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Subject: Re: Draft: Requirements for Usage of C++ Modules at Bloomberg
From: Bjarne Stroustrup (bjarne_at_[hidden])
Date: 2021-06-18 18:48:17


Let's try not to confuse familiarity with simplicity. Modules are far
simpler than header files and will make life simpler for all C++
developers for decades to come. However, we have lived with the
fundamentally messy header files for almost 5 decades and have pretty
good ideas of how to deal with them, so they can seem simple and fit
with many tool chains. We should not compromise the future by burdening
it with backwards-looking standard rules that will become pure overhead.

On 6/18/2021 7:19 PM, David Blaikie via SG15 wrote:
> I agree that the N+1 problem is a real concern - hence bringing up
> whether pkg-config might be the wagon we could hitch our cart to in
> this regard (or some other existing solution that could be fleshed out
> and more strongly encouraged (than in the past) as "the" tool to use
> here, with some expectation/desire/buy-in from other tool vendors
> (like cmake) that they'd be on board with that direction).
>
> Be good to get some of the relevant tooling folks (the cmakes, etc, of
> the world) to see what they think would be a good path forward for them.
>
> On Fri, Jun 18, 2021 at 3:41 PM Gabriel Dos Reis via SG15
> <sg15_at_[hidden] <mailto:sg15_at_[hidden]>> wrote:
>
> * Module interfaces on paper fit this model perfectly, except
> for the part of actually getting builds to work.
>
> Module interfaces have not yet had the many decades afforded to
> header files to work with the many  build systems.  From
> experience with trying to stay above the “build systems garden
> variety” and yet have sufficient hands-on experience/involvement
> to understand each one’s challenged assumptions, I don’t think
> we’ve hit a point where they (modules) can’t work with existing
> build systems.  The real thorny issue is whether we continue with
> ad-hoc solutions as we’ve always had in the last 50 years, or if
> we shoot for a common solution which hits the “N+1 standards problem”.
>
> For example, we know from empirical data and experience that C++
> Modules require less of build systems than Fortran does – this is
> not a contest of who can require more.
>
> -- Gaby
>
> *From:* SG15 <sg15-bounces_at_[hidden]
> <mailto:sg15-bounces_at_[hidden]>> *On Behalf Of *Steve
> Downey via SG15
> *Sent:* Friday, June 18, 2021 3:31 PM
> *To:* ISO C++ Tooling Study Group <sg15_at_[hidden]
> <mailto:sg15_at_[hidden]>>
> *Cc:* Steve Downey <sdowney_at_[hidden] <mailto:sdowney_at_[hidden]>>
> *Subject:* Re: [SG15] Draft: Requirements for Usage of C++ Modules
> at Bloomberg
>
> Also, although Daniel's written this from the standpoint of
> Bloomberg, that's because we know for certain we can speak for our
> requirements. We really do think we share requirements with many
> other development organizations, though. We ship packages to each
> other, literally in dpkg, on all our posixy platforms. We can see
> each other's source code, but don't usually build it. Our shared
> model is basically a debian unstable, `sid`, shipped as fast as we
> can turn the crank. We're managing large scale reuse, with
> dependency graph depths of 40 or more, involving hundreds of
> packages, in thousands of applications. Module interfaces on paper
> fit this model perfectly, except for the part of actually getting
> builds to work.
>
> We really want modules to work for us, and we think if they work
> for us, they work for a large class of developer orgs.
>
> On Fri, Jun 18, 2021 at 5:25 PM Steve Downey <sdowney_at_[hidden]
> <mailto:sdowney_at_[hidden]>> wrote:
>
> With header files (except for generated ones) there's not the
> requirement of a build DAG, just out-of-date information. That's
> why we can generate the dependencies as part of the compilation
> step. With modules, at least with the current physical model, we
> have to make sure the interface has been translated before it can
> be consumed. Incremental builds are going to break for a lot of
> people in subtle ways. And yes, this is very much like the past
> and using tools like `makedeps`.
>
> What we learned then is that it's very very difficult to precisely
> reproduce the preprocessor and include searching. There's a lot of
> poorly specified and undefined behavior in the early phases of
> translation. If the tools disagree, build problems ensue.
>
> I don't believe we have made the situation impossible. A tool
> should be able to proceed sufficiently with preprocessing in the
> face of missing named modules and be able to report all of the
> requirements of a TU. It wouldn't for header imports, but that's a
> more well understood problem, as those map to filenames in a well
> agreed on way.
>
> What module an interface provides is also discoverable, but that
> may not be as useful in an open universe situation. The question
> of which instance of the primary module interface out of many is
> hard. We have that today with headers, of course, and we try to
> deal with it via ordering rules for search paths, but it's still a
> source of error. Duplicates and collisions are harder for named
> modules, even if figuring out what an interface provides is
> straight forward.
>
> On Fri, Jun 18, 2021 at 2:12 PM David Blaikie via SG15
> <sg15_at_[hidden] <mailto:sg15_at_[hidden]>> wrote:
>
> On Fri, Jun 18, 2021 at 11:01 AM Gabriel Dos Reis
> <gdr_at_[hidden] <mailto:gdr_at_[hidden]>> wrote:
>
> * but you cannot discover the graph with a simple lexer
>
> Yes, and I have not found that to be an actual problem in
> practice.  In fact there is really nothing new here (other
> than recognizing the ‘import’ directives and friends) in the
> non-module world: the ol’ C and C++ standards allow
>
>       #include HEADER
>
> where HEADER comes from macro expansions and all the other
> stuff that you mention about inability to use “simple lexer”,
> conditional inclusion, etc.  As a PSA, I would like to
> disabuse anyone who ever thought that grep was sufficient to
> compute dependency of C or C++ source files pre-C++20.
>
> Maybe it is my dinosaurus status showing bleeding out here but
> are we rehashing the same arguments from the past?
>
> The one thing we can do for the community is to have a library
> readily available to anyone who ever thought that a simple
> lexer is sufficient to compute C++ source file dependency. 
> That would be a major step forward.
>
>
> I don't think Daniel is suggesting/has mentioned the idea that a
> simple lexer is sufficient for either headers or modules. But
> expected/would like it if we could simplify the situation further
> than module files alone currently allow in this regard.
>
> I do lean towards "maybe it's simple enough, even if it doesn't
> allow a simple lexer to do this operation" - at least I don't
> think that's an ironclad proof that we need the dependency
> information in a more immediately legible format such as in the
> pkg-config files or equivalent. I wouldn't rule it out either, though.
>
> Like, I wouldn't be averse to pkg-config or equivalent files
> supporting this as an optional feature - so that folks could
> explore whether it significantly improves performance. If it's
> more a matter of software complexity (since build tools that don't
> know anything about parsing C++ are going to want to discover this
> dependency graph) - yeah, maybe compilers could vend a command
> line mode that reports the dependencies as quickly as possible -
> ala clang-scan-deps & for library level uses (llvm-buildozer,
> background running build systems/IDE type situations) there could
> be a library for that.
>
> But, going on past experience, those kinds of
> utilities/dependencies don't necessarily eventuate and I can
> understand the concern about proliferation/variation/lack of
> ubiquity in such things & so a preference to bake it into a common
> config file. I guess all I have on that is that I think it'd be
> unfortunate for developers to have to repeat this information in
> multiple places like that.
>
> - Dave
>
> -- Gaby
>
> *From:* Daniel Ruoso <daniel_at_[hidden] <mailto:daniel_at_[hidden]>>
> *Sent:* Friday, June 18, 2021 10:30 AM
> *To:* Gabriel Dos Reis <gdr_at_[hidden]
> <mailto:gdr_at_[hidden]>>
> *Cc:* sg15_at_[hidden] <mailto:sg15_at_[hidden]>;
> David Blaikie <dblaikie_at_[hidden] <mailto:dblaikie_at_[hidden]>>
> *Subject:* Re: [SG15] Draft: Requirements for Usage of C++
> Modules at Bloomberg
>
> On Fri, Jun 18, 2021 at 1:20 PM Gabriel Dos Reis
> <gdr_at_[hidden] <mailto:gdr_at_[hidden]>> wrote:
>
> Also notice that while a dependency scanner needs to chase
> #included or imported files (via the appropriate
> implementation-defined mapping), it actually does not
> need  to fully preprocess the source code found therein;
>
> the spec has been carefully crafted to that effect.
>
> It doesn't need to preprocess the complete file, but it needs
> to significantly preprocess the early sections of the file.
> While the spec has carefully prevented conditional
> specification of the keywords, it still allows macros to be
> used, which requires support for conditional evaluation,
> variable expansion, and string concatenation.
>
> In other words, the only optimization granted by the standard
> today is on the discovery of whether or not a source file
> needs to participate on the construction of the DAG for
> processing modules, but you cannot discover the graph with a
> simple lexer, you need the full expression evaluation logic of
> the preprocessor.
>
> daniel
>
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