[llvm-dev] [RFC] Pass return status

Thu Jul 16 04:38:44 PDT 2020

+1 This is definitely a useful feature.

Hashing functions would definitely get tricky over time. One way to encode
it would be encoding CFG structure. Order of BBs in CFG coupled with order
of instructions in each BB would be fairly fine, IMHO. Of course, such a
hash function should be invoked when we want to preserve CFG. In addition
to this, it could also be a post-order + pre-order traversal of DOM trees.
However, more importantly, I think one hash function may not serve all the
purpose. A hash function can be a part of AnalysisPass. Clients would
invoke such  a top-level wrapper to verify sanity of the analysis pass
which in-turn calls pass specific hash function to do the job.

On Thu, Jul 16, 2020 at 4:45 PM Serge Guelton via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> > Out of curiosity, does change here include changes to names, and other
> semantically-irrelevant changes (e.g., changing the order of operands in a
> PHI)?
>
> The hashing function used to detect changes is currently very simple: it
> only accounts for instruction opcode and order. So
> some semantically-irrelevant changes are ignored (as well as some relevant
> changes), and some are not.
> Permuting two independant instructions is not ignored, while permuting the
> operands of a sub is ignored.
>
> On Wed, Jul 15, 2020 at 4:55 PM Hal Finkel <hfinkel at anl.gov> wrote:
>
>>
>> On 7/15/20 3:33 AM, Serge Guelton via llvm-dev wrote:
>>
>> Hi folks,
>>
>> some more information on this feature -  as a reminder I started one
>> month ago to work on an expensive check that would verify that pass return
>> status is correctly reported by passes, i.e. no pass return « IR not
>> modified » while actually modifying it.
>> It took ~20 pass fixes to achieve that goal, as many passes were not
>> respectful of that contract, but as
>> of 3667d87a33d3c8d4072a41fd84bb880c59347dc0,  https://reviews.llvm
>> .org/D80916 has been merged in master and the check is active, which
>> should prevent further regression on that topic.
>>
>> Thanks a lot to @foad, @jdoerfert, @fhahn, @calixte (and others I'm sorry
>> to forgot) for their help during the reviews.
>>
>>
>> This is great news!
>>
>> Some years ago, we did some experiments on whether we could develop more
>> fixed-point optimization within LLVM's pipeline. This was not the only
>> impediment we identified, but it was a major one.
>>
>> Out of curiosity, does change here include changes to names, and other
>> semantically-irrelevant changes (e.g., changing the order of operands in a
>> PHI)?
>>
>>  -Hal
>>
>>
>>
>>
>>
>> On Fri, Jun 12, 2020 at 11:24 PM Mehdi AMINI <joker.eph at gmail.com> wrote:
>>
>>>
>>>
>>> On Thu, Jun 11, 2020 at 8:42 AM Serge Guelton via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hi folks,
>>>>
>>>> Per the documentation[0], whenever an LLVM pass doesn't modify the IR
>>>> it's run on, it
>>>> should return `false`--it's okay to return `true` if no change happen,
>>>> just less
>>>> optimal. In the New PM area, this is generally translated into a
>>>> `PreservedAnalyses::all()`.
>>>>
>>>> https://reviews.llvm.org/D80916 provides an `EXPENSIVE_CHECK` that
>>>> computes a
>>>> hash of the IR before and after the pass, and checks that any change is
>>>> correctly reported. The hash is currently incomplete (on purpose, let's
>>>> start
>>>> small), but it turns out a dozen of passes do not satisfy that
>>>> requirement.
>>>>
>>>> This could lead to various category of bugs (dangling references,
>>>> inconsistent
>>>> state, etc). This affects both New and Legacy PM, as passes tend to
>>>> wrap functions
>>>> that report their status.
>>>>
>>>> I wrote a bunch of patches for all failure detected by this check, as I
>>>> cannot land the
>>>> check now, it would break the buildbots :-) Any help to review the
>>>> remaining
>>>> ones [1] is appreciated.
>>>>
>>>> Once that check lands and we're relatively confident on the quality of
>>>> the
>>>> return status, some more optimizations could be triggered, like
>>>> https://reviews.llvm.org/D80707.
>>>>
>>>
>>> Awesome feature! I am really fond of these pieces of infrastructure that
>>> can defend against human mistakes and save countless hours of debugging
>>> when subtle issues arise.
>>>
>>> Thanks Serge,
>>>
>>> --
>>> Mehdi
>>>
>>>
>>>
>>>>
>>>>
>>>> [0] https://llvm.org/docs/WritingAnLLVMPass.html#the-runonmodule-method
>>>> [1] https://reviews.llvm.org/D81230
>>>>     https://reviews.llvm.org/D81236
>>>>     https://reviews.llvm.org/D81256
>>>>     https://reviews.llvm.org/D81238
>>>>     https://reviews.llvm.org/D81225
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
>> _______________________________________________
>> LLVM Developers mailing listllvm-dev at lists.llvm.orghttps://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>> _______________________________________________
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-- 
*Disclaimer: Views, concerns, thoughts, questions, ideas expressed in this
mail are of my own and my employer has no take in it. *
Thank You.
Madhur D. Amilkanthwar
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