llvm/docs/CodeReview.rst - llvm-project - Git at Google

 =====================================
 LLVM Code-Review Policy and Practices
 =====================================

 LLVM's code-review policy and practices help maintain high code quality across
 the project. Specifically, our code review process aims to:

  * Improve readability and maintainability.
  * Improve robustness and prevent the introduction of defects.
  * Best leverage the experience of other contributors for each proposed change.
  * Help grow and develop new contributors, through mentorship by community leaders.

 It is important for all contributors to understand our code-review
 practices and participate in the code-review process.

 General Policies
 ================

 What Code Should Be Reviewed?
 -----------------------------

 All developers are required to have significant changes reviewed before they
 are committed to the repository.

 Must Code Be Reviewed Prior to Being Committed?
 -----------------------------------------------

 Code can be reviewed either before it is committed or after. We expect
 significant patches to be reviewed before being committed. Smaller patches
 (or patches where the developer owns the component) that meet
 likely-community-consensus requirements (as apply to all patch approvals) can
 be committed prior to an explicit review. In situations where there is any
 uncertainty, a patch should be reviewed prior to being committed.

 Please note that the developer responsible for a patch is also
 responsible for making all necessary review-related changes, including
 those requested during any post-commit review.

 .. _post_commit_review:

 Can Code Be Reviewed After It Is Committed?
 -------------------------------------------

 Post-commit review is encouraged, and can be accomplished using any of the
 tools detailed below. 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 :ref:`reverted <revert_policy>`.

 If a community member expresses a concern about a recent commit, and this
 concern would have been significant enough to warrant a conversation during
 pre-commit review (including around the need for more design discussions),
 they may ask for a revert to the original author who is responsible to revert
 the patch promptly. Developers often disagree, and erring on the side of the
 developer asking for more review prevents any lingering disagreement over
 code in the tree. This does not indicate any fault from the patch author,
 this is inherent to our post-commit review practices.
 Reverting a patch ensures that design discussions can happen without blocking
 other development; it's entirely possible the patch will end up being reapplied
 essentially as-is once concerns have been resolved.

 Before being recommitted, the patch generally should undergo further review.
 The community member who identified the problem is expected to engage
 actively in the review. In cases where the problem is identified by a buildbot,
 a community member with access to hardware similar to that on the buildbot is
 expected to engage in the review.

 Please note: The bar for post-commit feedback is not higher than for pre-commit
 feedback. Don't delay unnecessarily in providing feedback. However, if you see
 something after code has been committed about which you would have commented
 pre-commit (had you noticed it earlier), please feel free to provide that
 feedback at any time.

 That having been said, if a substantial period of time has passed since the
 original change was committed, it may be better to create a new patch to
 address the issues than comment on the original commit. The original patch
 author, for example, might no longer be an active contributor to the project.

 What Tools Are Used for Code Review?
 ------------------------------------

 Pre-commit code reviews are conducted on our web-based code-review tool (see
 :doc:`Phabricator`). Post-commit reviews can be done on Phabricator, by email
 on the relevant project's commit mailing list, on the project's development
 list, or on the bug tracker.

 When Is an RFC Required?
 ------------------------

 Some changes are too significant for just a code review. Changes that should
 change the LLVM Language Reference (e.g., adding new target-independent
 intrinsics), adding language extensions in Clang, and so on, require an RFC
 (Request for Comment) email on the project's ``*-dev`` mailing list first. For
 changes that promise significant impact on users and/or downstream code bases,
 reviewers can request an RFC achieving consensus before proceeding with code
 review. That having been said, posting initial patches can help with
 discussions on an RFC.

 Code-Review Workflow
 ====================

 Code review can be an iterative process, which continues until the patch is
 ready to be committed. Specifically, once a patch is sent out for review, it
 needs an explicit approval before it is committed. Do not assume silent
 approval, or solicit objections to a patch with a deadline.

 Acknowledge All Reviewer Feedback
 ---------------------------------

 All comments by reviewers should be acknowledged by the patch author. It is
 generally expected that suggested changes will be incorporated into a future
 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 a new patch
 does not address all outstanding feedback, the author should explicitly state
 that when providing the updated patch. When using the web-based code-review
 tool, such notes can be provided in the "Diff" description (which is different
 from the description of the "Differential Revision" as a whole used for the
 commit message).

 If you suggest changes in a code review, but don't wish the suggestion to be
 interpreted this strongly, please state so explicitly.

 Aim to Make Efficient Use of Everyone's Time
 --------------------------------------------

 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 and reviewers to make the most efficient
 use of their time.

 LGTM - How a Patch Is Accepted
 ------------------------------

 A patch is approved to be committed when a reviewer accepts it, and this is
 almost always associated with a message containing the text "LGTM" (which
 stands for Looks Good To Me). Only approval from a single reviewer is required.

 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.

 Note that, if a reviewer has requested a particular community member to review,
 and after a week that community member has yet to respond, feel free to ping
 the patch (which literally means submitting a comment on the patch with the
 word, "Ping."), or alternatively, ask the original reviewer for further
 suggestions.

 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 of days in case others wish to review"). If approval is received very
 quickly, a patch author may also elect to wait before committing (and this is
 certainly considered polite for non-trivial patches). Especially given the
 global nature of our community, this waiting time should be at least 24 hours.
 Please also be mindful of weekends and major holidays.

 Our goal is to ensure community consensus around design decisions and
 significant implementation choices, and one responsibility of a reviewer, when
 providing an overall approval for a patch, is to be reasonably sure that such
 consensus exists. If you're not familiar enough with the community to know,
 then you shouldn't be providing final approval to commit. A reviewer providing
 final approval should have commit access to the LLVM project.

 Every patch should be reviewed by at least one technical expert in the areas of
 the project affected by the change.

 Splitting Requests and Conditional Acceptance
 ---------------------------------------------

 Reviewers may request certain aspects of a patch to be broken out into separate
 patches for independent review. Reviewers may also 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). Moreover, reviewers can 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.

 Don't Unintentionally Block a Review
 ------------------------------------

 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.

 Who Can/Should Review Code?
 ===========================

 Non-Experts Should Review Code
 ------------------------------

 You do not need to be an expert in some area of the code base 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. Please
 remember that it is not in the long-term best interest of the community to have
 components that are only understood well by a small number of people. Extra
 comments and/or test cases can often help (and asking for comments in the test
 cases is fine as well).

 Moreover, authors are encouraged to interpret questions as a reason to reexamine
 the readability of the code in question. Structural changes, or further
 comments, may be appropriate.

 If you're new to the LLVM community, you might also find this presentation helpful:
 .. _How to Contribute to LLVM, A 2019 LLVM Developers' Meeting Presentation: https://youtu.be/C5Y977rLqpw

 A good way for new contributors to increase their knowledge of the code base is
 to review code. It is perfectly acceptable to review code and explicitly
 defer to others for approval decisions.

 Experts Should Review Code
 --------------------------

 If you are an expert in an area of the compiler affected by a proposed patch,
 then you are highly encouraged to review the code. If you are a relevant code
 owner, and no other experts are reviewing a patch, you must either help arrange
 for an expert to review the patch or review it yourself.

 Code Reviews, Speed, and Reciprocity
 ------------------------------------

 Sometimes code reviews will take longer than you might hope, especially for
 larger features. Common ways to speed up review times for your patches are:

 * Review other people's patches. If you help out, everybody will be more
   willing to do the same for you; goodwill is our currency.
 * Ping the patch. If it is urgent, provide reasons why it is important to you to
   get this patch landed and ping it every couple of days. If it is
   not urgent, the common courtesy ping rate is one week. Remember that you're
   asking for valuable time from other professional developers.
 * Ask for help on IRC. Developers on IRC will be able to either help you
   directly, or tell you who might be a good reviewer.
 * Split your patch into multiple smaller patches that build on each other. The
   smaller your patch is, the higher the probability that somebody will take a quick
   look at it. When doing this, it is helpful to add "[N/M]" (for 1 <= N <= M) to
   the title of each patch in the series, so it is clear that there is an order
   and what that order is.

 Developers should participate in code reviews as both reviewers and
 authors. If someone is kind enough to review your code, you should return the
 favor for someone else. Note that anyone is welcome to review and give feedback
 on a patch, but approval of patches should be consistent with the policy above.
	=====================================
	LLVM Code-Review Policy and Practices
	=====================================

	LLVM's code-review policy and practices help maintain high code quality across
	the project. Specifically, our code review process aims to:

	* Improve readability and maintainability.
	* Improve robustness and prevent the introduction of defects.
	* Best leverage the experience of other contributors for each proposed change.
	* Help grow and develop new contributors, through mentorship by community leaders.

	It is important for all contributors to understand our code-review
	practices and participate in the code-review process.

	General Policies
	================

	What Code Should Be Reviewed?
	-----------------------------

	All developers are required to have significant changes reviewed before they
	are committed to the repository.

	Must Code Be Reviewed Prior to Being Committed?
	-----------------------------------------------

	Code can be reviewed either before it is committed or after. We expect
	significant patches to be reviewed before being committed. Smaller patches
	(or patches where the developer owns the component) that meet
	likely-community-consensus requirements (as apply to all patch approvals) can
	be committed prior to an explicit review. In situations where there is any
	uncertainty, a patch should be reviewed prior to being committed.

	Please note that the developer responsible for a patch is also
	responsible for making all necessary review-related changes, including
	those requested during any post-commit review.

	.. _post_commit_review:

	Can Code Be Reviewed After It Is Committed?
	-------------------------------------------

	Post-commit review is encouraged, and can be accomplished using any of the
	tools detailed below. 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 :ref:`reverted <revert_policy>`.

	If a community member expresses a concern about a recent commit, and this
	concern would have been significant enough to warrant a conversation during
	pre-commit review (including around the need for more design discussions),
	they may ask for a revert to the original author who is responsible to revert
	the patch promptly. Developers often disagree, and erring on the side of the
	developer asking for more review prevents any lingering disagreement over
	code in the tree. This does not indicate any fault from the patch author,
	this is inherent to our post-commit review practices.
	Reverting a patch ensures that design discussions can happen without blocking
	other development; it's entirely possible the patch will end up being reapplied
	essentially as-is once concerns have been resolved.

	Before being recommitted, the patch generally should undergo further review.
	The community member who identified the problem is expected to engage
	actively in the review. In cases where the problem is identified by a buildbot,
	a community member with access to hardware similar to that on the buildbot is
	expected to engage in the review.

	Please note: The bar for post-commit feedback is not higher than for pre-commit
	feedback. Don't delay unnecessarily in providing feedback. However, if you see
	something after code has been committed about which you would have commented
	pre-commit (had you noticed it earlier), please feel free to provide that
	feedback at any time.

	That having been said, if a substantial period of time has passed since the
	original change was committed, it may be better to create a new patch to
	address the issues than comment on the original commit. The original patch
	author, for example, might no longer be an active contributor to the project.

	What Tools Are Used for Code Review?
	------------------------------------

	Pre-commit code reviews are conducted on our web-based code-review tool (see
	:doc:`Phabricator`). Post-commit reviews can be done on Phabricator, by email
	on the relevant project's commit mailing list, on the project's development
	list, or on the bug tracker.

	When Is an RFC Required?
	------------------------

	Some changes are too significant for just a code review. Changes that should
	change the LLVM Language Reference (e.g., adding new target-independent
	intrinsics), adding language extensions in Clang, and so on, require an RFC
	(Request for Comment) email on the project's ``*-dev`` mailing list first. For
	changes that promise significant impact on users and/or downstream code bases,
	reviewers can request an RFC achieving consensus before proceeding with code
	review. That having been said, posting initial patches can help with
	discussions on an RFC.

	Code-Review Workflow
	====================

	Code review can be an iterative process, which continues until the patch is
	ready to be committed. Specifically, once a patch is sent out for review, it
	needs an explicit approval before it is committed. Do not assume silent
	approval, or solicit objections to a patch with a deadline.

	Acknowledge All Reviewer Feedback
	---------------------------------

	All comments by reviewers should be acknowledged by the patch author. It is
	generally expected that suggested changes will be incorporated into a future
	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 a new patch
	does not address all outstanding feedback, the author should explicitly state
	that when providing the updated patch. When using the web-based code-review
	tool, such notes can be provided in the "Diff" description (which is different
	from the description of the "Differential Revision" as a whole used for the
	commit message).

	If you suggest changes in a code review, but don't wish the suggestion to be
	interpreted this strongly, please state so explicitly.

	Aim to Make Efficient Use of Everyone's Time
	--------------------------------------------

	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 and reviewers to make the most efficient
	use of their time.

	LGTM - How a Patch Is Accepted
	------------------------------

	A patch is approved to be committed when a reviewer accepts it, and this is
	almost always associated with a message containing the text "LGTM" (which
	stands for Looks Good To Me). Only approval from a single reviewer is required.

	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.

	Note that, if a reviewer has requested a particular community member to review,
	and after a week that community member has yet to respond, feel free to ping
	the patch (which literally means submitting a comment on the patch with the
	word, "Ping."), or alternatively, ask the original reviewer for further
	suggestions.

	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 of days in case others wish to review"). If approval is received very
	quickly, a patch author may also elect to wait before committing (and this is
	certainly considered polite for non-trivial patches). Especially given the
	global nature of our community, this waiting time should be at least 24 hours.
	Please also be mindful of weekends and major holidays.

	Our goal is to ensure community consensus around design decisions and
	significant implementation choices, and one responsibility of a reviewer, when
	providing an overall approval for a patch, is to be reasonably sure that such
	consensus exists. If you're not familiar enough with the community to know,
	then you shouldn't be providing final approval to commit. A reviewer providing
	final approval should have commit access to the LLVM project.

	Every patch should be reviewed by at least one technical expert in the areas of
	the project affected by the change.

	Splitting Requests and Conditional Acceptance
	---------------------------------------------

	Reviewers may request certain aspects of a patch to be broken out into separate
	patches for independent review. Reviewers may also 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). Moreover, reviewers can 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.

	Don't Unintentionally Block a Review
	------------------------------------

	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.

	Who Can/Should Review Code?
	===========================

	Non-Experts Should Review Code
	------------------------------

	You do not need to be an expert in some area of the code base 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. Please
	remember that it is not in the long-term best interest of the community to have
	components that are only understood well by a small number of people. Extra
	comments and/or test cases can often help (and asking for comments in the test
	cases is fine as well).

	Moreover, authors are encouraged to interpret questions as a reason to reexamine
	the readability of the code in question. Structural changes, or further
	comments, may be appropriate.

	If you're new to the LLVM community, you might also find this presentation helpful:
	.. _How to Contribute to LLVM, A 2019 LLVM Developers' Meeting Presentation: https://youtu.be/C5Y977rLqpw

	A good way for new contributors to increase their knowledge of the code base is
	to review code. It is perfectly acceptable to review code and explicitly
	defer to others for approval decisions.

	Experts Should Review Code
	--------------------------

	If you are an expert in an area of the compiler affected by a proposed patch,
	then you are highly encouraged to review the code. If you are a relevant code
	owner, and no other experts are reviewing a patch, you must either help arrange
	for an expert to review the patch or review it yourself.

	Code Reviews, Speed, and Reciprocity
	------------------------------------

	Sometimes code reviews will take longer than you might hope, especially for
	larger features. Common ways to speed up review times for your patches are:

	* Review other people's patches. If you help out, everybody will be more
	willing to do the same for you; goodwill is our currency.
	* Ping the patch. If it is urgent, provide reasons why it is important to you to
	get this patch landed and ping it every couple of days. If it is
	not urgent, the common courtesy ping rate is one week. Remember that you're
	asking for valuable time from other professional developers.
	* Ask for help on IRC. Developers on IRC will be able to either help you
	directly, or tell you who might be a good reviewer.
	* Split your patch into multiple smaller patches that build on each other. The
	smaller your patch is, the higher the probability that somebody will take a quick
	look at it. When doing this, it is helpful to add "[N/M]" (for 1 <= N <= M) to
	the title of each patch in the series, so it is clear that there is an order
	and what that order is.

	Developers should participate in code reviews as both reviewers and
	authors. If someone is kind enough to review your code, you should return the
	favor for someone else. Note that anyone is welcome to review and give feedback
	on a patch, but approval of patches should be consistent with the policy above.