[LLVMdev] Contribute a new precise pointer analysis to LLVM

[lian li]

On Wed, Oct 23, 2013 at 3:07 AM, Chandler Carruth <chandlerc at google.com> wrote:
> On Tue, Oct 22, 2013 at 9:55 AM, Hal Finkel <hfinkel at anl.gov> wrote:
>>
>> ----- Original Message -----
>>
>> It might also be interesting to try with -O3; I don't know if we have any
>> significant vectorizable loops in the test suite with a large number of
>> arrays, but if we do, this kind of analysis should help.
>
>
> We perform loop vectorization at -O2 now?
>

We did an experimental with -O3, and the compilation overhead is about 32%.

The compilation time is reported by running the analysis once over the
linked bc file.
It turns out that for the LLVM test suite, running the analysis over
the linked bc file doesn't introduce much extra overhead.

Note that we run the analysis only once in the experiment. The
analysis is not rerun after it is invalidated by an optimization pass.

>>
>>
>> >
>> > The overall overhead of compiling the whole test suite using our
>> > analysis is 36.5%.
>>
>> Maybe a good candidate for -fexpensive-optimizations? (or -O4?)
>
>
> It would also be helpful to know what the execution performance impact is to
> provide context.

According to our experiments with SPEC, we haven't observed
significant impact in terms of execution performance.
In our experiments, we run a list of 8 optimizations that require
alias analysis (including -licm, -gvn, -bb-vectorize,...), and for
each optimization we rerun our analysis so that
its result can be used by the optimization pass.  The biggest
performance improvement we observed is close to 3%, but for some
benchmarks we also have a slow down of 2%.
The overall performance improvement is less than 0.5%.

Regards,
Lian


>
>>
>>
>> > The biggest overhead is  observed in
>> > "SingleSource/Benchmarks/Misc/flops-5", where we are 5 times slower:
>> > 0.07s (with our analysis) compared to 0.0133s(default).
>> > This may not be accurate as the compilation time in both cases are
>> > quite small.
>>
>> In my experience, compile-time measurements from a normal test-suite
>> configuration less than 0.1s are too noisy to use.
>>
>> >
>> > Remind that the compilation time is reported by running our analysis
>> > over individual files, the compilation time overhead will be larger
>> > if
>> > we link all individual compilation units together and run the
>> > analysis over the whole application.
>>
>> Yes, LTO tests would be interesting too.
>>
>> Thanks again,
>> Hal
>>
>> >
>> > Regards,
>> > Lian
>> >
>> > On Sat, Oct 19, 2013 at 7:56 AM, Evan Cheng <evan.cheng at apple.com>
>> > wrote:
>> > >
>> > > On Oct 17, 2013, at 10:51 PM, lian li <lianli at gmail.com> wrote:
>> > >
>> > >> Thanks, Chris.
>> > >>
>> > >> We are interested in contributing it to LLVM itself. Our manager
>> > >> agrees to commit resources for maintenance needs if it is accepted
>> > >> by
>> > >> the community.
>> > >
>> > > This is great. Please make sure Oracle legal sign off on explicitly
>> > > granting LLVM the use of the patents associated with the work.
>> > >
>> > > On the compile time front, can you do a comparison with the LLVM
>> > > default and your new AA? You can just build the LLVM test suite.
>> > >
>> > > Thanks,
>> > >
>> > > Evan
>> > >
>> > >>
>> > >> Regards,
>> > >> Lian
>> > >>
>> > >> On Fri, Oct 18, 2013 at 3:43 PM, Chris Lattner
>> > >> <clattner at apple.com> wrote:
>> > >>>
>> > >>> On Oct 17, 2013, at 5:20 PM, lian li <lianli at gmail.com> wrote:
>> > >>>
>> > >>> Hi All,
>> > >>>
>> > >>> This is Lian Li  from Oracle Labs in Brisbane Australia.
>> > >>>
>> > >>> We have developed a precise and highly efficient pointer analysis
>> > >>> framework on top of LLVM,  The approach is flow, context, and
>> > >>> field
>> > >>> sensitive, details are described in the two papers below:
>> > >>>
>> > >>> "Boosting the performance of flow-sensitive points-to analysis
>> > >>> using
>> > >>> value flow" (in ESEC-FSE 2011), and
>> > >>> "Precise and scalable context-sensitive pointer analysis via
>> > >>> value
>> > >>> flow graph" (in ISMM 2013).
>> > >>>
>> > >>> The analysis was initially developed for the purpose of bug
>> > >>> checking,
>> > >>> and is now extended to support compiler optimizations. We have
>> > >>> tested
>> > >>> it with existing compiler optimizations in LLVM, and have seen
>> > >>> promising results.
>> > >>>
>> > >>> We are now considering to make this analysis available to the
>> > >>> LLVM
>> > >>> community, and contribute resources for future maintenance needs,
>> > >>> provided that there is enough interest. We think that a new
>> > >>> precise
>> > >>> pointer analysis in LLVM can enable more new optimization and
>> > >>> analysis
>> > >>> techniques to be developed in LLVM.
>> > >>>
>> > >>> Any people interested in seeing a new precise pointer analysis in
>> > >>> LLVM?
>> > >>>
>> > >>>
>> > >>> This sounds very interesting.  Even if it isn't fast enough to be
>> > >>> used with
>> > >>> (for example) clang -O2 or -O3, having such a precise analysis
>> > >>> would be a
>> > >>> great baseline for doing other alias analysis work.
>> > >>>
>> > >>> Are you interested in contributing this to LLVM itself, or just
>> > >>> asking if
>> > >>> people would be interested to see the code in some other form?
>> > >>>
>> > >>> -Chris
>> > >>
>> > >>
>> > >>
>> > >> --
>> > >> // Copyright @ 2011 Authorized by ** LIAN LI **
>> > >> _______________________________________________
>> > >> LLVM Developers mailing list
>> > >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> > >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> > >
>> >
>> >
>> >
>> > --
>> > // Copyright @ 2011 Authorized by ** LIAN LI **
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>



-- 
// Copyright @ 2011 Authorized by ** LIAN LI **