[llvm-dev] [RFC] Pass return status

[Hal Finkel via llvm-dev]

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 <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 
> <mailto: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 <mailto: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
>         <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 <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
>         <https://llvm.org/docs/WritingAnLLVMPass.html#the-runonmodule-method>
>         [1] https://reviews.llvm.org/D81230
>         <https://reviews.llvm.org/D81230>
>         https://reviews.llvm.org/D81236 <https://reviews.llvm.org/D81236>
>         https://reviews.llvm.org/D81256 <https://reviews.llvm.org/D81256>
>         https://reviews.llvm.org/D81238 <https://reviews.llvm.org/D81238>
>         https://reviews.llvm.org/D81225 <https://reviews.llvm.org/D81225>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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