[llvm-dev] [GSoC 2016] Introduction & Feedback - Better Alias Analysis

Thu May 12 10:46:00 PDT 2016

> What's the magic rune to enable cfl-aa?

We have two magical opt flags: -use-cfl-aa and -use-cfl-aa-in-codegen :)

IIRC, both of them should cause CFLAA to be enabled as a last resort.

On Thu, May 12, 2016 at 12:38 AM, Daniel Berlin <dberlin at dberlin.org> wrote:

> (Just to note: the other issue i remember with CFL-AA is that it currently
> causes performance loss. This is quite common when you increase precision,
> because things move/change things they couldn't before, and often do so
> without the natural bounds imprecision provided before :P)
>
>
> On Thu, May 12, 2016 at 12:29 AM, James Molloy via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Yep, same. What's the magic rune to enable cfl-aa?
>>
>> James
>>
>> On Thu, 12 May 2016 at 05:58 Philip Reames via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> On 05/11/2016 05:16 PM, George Burgess IV via llvm-dev wrote:
>>>
>>> > After applying the patch on r267335 and bootstrap LLVM/clang with
>>> cfl-aa enabled on its own as well as behind basic-aa on an x86 machine, I
>>> ran test-suite with lit and saw no failed test cases
>>>
>>> Woohoo! This is great news. :D
>>>
>>> Awesome!
>>>
>>>
>>> I'm not sure how closely everyone is reading the intro emails, so we may
>>> get more help if we also send a slightly more targeted "Hey, CFLAA isn't
>>> obviously broken anymore. Please help us find any other problems/please
>>> report performance numbers to us," email. Whether we do that this very
>>> second, or when GSoC actually starts, is up to you.
>>>
>>> Once this lands in tree, I'll do a run on our codebase to see if I see
>>> anything obvious.  I suspect others can do the same.
>>>
>>>
>>> Philip
>>>
>>>
>>> On Wed, May 11, 2016 at 3:02 PM, Jia Chen via llvm-dev <
>>> <llvm-dev at lists.llvm.org>llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Dear LLVM community,
>>>>
>>>> I am a GSoC student this year working on the project of improving alias
>>>> analysis in LLVM.
>>>>
>>>> The proposal initially came from a discussion I had with various devs
>>>> on the mailing list some time ago [1]. The general goal of this project is
>>>> to make alias analysis (in particular, cfl-aa) "better", and to be more
>>>> concrete here is a list of objectives I had in mind:
>>>>
>>>> - Evaluating the current state of cfl-aa, and fix all known bugs.
>>>> - Improving the precision of cfl-aa. Although flow sensitivity may not
>>>> be very helpful for LLVM in its current form, field sensitivity should be
>>>> something important and I'll definitely try to add it to cfl-aa. Given the
>>>> complexity LLVM's memory model has, my guess is that I may need to spend
>>>> most of my summer on this task.
>>>> - Improving the performance of cfl-aa. (It is fairly efficient in its
>>>> current form, IMO. Further performance tuning may be needed if more
>>>> features such as field sensitivity were added.)
>>>> - Understanding how various clients interacts with cfl-aa, and
>>>> exploring various ways to minimize precision/performance losses caused by
>>>> the interaction.
>>>> - If time permits, maybe I can look at scev-aa and try to bring it back
>>>> to the compilation pipeline.
>>>>
>>>> I know these objectives are not as clear cut as other GSoC projects
>>>> people used to have, and it is hard to come up with a clear schedule as
>>>> well as a success metric. Nevertheless even if my contributions may seem
>>>> fragmented and incremental, I felt that as long as the work is useful to
>>>> the community, it is going to be the most valuable summer I've ever had as
>>>> a student.
>>>>
>>>> ** Current Status **
>>>>
>>>> I've submitted a simple patch last week (D19776) to fix a subtle bug in
>>>> cfl-aa. After applying the patch on r267335 and bootstrap LLVM/clang with
>>>> cfl-aa enabled on its own as well as behind basic-aa on an x86 machine, I
>>>> ran test-suite with lit and saw no failed test cases. I didn't time the
>>>> tests in any rigorous way, but it didn't look like cfl-aa add very
>>>> noticable performance overhead. It may be a good time, I think, to call for
>>>> people's help to test cfl-aa on their internal codebase. If everything goes
>>>> well, we should be able to safely turn on cfl-aa by default soon.
>>>>
>>>> Let me conclude this introduction by saying thank you for accepting my
>>>> proposal, and in particular I want to thank my mentors George and Hal for
>>>> the providing me with so much support and guidance. Please let me know if
>>>> you have any comments or suggestions.
>>>>
>>>> [1] http://lists.llvm.org/pipermail/llvm-dev/2016-March/096851.html
>>>>
>>>> --
>>>> Best Regards,
>>>>
>>>> --
>>>> Jia Chen
>>>> Department of Computer Science
>>>> University of Texas at Austinjchen at cs.utexas.edu
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>>
>>>
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>>
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