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

[David Blaikie via llvm-dev]

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
>>
>>
>> _______________________________________________
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>>
>>
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