Hi all,
As a Ph.D. student majored in Software Reliability, I have used LLVM in many of my projects, such as the Anticipating Invariant (http://james0zan.github.io/AI.html) and some other undergoing ones.
Thus, it would be a great pleasure for me if I could take this opportunity to contribute to this awesome project.
After reading the idea list (http://llvm.org/OpenProjects.html), I was most interested in the idea of improving the “Pointer and Alias Analysis” passes.
Could you please give me some more tips or advices on how to get started on working on the application?
Simultaneously, I also have another idea about using LLVM to detect runtime bloat, just like the ThreadSanitizer tool for data races.
If there is anyone here who would like to mentor this project, could you please find some time to review the more detailed proposal on gist and give me some feedbacks?
P.S.
I do prefer the bloat detection tool, but I’m not sure about whether it is suitable for GSoC.
Thus I will apply for the Alias Analysis one if it is not suitable.
Thanks!
Hello John,
Thank you for your advices and congratulations~
I’ll read the code of cfl-aa and Giri first and make the decision of which project to pursue.
The choice will be reported to this thread once I made the determination (hopefully within this week).
Thanks!
You should check for yourself, but I don’t think anything prevents you from submitting two proposals. If you have time to write two strong proposals, I see no problem with that. Just make sure that any proposal you write is strong: it provides a concrete explanation of what you want to do, some justification for why it would benefit the community (short or long term), and why you’re the person qualified to do it. Proposals should also include a set of milestones and expected dates for completing those milestones. Regards, John Criswell
Hello John,
According to the FAQ, I can submit two proposals although at most one of them can be accepted.
Thus I will prepare a proposal for each of the two projects.
And, after reading the code of cfl-aa and several related papers, I’ve listed four milestones for the AA project:
-
In order to use the fast algorithm described in PLDI’13 [1], cfl-aa makes a simplification on the CFL defined in POPL’08 [2], which will lead to a reduction on precision (I’ve confirmed this observation with the author).
Thus a quantitative measurement on how much is the reduction is needed.
-
In cfl-aa, different fields of a same struct and the whole array are represented by a single node.
This is the reason of the problem 2, 4 listed in http://llvm.org/OpenProjects.html.
We should split these large nodes.
-
Handling special global variables, such as errno.
-
It seems that the current version of cfl-aa is an intraprocedural analysis.
If the time is enough, I think we may extend it to an interprocedural analysis.
The algorithm described in [3] can be applied to scaling it.
As for the bloat-detection project, the final result should be a tool that is verified by known bugs and a set of newly detected bugs.
Do you have any suggestions on these objectives?
Thanks!
[1] Fast Algorithms for Dyck-CFL-Reachability with Applications to Alias Analysis. PLDI’13
[2] Demand-Driven Alias Analysis for C. POPL’08
[3] Demand-Driven Context-Sensitive Alias Analysis for Java. ISSTA’11
Correct. Only one proposal will be accepted. I think the real question is whether the loss of precision matters, and if so, to which uses of alias analysis. SAFECode, for example, wants field information to determine type safety (so that it can optimize away type-safe loads and stores), so field sensitivity matters. Perhaps field sensitivity doesn’t matter for other applications (e.g., optimization). There’s no point in improving precision if it doesn’t help the analyses that people care about most. As part of your project, I think you should state the uses of alias analysis/points-to analysis that you’re aiming to improve and understand whether your proposed improvements will help that use. I would also recommend picking a use that matters to a significant portion of the LLVM community. For the bloat detection tool, I would like to be convinced that dynamic tracing will be, or can be, sufficiently efficient to be practical. I hate to ask, but I think you need to run an experiment with Giri to show that dynamic slicing is going to be practical for the executions that you expect to analyze. Either that, or you need to explain how you can use something more efficient than dynamic slicing (note that dynamic slicing and dynamic tracing are not the same, so be sure you’re correctly stating which one you need). In your proposal, be sure to include a set of milestones and how long you think you will need to achieve those milestones. I may have said that before, but it’s worth repeating. Regards, John Criswell
Got it.
I’ll try to find the applications for field-sensitivity (and interprocedural, etc) AA analysis.
And I’ll do some preliminary evaluation on the tracing/slicing part for the bloat detection.
Thanks!
Hello John,
I’ve finished the first version of my proposal on enhancing alias analysis.
The proposal can be downloaded at http://james0zan.github.io/resource/GSoC15-Proposal-AA.pdf.
I hope I’ve successfully justified the necessity and benefits of this project.
If possible, please find some time to review it and give me some more feedbacks.
Thank you very much!
P.S. I’m working on the other proposal, a couple of days is needed.
A few notes:
- “But these standard LLVM AA passes either take a large amount of time (Anderson Analysis at cubic time and large memory requirements)”
Neither of these is correct. Andersen’s is not cubic in practice, or large memory in practice, when implemented properly. GCC uses it by default as the points-to implementation, and it’s never even close to the top of the profile.
It takes about 1 second to do a million lines of code.
And one can do better (gcc’s impl is no longer state of the art).
-
The approach to field sensitivity you mention is not going to work very well, given how much casting occurs (everything is casted). I would suggest using the approach in http://dl.acm.org/citation.cfm?id=996835
-
George, cc’d, is planning on implementing both context sensitive and context-insensitive interprocedural analysis in cfl-aa the next month or two.
-
Using a BDD cloning approach for CFL-AA doesn’t make much sense, the whole point of CFL is not having to generate explicit points-to sets if you don’t want to. Plus, followup papers and researchers have never been able to reproduce the scalability of Whaley’s work.
Not to mention it’s done on Java. Java is a language where doing things like field-sensitivity always increase precision, which is not true for C.
If you really want to attempt this, I would suggest using one of the demand driven context-sensitive approaches that will be easy to fit in CFL.
Hello Daniel,
Thank you for your comments and sorry for my mistakes, I’ll revise them.
And I’ll for sure read the paper you mentioned and survey the recent researches before deciding the implementation technique.
To George:
May I know the exact plan of your attempt for making cfl-aa interprocedural?
I do think that this is the most valuable part of my proposal, but that makes no sense to do it twice.
Maybe I can work on the porting of the flow-sensitive method proposed by Prof. Ben Hardekopf at CGO11.
It is declared in his homepage that the published source code “is written for a pre-release version of LLVM 2.5 and does not work in current versions of LLVM”
Thanks!
Dear Mingxing,
I've attached comments to the PDF document and attached the document to this email. You may need to use an option in your PDF reader to see the comments (I made the comment using Apple's Preview application).
The most significant thing you need to do is to provide better justification for the work. What I (and I think others) want to see is an application of alias analysis that LLVM (or one of its sub-projects) is *already* using and how your project will improve that application of alias analysis.
You should also improve and fix the grammar and spelling (not so much for getting your proposal accepted but to practice doing well at that; it's something every Ph.D. student needs to practice).
Regards,
John Criswell
GSoC15-Proposal-AA.pdf (188 KB)
CFLAA already has some basic interprocedural analysis built in (see: tryInterproceduralAnalysis; it basically boils down to CFLAA grabbing the StratifiedSets for each function call and unifying sets based off of that instead of unifying everything ever). The only real changes I had in mind for it were:
- Adding context sensitivity (which kind of requires adding context sensitivity to CFLAA first)
- Making it less terrible for {mutually,indirectly,} recursive functions (While we’re building the sets for some function A(), the sets for A() aren’t accessible, so any call to A() from a function called by A() looks opaque).
If you want to take a stab at making IPA better/adding context sensitivity, you’re more than welcome to do so. I’m happy to work on other things in the meantime 
George
Thank you very much for all your advices!
I’ll revise the proposal according to them.
To George,
As mentioned in the former emails of this thread, I intend to prepare two proposals for the AA project listed in the idea list and the bloat detection project proposed by myself respectively and at most one of them will be accepted by GSoC.
Personally, I do prefer the second project since I’m more familiar with that field and the technique (static instrumentation) it uses.
According to the timeline of GSoC, the accepted proposals will only be announced at 27 April. Is it too late since I do not know which project will be selected or even none of them will be accepted until then?
Otherwise, will the mentors know the decision earlier (e.g., at 15 April after the slot is allocated to organizations)?
To John,
The proposal for bloat detection is also available now at http://james0zan.github.io/resource/GSoC15-Proposal-BloatDetection.pdf (not completely finished yet).
Some preliminary evaluation results on overhead and detecting ability based on a simple prototype are given.
(Actually I came up with this idea during my visitation in Columbia U and the prototype is also implemented at those days, but the project is paused until recent days due to my internship in Google and some other works.)
P.S. The tex template is downloaded at http://www.latextemplates.com/template/large-colored-title-article.
Once again, thank you for your time!
According to the timeline of GSoC, the accepted proposals will only be
announced at 27 April. Is it too late since I do not know which project will
be selected or even none of them will be accepted until then?
That's correct
Otherwise, will the mentors know the decision earlier (e.g., at 15 April
after the slot is allocated to organizations)?
It's against GSoC rules to disclose any information before the
official annoucement. So, "no".
I see…
FYI. I’ve submitted my proposal on bloat detection to GSoC.
And I’m still revising the other proposal on enhancing AA passes.
Thanks!
Hello John,
In fact I’m rushing toward a paper submission in recent days.
Thus I’m very sorry that I do have enough time to re-write a detailed timeline.
However, I’ve revised the “Preliminary Results” section and add some more information about the current prototype
(updated both on google-melange and the url http://james0zan.github.io/resource/GSoC15-Proposal-BloatDetection.pdf ).
I hope it will address your main concerns.
Thank you very much!
I understand. I provide feedback on proposals that interest me so that they have the greatest chance of getting accepted. That means that they should convince not only me but other potential reviewers as well, so I keep pushing for improvements. It’s also good exercise for writing fellowship and grant proposals. Competing for free money is a tough business. Regards, John Criswell