llvm dev - Sep 2019 - [RFC] changing variable naming rules

Personally I think the benefit-cost ratio is quite low here, and changing the
rules would bring little value to me.
Maybe I just don’t see that much benefits of changing the naming rules, having
got used to the current naming scheme after a few years with LLVM.


I'm a little bit curious to hear more about the experiences from the changes
in LLD.
I’m thinking about things like:

- ongoing reviews in Phabricator that suddenly needs major rebase

- merging of bugfixes to older release branches (I doubt that the script
supports doing the reverse rewrite as well)


When it comes to git blame and the ignore sweeping change file:

- Will that work when I do blame inside github (I assume it would need to be
configured to find the ignore file in some way)?
  Anyway, ignoring sweeping commits should probably not be the default in
github, since sometimes you want to find the sweeping commit.

- There are several more tools that also has blame functionality (e.g. Gerrit
Code Review that could be used by downstream forks).
  Just because there is a --ignore-revs option in newer git versions, it
wouldn't be used by all my tools. So I'm not sure it can be considered
as a solution.
 

Blame is also just one potential problem. I'd still end up with lots of
(IMHO) "irrelevant" changes when doing "git diff". For
example when trying to figure out what happened between LLVM 8.0.0 and LLVM
9.0.0 in some part of the code, by actually looking at code diffs and not just
the commit log.


Kind Regards,
Björn


From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Chris
Lattner via llvm-dev
Sent: den 5 september 2019 18:09
To: Rui Ueyama <ruiu at google.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [RFC] changing variable naming rules

Hi Rui,

FWIW, I’m very +1 on this.  I think you did a great job with the LLD migration,
and I’m really happy you used that as a relatively small scale proof point to
develop the methodology and see what issues shake out.

Thank you for driving this!

-Chris



On Sep 3, 2019, at 8:14 PM, Rui Ueyama via llvm-dev <mailto:llvm-dev at
lists.llvm.org> wrote:

Hi all,

To get wider visibility, build a broader consensus and address concerns on this
topic, I'm again raising this as an RFC. This is a proposal to change the
rule for variable names from CamelCase to camelBack _really this time_.

Background:

This has been proposed several times on this mailing list in the past. Most
recent one was by Michael Platings in February this year [1], and there seems to
be a general consensus that the status quo is not ideal.

In the previous RFC thread, I nominated lld [2] as a starter project for
renaming and made a sweeping change to rename variables in a few commits. This
renaming went well -- even though it broke buildbots, I managed to unbreak them
in a timely manner, and more importantly, it has been reported that several
downstream repos have successfully migrated to the new naming scheme using a
tool that I wrote to create sweeping changes. That being said, some claimed that
the renaming attempt didn't get enough attention, despite being discussed in
a thread that has 100+ emails. So I'm raising this topic as a new thread.

I propose we do the same thing to another relatively small subproject,
clang-tools-extras, to gain more experience, and then migrate the entire LLVM
codebase to the new style. It seems technically doable, and even though it would
cause a short-term pain, people seem to feel more comfortable with the new
naming scheme than the current one. I also believe that the migration won't
be that painful.

Objectives:

 - Migrating the entire LLVM repo including subprojects to the new naming scheme
without breaking them.
 - Many projects, especially LLVM and Clang have lots of out-of-tree downstream
repos. We need to provide a tool to rebase such repos to a commit after a
renaming.
 - The sweeping change shouldn't break `git blame`.

What I learned from the lld's naming scheme change:

 - There are many member variables in the LLVM/lld codebase that have the same
name as accessors ignoring case (i.e. many classes define foo() as an accessor
to a member variable Foo). Such variables would conflict with functions after
renaming, so we had to rename accessors by prepending `get`.

 - A single large sweeping change seemed to work better than small incremental
changes for downstream repos. Downstream repo maintainers rebased their trees to
a commit just prior to the sweeping change, apply my tool to rename all
variables in their trees, and then rebase the trees onto the sweeping change.
Because the tool creates the same diffs for existing code, downstream
maintainers basically only had to merge their diffs at the last step.
 - Even though my tool worked satisfactory, it couldn't rewrite code that
are excluded by #if, #ifdef and the like, because the clang-based tool
doesn't really see the code excluded by the preprocessor. That caused
several buildbot breakages.
 - git 2.23 (released in August) added a new option `--ignore-revs` to `git
blame` so that the command can take a list of commits that need to be ignored by
blame. Developers can set a default ignore file (typically named
`.git-blame-ignore-revs`) using `git config` so that blame automatically ignores
commits listed in the file. As far as I tried, that command worked pretty well
to ignore the sweeping change I made to lld, so the `git blame` issue seems a
solved problem now.

Migration plan:

Given the above findings, I propose we migrate to the new coding style in the
following steps.

 1. Change the codebase to eliminate name duplication between accessors and
members. This can be done incrementally with as many commits as we want.
 2. Complete the tool and apply it to the entire LLVM tree. I'll publish it
at GitHub so that people can take a look and try it out.
 3. Setup buildbots so that they checkout my GitHub tree, build it and run its
tests, to make sure that a sweeping change won't break them. (I don't
know how to configure buildbots, but I presume this step is doable.)
 4. Give a heads-up and submit a sweeping change to clang-tools-extras, and make
sure that that won't break anything.
 5. Give a heads-up and submit a sweeping change to the entire LLVM.

I'd like to submit a sweeping change after LLVM migrates to GitHub to
minimize confusion.

[1] http://lists.llvm.org/pipermail/llvm-dev/2019-February/130083.html
[2]
https://protect2.fireeye.com/url?k=ea62499f-b6f04480-ea620904-0cc47ad93db4-ecd933b4f617d714&q=1&u=https%3A%2F%2Fgithub.com%2Fllvm%2Fllvm-project%2Ftree%2Fmaster%2Flld
[3]
https://protect2.fireeye.com/url?k=3251f903-6ec3f41c-3251b998-0cc47ad93db4-9f9d4ccc78551a27&q=1&u=https%3A%2F%2Fgithub.com%2Fllvm%2Fllvm-project%2Fcommit%2F3837f4273fcc40cc519035479aefe78e5cbd3055
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On Fri, Sep 6, 2019 at 6:17 PM Björn Pettersson A via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I'm a little bit curious to hear more about the experiences from the
changes in LLD.
> I’m thinking about things like:
[..]
> - merging of bugfixes to older release branches (I doubt that the script
supports doing the reverse rewrite as well)
As someone who does a lot of these merges, this is the aspect that
worries me the most.

On Mon, Sep 9, 2019 at 8:25 PM Hans Wennborg <hans at chromium.org> wrote:
> On Fri, Sep 6, 2019 at 6:17 PM Björn Pettersson A via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > I'm a little bit curious to hear more about the experiences from
the
> changes in LLD.
> > I’m thinking about things like:
> [..]
> > - merging of bugfixes to older release branches (I doubt that the
script
> supports doing the reverse rewrite as well)
>
> As someone who does a lot of these merges, this is the aspect that
> worries me the most.
>
How large is a typical patch to merge that is backported from the next
major release to a previous dot release?
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> On Sep 9, 2019, at 4:24 AM, Hans Wennborg via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> On Fri, Sep 6, 2019 at 6:17 PM Björn Pettersson A via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> I'm a little bit curious to hear more about the experiences from
the changes in LLD.
>> I’m thinking about things like:
> [..]
>> - merging of bugfixes to older release branches (I doubt that the
script supports doing the reverse rewrite as well)
> 
> As someone who does a lot of these merges, this is the aspect that
> worries me the most.

Yes, I suspect you’re personally the one who is hit the worst by this.  I’m
super sensitive to that, because I really really really want to make sure that
releases are smooth and successful.

That said, are you aware of Rui’s work on lld?  He developed a process in which
people with significant out of tree patches were able to catch up, merge the
mega patch and move forward without significant churn.  I agree it is non-zero,
but it is very low, and the cost seems worth the price of forward progress and
avoiding perpetual stagnation.

-Chris