[LLVMdev] Range Analysis GSoC 2011 Proposal

John Criswell
Wed, 23 Mar 2011 22:24:59 -0000

Dear Douglas,

Comments below.

On 3/23/11 8:06 AM, Douglas do Couto Teixeira wrote:
> Dear LLVM community,
>
> I would like to contribute to LLVM in the Google Summer of Code
> project. My proposal is listed below. Please let me know your comments.
>
>
>   Adding Range Analysis to LLVM
>
>
>
> Abstract
>
> The objective of this work is patch our implementation of range
> analysis into LLVM. I have a running implementation of range

Say "working implementing" instead of "running
implementation."

> analysis in LLVM, but it is not currently part of the main
> distribution. I propose to integrate our range analysis implementation
> into the Lazy Value Info (LVI) interface that LLVM provides. Range
> analysis finds the intervals of values that may be bound to the
> integer variables during the execution of programs and is useful in
> several scenarios: constant propagation, detection of potential buffer
> overflow attacks, dead branch elimination, array bound check
> elimination, elimination of overflow tests in scripting languages such
> as JavaScript and Lua, etc.
>
>
> Objective
>
> The objective of this project is to augment LLVM with range analysis.
> We will do this integration by patching the current implementation of
> range analysis that we have onto the Lazy Value Info (LVI) interface
> that LLVM already provides. In addition, we will develop new
> optimizations using LVI. In particular, we will provide a pass that
> performs conditional constant propagation [5], and elimination of
> dead-branches.
>
> Criteria of Success
>
>     * To improve substantially the precision of the current
>       implementation of LVI. Currently, LVI’s interface only allows a
>       client to know if a variable contains a constant. We want to
>       allow LVI to report that a variable either contains a constant,
>       or a known-range.
>     * To improve the current implementation of constant propagation
>       that LLVM uses. We hope to obtain a small performance gain on
>       the C benchmarks in the LLVM test suite, and a larger gain on
>       Java programs that are compiled using VMKit.
>     * To improve the implementation of JumpThreading, in such a way
>       that more dead-branches will be eliminated. Again, we hope to
>       achieve a small speed-up on the C benchmarks, and a larger
>       speed-up on the Java benchmarks.
>
>
>
> Background
>
> Range Analysis is a technique that maps integer variables to the
> possible ranges of values that they may assume through out

throughout is a single word in this instance.

> the execution of a program. Thus, for each integer variable, a range
> analysis determines its lower and upper limits. A very simple range
> analysis would, for instance, map each variable to the limits imposed
> by its type. That is, an 8-bit unsigned integer variable can be
> correctly mapped to the interval [0, 255], and an 16-bit signed
> integer can be mapped to [-32767, 32766].

You probably want to be consistent in how numbers are interpreted.  Use
the unsigned or signed interpretations in both examples above.

> However, the precision of this analysis can greatly be improved from
> information inferred from the program text.
>
> Ideally this range should be as constrained as possible, so that an
> optimizing compiler could learn more information about each variable.
> However, the range analysis must be conservative, that is, it will
> only constraint the range of a variable if it can

"constrain the range"

> prove that it is safe to do so. As an example, consider the program:
>
> i = read();
> if (i < 10) {
>  print (i + 1);
> else {
>  print(i - 1);
> }
>
> In this program we know, from the conditional test, that the value of
> ‘i’ in the true side of the branch is in the range [-INF, 9], and in
> the false side is in the range [10, +INF].
>
> During the Summer of Code 2010 I have designed and implemented, under
> the orientation of Duncan Sands, a non-iterative

Remove the word "have" in the sentence above.

> range analysis algorithm. Our implementation is currently fully
> functional, been able to analyze the whole LLVM test suite. For more
> details, see [4]. However, this implementation has never been
> integrated into the LLVM main trunc, for two reasons:
>
>    1. We use an intermediate representation called extended static
>       assignment form [6], which the LLVM contributors were reluctant
>       to use;
>    2. During the SoC 2010 we did not have time to completely finish
>       our implementation, and runtime numbers were available only by
>       the end of 2010.
>    3. There was not really an infra-structure already in place in LLVM
>       to take benefit of our analysis.
>

I think your text is a little unclear about point #3.  From your text
below, it sounds like LLVM lacks optimizations that utilize value range
analysis.  If that is the case, then you should state below (like you do
in the beginning of your proposal) what optimizations you'll write.

As an aside, we'd be interested in using trying out value-range analysis
in SAFECode (http://safecode.cs.illinois.edu) for use in static array
bounds checking.  Creating a static array bounds checking pass for
SAFECode using your analysis would probably be trivial.  The only
difficulty is that SAFECode is currently built using LLVM 2.7, and I'm
guessing that you're using a newer version of LLVM.


>
>
> In order to address the first item, we propose to integrate the
> intermediate representation directly into our analysis, yet, as a
> module that can be used in separate by other clients, if necessary.

This part isn't completely clear to me.  It sounds like what you're
suggesting is to write one analysis pass that internally constructs
e-SSA form and a second analysis that actually does the value-range
analysis.  It also sounds like you're writing the value-range analysis
so that it can be used with and without e-SSA form.  Is this correct, or
have I misinterpreted what you're saying?

Either way, I think the text above and the paragraph below about e-SSA
form could be made a little more clear.

> We are splitting the live ranges of variables using single-arity
> phi-functions, which are automatically handled by the SSA elimination
> pass that LLVM already includes. This live range splitting is only
> necessary for greater precision. We can do our live range analysis
> without it, although the results are less precise. A previous Summer
> of Code, authored by Andre Tavares, has shown that the e-SSA form
> increases the number of phi-functions in the program code by less than
> 10%, and it is very fast to build [6].
>
> The second item of our list of hindrances is no longer a problem. Our
> implementation is ready for use. We have been able to analyze the
> whole LLVM test suite, plus SPEC CPU 2006 - over 4 million LLVM
> bytecoes - in 44 seconds on a 2.4GHz machine. We obtain non-trivial
> bit size reductions for the small benchmarks, having results that
> match those found by previous, non-conservative works, such as
> Stephenson’s et al’s [2].

What kind of improvements do you see in large benchmarks?  If small
benchmarks yield good results and large benchmarks yield poor results,
then your analysis probably needs more work to be useful for real-world
programs.


> Moreover, our implementation is based on a very modern algorithm, by
> Su and Wagner [1], augmented with Gawlitza’s technique to handle
> cycles [3]. We believe that this is the fastest implementation of such
> an analysis.
>
> Finally, the third item is also no longer a problem. Presently LLVM
> offers the Lazy Value Info interface that reports when variables are
> constants. The current LVI implementation also provides
> infra-structure to deal with ranges of integer intervals. However, it
> does not contain a fully functional implementation yet, an omission
> that we hope to fix with this project.

Again, you need to be clear about whether any optimizations use this for
anything beyond seeing if a value is constant, and if not, describe what
optimizations you plan to write to fix that.

>
> Timeline and Testing Methodology
>
>    1. Change the LVI interface, adding a new method to it:
>       getRange(Value *V), so that we can, not only know if a variable is
>

No comma after can.

>    1. a constant, but also know its range of values whenever it is not
>       a constant.
>    2. Change the implementation of the prototype range analysis that
>       LVI already uses, so that it will use our implementation.
>    3. Implement the sparse conditional constant propagation to use
>       LVI. This, in addition to JumpThreading will be another client
>       that will use LVI.
>

Is there an advantage to using your analysis for conditional constant
propagation?  I see the value in range analysis, but I don't see what
your algorithm adds above what is already there.  Please specify.

>    1. Run experiments on the LLVM test suite to verify that our new
>       optimization improves on the current dead branch elimination
>       pass that LLVM already uses.
>    2. Run experiments using VMKit, to check the impact of dead branch
>       elimination on Java programs. We hope to deliver better
>       performance numbers in this case because Java, as a memory safe
>       language, is notorious for using many checks to ensure that
>       memory is used only in ways that obey the contract of the
>       variable types.
>

You may want to check that VMKit is working with the version of LLVM
that you're using.

>
>
> Biograph

Biography

>
> I am currently a third year Computer Science student at theFederal
> University of Minas Gerais <http://www.dcc.ufmg.br/dcc/>, Brazil, and
> I work as a research assistant at theProgramming Languages Lab
> <http://www2.dcc.ufmg.br/laboratorios/llp/>(LLP), in that university.

You might want to state more explicitly whether you're an undergraduate
or graduate student.  I'm guessing your an undergraduate but am not sure.

>
> I believe I am a good candidate to work in the proposed project
> because I have already a good knowledge of LLVM. I have

"on the proposed project" and "because I am already knowledgeable
about
LLVM"

> implemented range analysis, and currently it can find the ranges of
> integer variables used in the programs. In addition to this, I have
> been working as a programmer for three years, before joining the LLP
> as a research assistant, and I am experienced

I think you want to say that you worked as a programmer for three years
before joining LLP.

> with C++, the language in which LLVM is implemented. Furthermore, I am
> a student at a very good university (UFMG). To ground this statement I
> would like to point that the Department of Computer Science of UFMG
> got thebest mark <http://www.ufmg.br/online/arquivos/012977.shtml>in
the

"To justify" instead of "to ground" and "point
out" instead of just "point"

> Brazilian National Undergrad Exam (ENADE). Finally, I work on a lab in
> which three other students work with LLVM. We had

work in a lab

> three Summer of Codes on LLVM in the past, and a number of papers have
> been published out of these experiences.

I think you should just point out the SoC's that you've been involved
in.  The ones from your lab mates seems less relevant to me.

All in all, I think your wrote a good proposal.  You may want to send a
revised version to the list; others may want to comment on the things
that I think need to be clarified.

BTW, are you looking for a mentor, or has Duncan volunteered for this
year again?

Good luck!

-- John T.

>
> References
>
> 1. A Class of Polynomially Solvable Range Constraints for Interval
> Analysis without Widenings and Narrowings, Zhendong Su and David
> Wagner, In Theoretical Computer Science, Volume 345 , Issue 1, 2005.
>
> 2. Bitwidth Analysis with Application to Silicon Compilation, Mark
> Stephenson, Jonathan Babb, and Saman Amarasinghe, In Proceedings of
> the SIGPLAN conference on Programming Language Design and
> Implementation, 2000.
>
> 3. Polynomial Precise Interval Analysis Revisited.Thomas Gawlitza,
> Jérôme Leroux, Jan Reineke, Helmut Seidl, Grégoire Sutre, Reinhard
> Wilhelm: Efficient Algorithms 2009: 422-437
>
> 4. Linear Time Range Analysis with Affine Constraints.Douglas do Couto
> Teixeira and Fernando Magno Quintao Pereira
>
(http://homepages.dcc.ufmg.br/~douglas/projects/RangeAnalysis/RangeAnalysis.paper.pdf
>
<http://homepages.dcc.ufmg.br/%7Edouglas/projects/RangeAnalysis/RangeAnalysis.paper.pdf>)
>
> 5. Constant propagation with conditional branches. Mark N. Wegman and
> F. Kenneth Zadeck, In ACM Transactions on Programming Languages and
> Systems (TOPLAS), 181-210, 1991.
>
> 6. Efficient SSI Conversion.André Luiz C. Tavares, Fernando Magno
> Quintão Pereira, Mariza A. S. Bigonha and Roberto S. Bigonha. Simpósio
> Brasileiro de Linguagens de Programação. 2010.
>
> 7. ABCD: eliminating array bounds checks on demand, Rajkslav Bodik,
> Rajiv Gupta and Vivek Sarkar, In Proceedings of the SIGPLAN conference
> on Programming Language Design and Implementation, 2000.
>
> Contact Info
>
> Name:Douglas do Couto Teixeira
> e-mail 1:douglas at dcc dot ufmg dot br
> e-mail 2: douglasdocouto at gmail dot com
>
>
>
> --
> Douglas do Couto Teixeira

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