[llvm-dev] Layering Requirements in the LLVM Coding Style Guide

[David Blaikie via llvm-dev]

Sent out the code review: https://reviews.llvm.org/D42771 & CC'd everyone
who commented on this thread so they can easily follow along there.

On Wed, Jan 17, 2018 at 2:53 PM David Blaikie <dblaikie at gmail.com> wrote:

> On Wed, Jan 17, 2018 at 1:27 PM Chandler Carruth <chandlerc at gmail.com>
> wrote:
>
>> On Tue, Jan 16, 2018 at 11:35 AM Philip Reames <listmail at philipreames.com>
>> wrote:
>>
>>>
>>>
>>> On 01/16/2018 09:21 AM, David Blaikie via llvm-dev wrote:
>>>
>>> Context: I've been looking at experimenting with using Modular Code
>>> Generation (My talk at last year's LLVM dev meeting
>>> https://www.youtube.com/watch?v=lYYxDXgbUZ0 is about the best reference
>>> at the moment) when building the LLVM project, as a good experiment for the
>>> feature. This can/does enforce a stronger layering invariant than LLVM has
>>> historically been enforced. So I'm curious to get buy-in and maybe document
>>> this if it's something people like the idea of.
>>>
>>> I'm starting this discussion here rather than in an actual code review
>>> on llvm-commits since it seems like it could do with a bit of a wider
>>> discussion, but once/if the general direction is agreed on, I'll send a
>>> patch for review of specific wording for the LLVM Coding Standards.
>>>
>>>
>>> Currently the LLVM Coding Standards
>>> <https://llvm.org/docs/CodingStandards.html> doesn't say much/anything
>>> about layering. 'A Public Header File *is* a Module'
>>> <https://llvm.org/docs/CodingStandards.html#a-public-header-file-is-a-module> section
>>> talks about modules of functionality, mostly trying to describe why a
>>> header file should be self contained - but uses anachronistic language
>>> about modules that doesn't line up with the implicit or explicit modules
>>> concepts in use today, I think.
>>>
>>> I propose making this wording a bit more explicit, including:
>>>
>>> 1) Headers should be standalone (include all their dependencies - this
>>> is mentioned in the "is a Module" piece, by way of a technique to help
>>> ensure this, but not explicit as a goal itself).
>>>
>>> 2) Files intended to be included in a particular context (that aren't
>>> safe/benign to include multiple times, in multiple .cpp files, etc) should
>>> use a '.inc' or '.def' (.def specifically for those "define a macro,
>>> include the header which will reference that macro" style setups we have in
>>> a few places).
>>>
>>> Everything up to here seems non-controversial.  We should document this
>>> and ideally identify tooling suitable to enforce it.
>>>
>>
>> +1
>>
>>
>>>
>>>
>>> And the actual layering issue:
>>> 3) Each library should only include headers or otherwise reference
>>> entities from libraries it depends on. Including in headers and inline
>>> functions. A simple/explicit way to put this: every inline function should
>>> be able to be moved into a .cpp file and the build (with a unix linker -
>>> one that cannot handle circular library dependencies) should still succeed.
>>>
>>>
>>> This last point is the most interesting - and I hope one that people
>>> generally find desirable, so it might not be immediately obvious why it may
>>> be contentious or difficult:
>>>
>>> LLVM violates this constraint by using inline functions in headers to
>>> avoid certain layering constraints that might otherwise cause the build to
>>> fail. A couple of major examples I've hit are:
>>>
>>> TargetSelect.h
>>> <http://lists.llvm.org/pipermail/llvm-dev/2017-December/119494.html>and
>>> similar: This one's especially tricky - the header is part of libSupport,
>>> but each function in here depends on a different subset of targets
>>> (creating a circular dependency) - to call the given function the
>>> programmer needs to choose the right dependencies to link to or the program
>>> will not link.
>>> Clang Diagnostics <https://reviews.llvm.org/D41357> (work in progress):
>>> The diagnostics for each component are in their own component directories,
>>> but are then all included from libClangBasic, a library none of those
>>> components depends on. (so this isn't so much an inlining case as #include
>>> based circular dependency)
>>>
>>>
>>> Generally I'd like to get buy-in that stricter layering is desirable,
>>> and that these few cases are at least sub-optimal, if accepted for now.
>>>
>>> I have no strong opinion on this topic.  My experience has been that
>>> it's often far harder to unwind these types of inline dependencies than it
>>> first seems and that the value in doing so is often unclear.  I'm not
>>> opposed, but I'm also not signing up to help.  :)
>>>
>>
>> While I'm also not in a position to help a lot, I think there is a
>> question we should ask here:
>>
>> Should we hold new code to this standard? Should we declare that this is
>> what we want?
>>
>> For me, I say emphatically "yes" and we should put it into the coding
>> standards. I think cleaning up the existing code is a good thing to do and
>> we can let people who have a reason actually drive that, but I don't want
>> that to be necessarily finished in order for us to establish reasonable
>> guidelines going forward.
>>
>
> Yep, that's where I am too - I want it to be our standard going forward
> but, like naming conventions and other things, realize that not all
> existing code in the project will conform to this constraint.
>
> - Dave
>
>
>>
>>
>>>
>>> Happy to go into more details about any of this, examples, etc, but I
>>> realize this is already a bit long.
>>> - Dave
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
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