[llvm-dev] [RFC] Pass return status (original) (raw)

Serge Guelton via llvm-dev llvm-dev at lists.llvm.org
Wed Jul 15 01:33:29 PDT 2020

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.

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 EXPENSIVECHECK 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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