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

Robinson, Paul via llvm-dev llvm-dev at lists.llvm.org
Tue Jan 16 10:15:38 PST 2018

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.

From: llvm-dev [mailto: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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