[cfe-dev] [RFC] C++20 modules dependency discovery

Thu Aug 15 17:27:16 PDT 2019

On Mon, Aug 12, 2019 at 6:37 PM Michael Spencer via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>
> C++20 is coming and we need to decide how clang will handle dependency discovery for modules.  In the following, module means compiled C++20 module interface unit, and I will use header unit to refer to the thing generated by a clang module map.
>
> There are two different modes we care about when it comes to module dependencies:  implicit and explicit.
>
> Implicit Modules
> ================
>
> For implicit modules the build system doesn’t know anything about them, and thus can’t care about any intermediate files.  It needs to know about all source files that if changed should cause a rebuild of this translation unit.
>
> For this case clang needs to output the full transitive set of dependencies, excluding any intermediate temporaries.  This also means that we can’t get the full set of dependencies without actually at least preprocessing every module transitively referenced.  This means that `-E -MD` should fail if it can’t find a module or header unit.
>
> Explicit Modules
> ================
>
> For explicit modules we only need to know the direct dependencies, as the build system will handle the transitive set.
>
> For preprocessing we still need to import header units (but only their preprocessor state), but not normal modules.  For this case it’s ok if `-E -MD` fails to find a module.  But it does still need to be able to find header units and module maps.  Additionally the normal Make output syntax is not sufficient to represent the needed information unless the driver decides how modules and header units should be built and where intermediate files should go.  There’s currently a json format working its way through the tooling subgroup of the standards committee that I think we should adopt for this.
>
> I think we need separate modes in clang for these along with support for scanning through header units without actually building a clang module for them. clang-scan-deps will make use of the explicit mode.  The question I have is how should we select this mode, and what clang options do we need to add?
>
> Proposal
> ========
>
> As a rough idea I propose the following:
>
> * `-M?` means output the json format which can correctly represent dependencies on a module for which we don’t know what the final file path will be.
> * `clang++ -std=c++20 -E -MD -fimplicit-header-units` should implicitly find header unit sources, but not modules (as we've not given it any way to look up how to build modules).
>     * This means that the dep file will contain a bunch of `.h`s, `.modulemap`s, and any `.pcm`s explicitly listed on the command line.
>     * This also means erroring on unknown imported modules as we don't know what to put in the dep file for them.
> * `clang++ -std=c++20 -E -MD -fimplicit-header-units -fimplicit-module-lookup=?`  should do the same as the above, except that it does know how to find modules, and should list all of the transitive dependencies of any modules it finds.
> * `clang++ -std=c++20 -E -MD` should fail if it hits a module or header unit, and should never do implicit lookup.
> * `clang++ -std=c++20 -E -M?` should scan through header units without actually building clang modules for them (to get the macros it needs), and should note all module imports.
>     * This means that the dep file will contain only `.h`s that it includes, and use the json representation of header units and modules.
>     * It will also be shallow, with only direct dependencies.

Very nice break down of the different things we can get!

This might be a good opportunity to have more descriptive names than
-M<> related stuff. I really like the -fdep-* approach as pointed out
by Ben, open the opportunity for nice customization. As an alternative
approach on top of your idea, we could introduce a -fdep-mode=<mode>,
such as:

... -std=c++20 -E -fdep-mode=headerunit # `... -std=c++20 -E -MD
-fimplicit-header-units`
... -std=c++20 -E -fdep-mode=transitive # `... -std=c++20 -E -MD
-fimplicit-header-units -fimplicit-module-lookup=?`
... -std=c++20 -E -fdep-mode=shallow # `... -std=c++20 -E -M?`

WDYT?

Ben, how do you express different modes in GCC? Does it have more than
one currently?

> Additionally, we should (eventually) make:
>
> `$ clang++ -std=c++20 a.cpp b.cpp c.cpp a.cppm -o program`
>
> Work without a build system, even in the presence of modules.  To do this we will need to prescan the files to determine the module dependencies between them and then build them in dependency order.  This does mean adding a (simple) build system to the driver (maybe [llbuild](https://github.com/apple/swift-llbuild)?), but I think it’s worth it to make simple cases simple.  It may also make sense to actually push this work out to a real build system.  For example have clang write a temporary ninja file and invoke ninja to perform the build.
>
> - Michael Spencer
>
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-- 
Bruno Cardoso Lopes
http://www.brunocardoso.cc