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

[Chandler Carruth via llvm-dev]

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.


>
> 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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