

> On May 12, 2016, at 7:17 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> From: "Adam Nemet" <anemet at apple.com <mailto:anemet at apple.com>>
> To: "Hal Finkel" <hfinkel at anl.gov <mailto:hfinkel at anl.gov>>
> Cc: "llvm-dev (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>>, "John McCall" <rjmccall at apple.com <mailto:rjmccall at apple.com>>
> Sent: Wednesday, May 11, 2016 12:45:32 PM
> Subject: Re: Filter optimization remarks by the hotness of the code region
> 
> 
> On May 11, 2016, at 3:37 AM, Hal Finkel <hfinkel at anl.gov <mailto:hfinkel at anl.gov>> wrote:
> 
> 
> From: "Adam Nemet" <anemet at apple.com <mailto:anemet at apple.com>>
> To: "Hal Finkel" <hfinkel at anl.gov <mailto:hfinkel at anl.gov>>
> Cc: "llvm-dev (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>>
> 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 <mailto: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?
> This is on my TODO list, but I've not started yet. Next few days seem unlikely too.
> 
> One thing we should now thing about is where this YAML encoding happens. It could be in the backend, or it could be in the remark handler in the frontend. If they'll be no real programmatic interaction in the frontend with the remark content (other than serializing it into YAML), it might make sense to make the YAML-serialization capability a property of the remark itself handled by its implementation in the backend.

I was under the impression that your notion of source locations is opaquely encoded and so something in the frontend would need to interpret them.  If that's not the case, then it doesn't matter much, I think, as long as the format supports certain kinds of query.  For example, a tool should be able to start with a source location and find the interesting remarks about it + surrounding code.  As long as the format allows that query to occur without needing pass-specific knowledge of a remark's schema, it's probably not all that important who outputs the data.
> 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.
> I've also thought about this, and this is certainly something we'll need to keep an eye on. It is specifically why I thought at first that we'd prefer to put the report-generation functionality into Clang. I'm happy, however, to postpone this unless and until it proves to be a problem. There's certainly a benefit to separating the functionality like this.

Right, I think we can handle this as optional configuration data for the pass's remark generation: the default is to generate all remarks (if this is enabled at all), but we can evolve ways to filter that, e.g. opting specific places into more comprehensive remarks.

John.

> 
> Thanks again,
> Hal
> 
> 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 <mailto:anemet at apple.com>>
> To: "llvm-dev (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>>
> Cc: "Hal Finkel" <hfinkel at anl.gov <mailto: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 <http://lists.llvm.org/pipermail/llvm-dev/2016-April/098492.html>
> [2] http://lists.llvm.org/pipermail/cfe-dev/2016-April/048526.html <http://lists.llvm.org/pipermail/cfe-dev/2016-April/048526.html>
> [3]
> http://blog.llvm.org/2014/11/loop-vectorization-diagnostics-and.html <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
> 
> 
> 
> 
> -- 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory

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