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

Serge Guelton via llvm-dev llvm-dev at lists.llvm.org
Thu Jul 16 04:15:16 PDT 2020

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 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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