[llvm-dev] [RFC] High-Level Code-Review Documentation Update

Chris Bieneman via llvm-dev llvm-dev at lists.llvm.org
Mon Nov 18 13:38:04 PST 2019

One thing I wish our code review policy expressed was the intent of code reviews.

Personally, I believe the intent of code reviews is to improve the quality of code, to avoid the introduction of defects that another contributor could identify, and generally to benefit from the experience of other contributors.

One behavior that runs contrary to that intent (which has occasionally occurred in LLVM), is having your friend LGTM your patches as a rubber stamp. I'm not sure the right way to address that, but a clear statement of intent for code reviews could be a good first step.


> On Nov 18, 2019, at 9:54 AM, Philip Reames via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> All of the variations suggested seem reasonable to me.  I minorly prefer James' wording, but any of them are better than nothing.  
> Philip
> On 11/18/19 7:53 AM, Finkel, Hal J. wrote:
>> On 11/18/19 4:29 AM, James Henderson wrote:
>>> Only a single LGTM is required.  Reviewers are expected to only LGTM
>>> patches they're confident in their knowledge of.  Reviewers may review
>>> and provide suggestions, but explicitly defer LGTM to someone else. 
>>> This is encouraged and a good way for new contributors to learn the code.
>>> Whilst I get what you're trying to say, I'm not particularly comfortable with this particular suggestion as it stands: I regularly am one of two or three active reviewers on something, often spread out over different timezones, and I might be happy with it, in which case I'd signal that with an LGTM, but others might not be ready for it to be committed. Sometimes I'll ask for the author to wait to commit until one or more of the other reviewers are happy too, but other times I forget to explicitly say this. Perhaps a couple of sentences could be added to the end of this paragraph to capture this approach:
>>> "If multiple reviewers have been actively reviewing the patch, it is generally courteous to allow all of them a chance to give their LGTM before committing, after one LGTM has been received. The reviewer who gives the original LGTM may suggest an appropriate amount of time to wait before committing in this case."
>>> What I want to avoid is me (UK timezone) making some suggestions on a patch proposed by someone e.g. in the US, then a reviewer from the US getting into an active discussion, proposing a counter-suggestion, which gets adopted and LGTMed by that reviewer, resulting in a commit before I've had a chance to follow up on my comments etc. Obviously I can make post-commit requests, but sometimes it feels like the bar for suggestions post-commit is higher, and therefore my comments might not reach that level etc.
>> I agree with this. I was planning on proposing wording along the lines of the following, adding to the original suggestion:
>> When providing an unqualified LGTM (approval to commit), it is the responsibility of the reviewer to have reviewed all of the discussion and feedback from all reviewers ensuring that all feedback has been addressed and that all other reviewers will almost surely be satisfied with the patch being approved. If unsure, the reviewer should provide a qualified approval, (e.g., "LGTM, but please wait for @someone, @someone_else"). You may also do this if you are fairly certain that a particular community member will wish to review, even if that person hasn't done so yet (although don't wait for more than one week if that         person has not responded; if you think something is "must see" by a wider audience, it should have an RFC). If it is likely that others will want to review a recently-posted patch, especially if there might be objections, but no one else has done so yet, it is also polite to provide a qualified approval (e.g., "LGTM, but please wait for a couple days in case others wish to review").
>> Thoughts?
>>  -Hal
>>> James
>>> On Sat, 16 Nov 2019 at 16:37, Philip Reames via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>> + 1 in general, a couple of suggestions
>>> On 11/14/19 7:46 PM, Finkel, Hal J. via llvm-dev wrote:
>>> > Hi, everyone,
>>> >
>>> > I've been fielding an increasing number of questions about how our 
>>> > code-review process in LLVM works from people who are new to our 
>>> > community, and it's been pointed out to me that our documentation on 
>>> > code reviews is both out of date and not as helpful as it could be to 
>>> > new developers.
>>> >
>>> >    http://llvm.org/docs/DeveloperPolicy.html#code-reviews <http://llvm.org/docs/DeveloperPolicy.html#code-reviews>
>>> >
>>> > I would like to compose a patch to update this, but before I do that, I 
>>> > want to highlight some of my thoughts to get feedback. My intent is to 
>>> > capture our community best practices in writing so that people new to 
>>> > our community understand our processes and expectations. Here are some 
>>> > things that I would like to capture:
>>> >
>>> >   1. You do not need to be an expert in some area of the compiler to 
>>> > review patches; it's fine to ask questions about what some piece of code 
>>> > is doing. If it's not clear to you what is going on, you're unlikely to 
>>> > be the only one. Extra comments and/or test cases can often help (and 
>>> > asking for comments in the test cases is fine as well).
>>> Authors are encouraged to interpret questions as reasons to reexamine
>>> the readability of the code in question.  Structural changes, or further
>>> comments may be appropriate.
>>> >
>>> >   2. If you review a patch, but don't intend for the review process to 
>>> > block on your approval, please state that explicitly. Out of courtesy, 
>>> > we generally wait on committing a patch until all reviewers are 
>>> > satisfied, and if you don't intend to look at the patch again in a 
>>> > timely fashion, please communicate that fact in the review.
>>> >
>>> >   3. All comments by reviewers should be addressed by the patch author. 
>>> > It is generally expected that suggested changes will be incorporated 
>>> > into the next revision of the patch unless the author and/or other 
>>> > reviewers can articulate a good reason to do otherwise (and then the 
>>> > reviewers must agree). If you suggest changes in a code review, but 
>>> > don't wish the suggestion to be interpreted this strongly, please state 
>>> > so explicitly.
>>> >
>>> >   4. Reviewers may request certain aspects of a patch to be broken out 
>>> > into separate patches for independent review, and also, reviewers may 
>>> > accept a patch conditioned on the author providing a follow-up patch 
>>> > addressing some particular issue or concern (although no committed patch 
>>> > should leave the project in a broken state). Reviewers can also accept a 
>>> > patch conditioned on the author applying some set of minor updates prior 
>>> > to committing, and when applicable, it is polite for reviewers to do so.
>>> >
>>> >   5. Aim to limit the number of iterations in the review process. For 
>>> > example, when suggesting a change, if you want the author to make a 
>>> > similar set of changes at other places in the code, please explain the 
>>> > requested set of changes so that the author can make all of the changes 
>>> > at once. If a patch will require multiple steps prior to approval (e.g., 
>>> > splitting, refactoring, posting data from specific performance tests), 
>>> > please explain as many of these up front as possible. This allows the 
>>> > patch author to make the most-efficient use of his or her time.
>>> If the path forward is not clear - because the patch is too large to
>>> meaningful review, or direction needs to be settled - it is fine to
>>> suggest a clear next step (e.g. landing a refactoring) followed by a
>>> re-review.  Please state explicitly if the path forward is unclear to
>>> prevent confusions on the part of the author. 
>>> >
>>> >   6. Some changes are too large for just a code review. Changes that 
>>> > should change the Language Reference (e.g., adding new 
>>> > target-independent intrinsics), adding language extensions in Clang, and 
>>> > so on, require an RFC on *-dev first. For changes that promise 
>>> > significant impact on users and/or downstream code bases, reviewers can 
>>> > request an RFC (Request for Comment) achieving consensus before 
>>> > proceeding with code review. That having been said, posting initial 
>>> > patches can help with discussions on an RFC.
>>> >
>>> > Lastly, the current text reads, "Code reviews are conducted by email on 
>>> > the relevant project’s commit mailing list, or alternatively on the 
>>> > project’s development list or bug tracker.", and then only later 
>>> > mentions Phabricator. I'd like to move Phabricator to be mentioned on 
>>> > this line before the other methods.
>>> >
>>> > Please let me know what you think.
>>> >
>>> > Thanks again,
>>> >
>>> > Hal
>>> A couple of additional things:
>>> Only a single LGTM is required.  Reviewers are expected to only LGTM
>>> patches they're confident in their knowledge of.  Reviewers may review
>>> and provide suggestions, but explicitly defer LGTM to someone else. 
>>> This is encouraged and a good way for new contributors to learn the code. 
>>> There is a cultural expectation that at least one reviewer is from a
>>> different organization than the author of the patch.  If that's not
>>> possible, care should be taken to ensure overall direction has been
>>> widely accepted. 
>>> Post commit review is encouraged via either phabricator or email.  There
>>> is a strong expectation that authors respond promptly to post commit
>>> feedback and address it.  Failure to do so is cause for the patch to be
>>> reverted.  If substantial problems are identified, it is expected that
>>> the patch is reverted, fixed offline, and then recommitted (possibly
>>> after further review.)
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>> -- 
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
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