[llvm-dev] Existing studies on the benefits of pointer analysis

[George Burgess IV via llvm-dev]

As of late-August 2015, putting CFL-AA behind BasicAA caused miscompiles
when trying to bootstrap Clang/LLVM, yeah. It didn't seem that there were
many new errors (I think it caused ~10 tests to fail, where fail = either
segv or produce the wrong output), but it did end up breaking things. I
don't recall if standalone CFL-AA causes miscompiles, but I highly doubt
the breakages I observed were BasicAA's fault.

WRT speed, `time make -j14` on my box (6c/12t) didn't show a meaningful
increase in compile time when CFL-AA gets enabled (read: it got lost in the
noise). So, I agree that it's probably fast enough at the moment; if we
want to enhance it, we should focus on making it bootstrap
clang+LLVM/making it more accurate.

On Mon, Mar 21, 2016 at 12:26 PM, Hal Finkel <hfinkel at anl.gov> wrote:

>
> ------------------------------
>
> *From: *"Daniel Berlin via llvm-dev" <llvm-dev at lists.llvm.org>
> *To: *"Renato Golin" <renato.golin at linaro.org>, "George Burgess IV" <
> george.burgess.iv at gmail.com>
> *Cc: *"llvm-dev" <llvm-dev at lists.llvm.org>, "Jia Chen" <
> jchen at cs.utexas.edu>
> *Sent: *Monday, March 21, 2016 2:07:44 PM
> *Subject: *Re: [llvm-dev] Existing studies on the benefits of pointer
> analysis
>
>
>
> On Mon, Mar 21, 2016 at 12:05 PM, Renato Golin <renato.golin at linaro.org>
> wrote:
>
>> On 21 March 2016 at 18:59, Daniel Berlin <dberlin at dberlin.org> wrote:
>> > Which is why i've never mentioned it or used it in the community ;)
>>
>> Makes sense. :)
>>
>>
>> > I would rather see someone spend their time getting SCEV-AA on by
>> default or
>> > CFL-AA on by default than doing another evaluation.
>>
>> But those may not be simple enough for a GSOC, that's why I mentioned it.
>>
>>
> CFL-AA should just be fixing performance regressions, and maybe a little
> bug fixing, which is hopefully easy enough.  It's already fast enough as a
> pass.
>
>
> My understanding from George is that there are self-hosting miscompiles if
> you disable all AA except for CFL-AA. This is what is preventing us from
> enabling it by default. George, is that right?
>
>  -Hal
>
>
> SCEV-AA would be harder (must make SCEV-AA faster).
>
> The analysis could not only get us a birds view of the problem ahead,
>> but also introduce new developers to AA, which would make their future
>> work on SCEV-AA or CFL-AA easier. Kind of a teaching tool to get more
>> AA-savvy people.
>>
>
> Sure.
>
>>
>> cheers,
>> --renato
>>
>
>
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>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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