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

[via llvm-dev]

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.

To make this as painless as possible for all future generations of contributors, we should provide an in-tree script that will DTRT to set this up, and document it in the Getting Started page.  My understanding is that cloning can't set any config options like this automatically, so an easy one-time script is the next best thing.
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Rui Ueyama via llvm-dev
Sent: Tuesday, September 03, 2019 11:15 PM
To: llvm-dev
Subject: [llvm-dev] [RFC] changing variable naming rules

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://github.com/llvm/llvm-project/tree/master/lld
[3] https://github.com/llvm/llvm-project/commit/3837f4273fcc40cc519035479aefe78e5cbd3055
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