[llvm-dev] Profiling with LLVM.

Fri Feb 5 05:40:52 PST 2016

Dear Duncan,

I am generating branch-weights annotated IR files as described in the
documentation of LLVM, using profiling with instrumentation.
http://clang.llvm.org/docs/UsersManual.html#profiling-with-instrumentation

e.g.

llvm-profdata merge -output=$(BENCH).profdata default.profraw
> clang -S -emit-llvm -O3 -fprofile-instr-use=$(BENCH).profdata -o
> bench.prof.ll bench.c

The issue is that in some benchmarks I get crazy numbers in the annotated
metadata inside the generated *.ll files.

e.g.

!16 = !{!"branch_weights", i32 -2147483648, i32 0}
> !155 = !{!"branch_weights", i32 1075807200, i32 -1501637297}
> !181 = !{!"branch_weights", i32 -965299388, i32 218980800}

This should be a counter overflow.

Now the interesting thing is that by using these annotated files as input
for the BasicBlockFrequency analysis pass,the output seems to give correct
numbers, regarding the Frequency execution of each Basic Block, even though
few of the counters have overflowed.

This seems like a bug, unless I need to do specific configurations while
running the profiling part before the analysis.
>From your experience, would you say that the BasicBlockFrequency analysis
pass output is to be trusted? Is it known to be stable or do I need to be
really cautious and always inspect the output? Are there any common cases
of not having accurate profiling?

As I mentioned earlier, the analysis pass seems fine to me, but I have only
tested it for a number of benchmarks.

Best Regards.
Georgios Zacharopoulos

2016-02-04 18:13 GMT+01:00 Duncan P. N. Exon Smith <dexonsmith at apple.com>:

> It's up to the frontend (or whatever is generating the branch weights
> metadata) to scale the branch weights down appropriately.  You can have a
> look at how clang does it for an example (IIRC, it's in clang's
> lib/CodeGen/CodeGenPGO.cpp, but possibly just the caller is there and the
> scaling logic is somewhere in LLVM).
>
> When are your counters overflowing?  If they overflow during an
> optimization pass, that sounds like a bug.
>
> > On 2016-Feb-04, at 05:51, Giorgio Zacharo <giorgiozacharo at gmail.com>
> wrote:
> >
> > Dear Duncan,
> >
> > Thank you a lot for your feedback. I have a problem though. The branch
> weights counters overflow in some files and thus I get incorrect numbers.
> > Is there any way to find a workaround for that? Is is supposed to be a
> known bug or is it something that needs configuration on my part?
> >
> > Again, thank you a lot for your reply.
> >
> > Best Regards,
> > Georgios Zacharopoulos
> >
> > 2016-02-03 18:23 GMT+01:00 Duncan P. N. Exon Smith <dexonsmith at apple.com
> >:
> > (Ideally you would have sent this to llvm-dev at lists.llvm.org (and
> > CC'ed me) so that the answer is there for others to find in the
> > archive.  If you have any follow-up questions, please add llvm-dev
> > to the CC list.)
> >
> > Assuming you already have branch weights, you should use the
> > BlockFrequencyInfo analysis to calculate the block frequencies.  At
> > the Machine level, there's a MachineBlockFrequencyInfo analysis.
> >
> > The frequencies shouldn't be used in isolation, but with reference
> > to each other.
> >
> > Cheers,
> > Duncan
> >
> > > On 2016-Feb-03, at 00:43, Giorgio Zacharo <giorgiozacharo at gmail.com>
> wrote:
> > >
> > > Dear Duncan,
> > >
> > > My name is Georgios Zacharopoulos and I am a PhD student at University
> of Lugano, in Switzerland. I am currently working on a research project and
> for a part of it I need to use some profiling information generated by
> existing LLVM tools from the latest version of LLVM. In more detail, I am
> trying to map profiling information generated from LLVM (Profiling with
> instrumentation) to IR.
> > > (
> http://clang.llvm.org/docs/UsersManual.html#profiling-with-instrumentation
> )
> > >
> > > Specifically, I want to annotate each Basic Block with its respective
> execution frequency. In order to accomplish that I am using the branch
> weight metadata annotated to conditional branches in the terminator
> instructions of Basic Blocks. These branch weight information as well as
> the entry counter of each function are generated by the already existing
> profiling tools of LLVM.
> > >
> > > Are you aware of such frequency annotation for each BB already
> existing? Do you know whether this is feasible using the information of the
> branch weights and the entry counters of functions? I want to verify that I
> am doing it in the correct way, but unfortunately I have not come up with a
> clean way to do this. If you could give me any tips regarding that, it
> would be extremely helpful to me.
> > >
> > > Thank you a lot in advance.
> > >
> > > --
> > > Best Regards,
> > > Georgios Zacharopoulos
> > >
> > > PhD student
> > > Faculty of Informatics
> > > Università della Svizzera italiana
> > > Lugano, Switzerland
> >
> >
> >
> >
> > --
> > Best Regards,
> > Georgios Zacharopoulos
> >
> > PhD student
> > Faculty of Informatics
> > Università della Svizzera italiana
> > Lugano, Switzerland
>
>

-- 
Best Regards,
Georgios Zacharopoulos

PhD student
Faculty of Informatics
Università della Svizzera italiana
Lugano, Switzerland
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