[LLVMdev] GSoC - Range Analysis

[Victor Campos]

> What version of LLVM does your analysis use currently?

We are working with LLVM 3.0 (stable release)

> It sounds like your analysis is fast.  Can you show results on how fast it
> is on various programs?  Do you have measurements on how much memory it
> uses?  How large is the largest program you've compiled with it?

Yes, we have a very extensive report about it. Take a look into the
pdf at
http://code.google.com/p/range-analysis/source/browse/trunk/doc/Manuscript/paper.pdf

In particular, take a look into fig.10, that shows program size vs
runtime. We are truly linear on the program size. The largest program
that we have compiled is SPEC CPU 2006 gcc. It takes less than 10
seconds to analyze it. This is the largest program that we had here.
Its constraint graph has almost 1.5 million nodes.

> We in the SAFECode project would be interested in trying your analysis out
> for eliminating bounds checks.  We just haven't had the time to do that
yet
> (I'm assuming you're the same person that wanted to do Value Range
analysis
> last year).

No, that was Douglas, a friend of mine. He is in Texas nowadays. Now I
am responsible for the range analysis project at UFMG. There are two
other guys working with me on it too. And there are a few users,
outside our school that use the pass. If you guys are willing to give
our pass a try, we will be more than happy to help you. Notice that we
have put some detailed instructions at
http://code.google.com/p/range-analysis/wiki/HowToUseRangeAnalysisInAnotherPass
about how to use our range analysis.

> The idea of integrating your pass as a lazy value pass sounds good.  The
> question I have is what optimizations benefit from LazyInfo?  I only see
> one or two transforms that use it in LLVM 3.0.

Yes, I do not think there are many optimizations that use LazyInfo so
far. But maybe, backed by a more aggressive analysis, LazyInfo will be
more effective. I can implement dead-code elimination based on value
ranges, and I think it can be quite effective, even more in code that
is produced out of type-safe languages, such as Java, which is full of
safety checks.

2012/3/29 John Criswell <criswell at illinois.edu>

>  On 3/29/12 3:59 PM, Victor Campos wrote:
>
> Dear LLVMers,
>
>     I have been working on Douglas's range analysis, and today, after
> toiling with it for two years, we have a very mature and robust
> implementation, which is publicly available at
> http://code.google.com/p/range-analysis/. We can, at this point,
> perform range analysis on very large benchmarks in a few seconds. To
> give you an idea, we take less than 10 seconds to globally analyze
> SPEC 2006 gcc benchmark with function inlining enabled. And the
> analysis is fairly precise. We have a gallery of examples at
> http://code.google.com/p/range-analysis/wiki/gallery that will give
> you an idea of what kind of information we can find. Our analysis
> comes together with a dynamic profiler that points the minimum and
> maximum values that each variable assumes during program execution
> too. And it uses a live range splitting strategy to obtain data-flow
> sparsity that is lightning fast. It is more than 100x faster than the
> original implementation of SSI in LLVM 2.7, for instance. There are a
> number of LLVMers, outside my university, that use our analysis.
>
>
> What version of LLVM does your analysis use currently?
>
> It sounds like your analysis is fast.  Can you show results on how fast it
> is on various programs?  Do you have measurements on how much memory it
> uses?  How large is the largest program you've compiled with it?
>
>
>     So, I would like to propose a summer of code that consists in (i)
> integrating our infra-structure in the LLVM main tree, and (ii)
> writing a dead-code elimination pass that uses the analysis as a
> client. So far people have been using our analysis to check for buffer
> overflows, and to eliminate array bound checks.
>
>
> If possible, I think you should first implement the dead-code elimination
> optimization that uses your analysis to show how much of a performance win
> your analysis provides for LLVM.  If the optimization is sufficiently fast
> and provides enough of a speedup, then it can be integrated into LLVM
> (because you will have proof that it is a performance win; that will help
> convince current developers with commit access to review your code for
> inclusion).
>
> We in the SAFECode project would be interested in trying your analysis out
> for eliminating bounds checks.  We just haven't had the time to do that yet
> (I'm assuming you're the same person that wanted to do Value Range analysis
> last year).
>
>
>  Yet, I think there is
> a lot of potential to optimizations. Dead code elimination at branches
> would be one such optimization. Given that the analysis is pretty
> mature, I think that it would not be too difficult to integrate it in
> the current infra-structure that LLVM offers, e.g., Lazy Values. As
> for dead-code, we already can flag variables that have impossible
> intervals, in which the lower bound is larger than the upper bound.
> So, it is only a matter of adapting it to remove this code.
>
>
>
> Regarding a dead-code elimination algorithm, I can see something like an
> Aggressive Dead Code Elimination pass using your analysis to prove that
> certain branches are never taken.  One thing you could do would be to write
> a pass that looks for icmp instructions and uses your analysis to change
> them to true/false when possible.  SCCP, ADCE, and other optimizations
> would then take care of the rest.
>
> The idea of integrating your pass as a lazy value pass sounds good.  The
> question I have is what optimizations benefit from LazyInfo?  I only see
> one or two transforms that use it in LLVM 3.0.
>
>
>
>  I would like to hear from you what you think about this Summer of Code
> project. If you think it could be interesting, I will write a proposal
> richer in details.
>
>
> My interest in range analysis is for using it with SAFECode, but if it can
> be used for standard compiler optimization and gets integrated into LLVM,
> all the better for me.
>
> -- John T.
>
>
>  Sincerely yours,
>
>  Victor
>
>
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