[llvm-dev] Preparing BOLT for LLVM monorepo

[Mehdi AMINI via llvm-dev]

Hi,

The instructions to build Bolt mention:
-DLLVM_ENABLE_PROJECTS="clang;lld;bolt"

It seems that clang is just a dependency here because you build C++ sources
during lit-tests execution, but it's not clear to me why we don't use the
host compiler or a prebuilt release here? Is there an intrinsic coupling
between clang and Bolt at HEAD?

Having to build clang at HEAD just to run the bolt test seems like a
non-trivial overhead for the casual developer, if the host compiler (or
some clang pulled with apt-get or your favorite package manager) would just
work.
Also in CI, even if you have a beefy enough machine, you add risks for the
Bolt bot to be broken because Clang/LLVM itself would be broken: so instead
of tracking Bolt bugs you end up tracking clang issues. Also the fact that
clang being broken implicating you can't test Bolt means that in the
meantime new bugs can land in Bolt and complicate auto-bisection.
(The same question applies to lld I think?)

Thanks!

-- 
Mehdi







On Tue, Dec 7, 2021 at 5:36 PM Rafael Auler <rafaelauler at fb.com> wrote:

> Hi folks,
>
>
>
> https://github.com/facebookincubator/BOLT branch “main” contains a merge
> proposal of BOLT into llvm-project. This is llvm from Nov 30th with 1016
> commits on top of it corresponding to the BOLT project.
>
> These 1016 commits would ideally be committed in a merge commit, merging
> LLVM as the first parent and BOLT as the second, and would be there only
> for the purposes of preserving project history. In this way, they should be
> easily skippable during a bisect of LLVM in the same way as the merge
> commit of flang. These commits represent the linear history of BOLT on top
> of rebased LLVM, so most commits are not buildable (since we can’t build a
> very old version of BOLT on top of a recent LLVM base). That’s why this is
> for history/blame only.
>
>
>
> We have addressed the issues in
> https://github.com/facebookincubator/BOLT/issues/248 and we are happy to
> continue working on any extra suggestions.
>
> Would it be better if we put this branch as a PR into llvm-project as a
> way to make it easier for people to review it? I don’t think we can put
> this into phabricator. However, I guess github’s bot will probably
> auto-close the PR. Also feel free to open new issues against our
> facebookincubator/BOLT project as a way to review it.
>
> Thanks
>
>
>
> *From: *Hans Wennborg <hans at chromium.org>
> *Date: *Thursday, November 4, 2021 at 5:42 AM
> *To: *Maksim Panchenko <maks at fb.com>
> *Cc: *Xinliang David Li <xinliangli at gmail.com>, Fāng-ruì Sòng <
> maskray at google.com>, Sriraman Tallam <tmsriram at google.com>, Chris Lattner
> <clattner at llvm.org>, Andrey Bokhanko <andreybokhanko at gmail.com>, Rafael
> Auler <rafaelauler at fb.com>, vladislav.khmelevskyi at huawei.com <
> vladislav.khmelevskyi at huawei.com>, tstellar at redhat.com <
> tstellar at redhat.com>, joker.eph at gmail.com <joker.eph at gmail.com>,
> echristo at gmail.com <echristo at gmail.com>, Nick Desaulniers <
> ndesaulniers at google.com>, llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org
> >
> *Subject: *Re: Preparing BOLT for LLVM monorepo
>
> Hi Maksim,
>
> On Tue, Nov 2, 2021 at 9:52 PM Maksim Panchenko <maks at fb.com> wrote:
> > We are still working on finalizing the exact logistics of the merge.
> >
> > However, we expect to follow the Flang project's footsteps and run the
> >
> > "--no-ff" merge to preserve the history of ~1K commits. We would like to
> >
> > ask for help and coordination from the release managers Tom Stellard and
> >
> > Hans Wennborg.
>
> I'm no longer involved in release management, and don't really have
> any opinion on how to merge this.
>
> Thanks,
> Hans
>
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