[llvm-dev] bugpoint and the new pass manager

[Florian Hahn via llvm-dev]

> On Jan 26, 2021, at 22:32, Fangrui Song <maskray at google.com> wrote:
> 
> Forked the thread and changed the subject to talk about bugpoint specifically.
> CCed folks on https://bugs.llvm.org/show_bug.cgi?id=48813

Thanks!

> On 2021-01-26, Arthur Eubanks via llvm-dev wrote:
>> Hi all,
>> 
>> We've been fixing the various remaining issues in order to turn on the new
>> pass manager for the optimization pipeline, and it's about time to turn it
>> on. (Thanks to everyone who has helped with testing and fixing the new pass
>> manager!)
>> 
>> https://reviews.llvm.org/D95380 is the change that would happen, which sets
>> the CMake flag -DENABLE_EXPERIMENTAL_NEW_PASS_MANAGER=ON by default. This
>> affects anything that uses the LLVM_ENABLE_NEW_PASS_MANAGER macro, which
>> includes opt's handling of the `opt -instcombine` syntax, clang, and
>> ThinLTO in lld drivers. This does not affect the backend target-specific
>> codegen pipeline since that's mostly not been ported to use the new PM
>> infrastructure yet.
>> 
>> Here <https://bugs.llvm.org/show_bug.cgi?id=46649> is the umbrella bug for
>> turning on the new PM with blockers. The main one is loop unswitching on
>> divergent loop conditions is unsafe
>> <https://bugs.llvm.org/show_bug.cgi?id=48819>, which is being looked into.
>> There's also the LLVM C API and bugpoint that still use the legacy PM,
>> which can be ported later on and only block the removal of the legacy PM.
> 
> I have heard several users said they just use
> 
>  bugpoint -compile-custom -compile-command=./run a.ll
>  (this usage is similar to llvm-reduce)
> 
> but not any other command listed on https://llvm.org/docs/Bugpoint.html .
> (I am among them. https://llvm.org/docs/Bugpoint.html and
> https://llvm.org/docs/CommandGuide/bugpoint.html
> have no usage examples so I resorted to an external article
> http://logan.tw/posts/2014/11/26/llvm-bugpoint/ )

FWIW I often use bugpoint’s crashing pass reduction feature ( e.g. `bugpoint -O3 foo.bc` to reduce the sequence of passes exposing a crash) when investigating crashes in LLVM optimizations. This is unfortunately one of the features directly tied to the pass manager.

My main concern is that `opt` and `bugpoint` having different defaults seems to have potential to be quite confusing to user  (e.g. `opt -passes=default<O3>` crashes, but `bug point -passes=default<O3>` does not, `bug point -O3 ` works, but does not reproduce the failure, e.g. due to difference in the pipeline).

I don’t know how widely used bugpoint is, but it features prominently in the documentation, e.g. https://llvm.org/docs/HowToSubmitABug.html , which is linked from the getting started page and the documentation sidebar.

I think before switching, it would be good to at least update HowToSubmitABug. We should probably also add a note to Bugpoint’s docs and refer the the documentation for llvm-reduce. It is great if people take time to reduce the test cases for their bug reports and we should try to make the experience as smooth as possible

Cheers,
Florian