[llvm-dev] [RFC] Pass return status

[Nicolai Hähnle via llvm-dev]

On Wed, Jul 15, 2020 at 10:34 AM Serge Guelton via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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.
>

Awesome, thank you!

Cheers,
Nicolai


>
> Thanks a lot to @foad, @jdoerfert, @fhahn, @calixte (and others I'm sorry
> to forgot) for their help during the reviews.
>
>
> 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
>>>
>>> _______________________________________________
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>>> llvm-dev at lists.llvm.org
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>>>
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