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

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Tue Jan 16 10:50:43 PST 2018

On Tue, Jan 16, 2018 at 10:43 AM Matthias Braun <mbraun at apple.com> wrote:

> I would describe it from this angle: LLVM is layered just fine.

Yeah, in most cases/in general I agree.

> Usually the layering is enforced as we don't link all libraries to all
> targets and you will notice missing symbols if you violate it.

Actually even more than that - on unix, linkers don't resolve circular
dependencies (they start with a list of unresolved symbols and walk the
link line left to right - resolving any symbols they can iteratively while
looking at one library, then moving on to the next - never going back to an
earlier library to resolve a later dependency), this enforces quite a lot
of the layering constraints we usually think about. Google's build system
enforces things a bit more strictly, I think (even without modular codegen)
& that's generally been a good/accepted thing - though I don't have any
precise examples to point to, unfortunately.

> It just happens that you can violate the layering with header-only
> implementations of features that are not catched this way and sure enough
> we a handful of cases that violate the layering this way as David nicely
> explained here.
> I don't think there is a reason not to fix those layering violations. We
> just need a plan on how to fix them.

Yep - my goal is just to enshrine that understanding in documentation so
it's a bit more of an explicit/clear goal going forward.

I may be able to fix some of the existing violations - but mostly wanting
to sure up things for future changes.

- Dave

> - Matthias
> On Jan 16, 2018, at 10:15 AM, Robinson, Paul via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> I have found layering to be a particularly useful and beneficial model in
> past large software projects.
> Is LLVM's layering actually written down anywhere?  Last time I went
> looking, there was nothing.  If there's no spec, there's no verifiable
> conformance; you have to guess based on what other files do.
> --paulr
> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org
> <llvm-dev-bounces at lists.llvm.org>] *On Behalf Of *David Blaikie via
> llvm-dev
> *Sent:* Tuesday, January 16, 2018 9:22 AM
> *To:* llvm-dev; Richard Smith; Chandler Carruth; Reid Kleckner
> *Subject:* [llvm-dev] Layering Requirements in the LLVM Coding Style Guide
> 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).
> 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.
> 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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