[llvm-dev] bugpoint and the new pass manager

[Fangrui Song via llvm-dev]

Forked the thread and changed the subject to talk about bugpoint specifically.
CCed folks on https://bugs.llvm.org/show_bug.cgi?id=48813

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/ )

The few things tied to the legacy pass manager are these "other commands".
I hope the active users can

* help improve the documentation (several developers found bugpoint unintuitive
  to use and don't use features other than -compile-custom)
* Port legacy PM features to the new pass manager.
* If people think that new reducing strategies can be ported to llvm-reduce, and keep
   bugpoint as the few misc features that'll also look good:)

>The C API can be worked through (we may need to introduce replacements to
>the legacy pass manager APIs), but bugpoint will be tricky since it has so
>many legacy PM-specific hacks and we may need to trim it down if we want it
>to work with the new PM. Anyway, I don't think any of the remaining
>blockers are large enough to block the switch (but comments welcome).
>
>I'd like to turn on the new PM by default soonish, after the 12.x branch.
>Perhaps roughly a week from now barring any major newly discovered
>regressions?
>
>As for potential issues only uncovered after the switch, if there is a
>large issue I will roll it back, but for smaller issues I'd rather ask
>users to pin to the legacy PM while we fix the issues, either via the CMake
>flag -DENABLE_EXPERIMENTAL_NEW_PASS_MANAGER=OFF, or the corresponding
>compiler flags, like -flegacy-pass-manager for clang.
>
>Any concerns/comments?

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