C++ Logo

SG15

Advanced search

Subject: Re: Draft: Requirements for Usage of C++ Modules at Bloomberg
From: David Blaikie (dblaikie_at_[hidden])
Date: 2021-06-18 13:12:05


On Fri, Jun 18, 2021 at 11:01 AM Gabriel Dos Reis <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]>
> *Sent:* Friday, June 18, 2021 10:30 AM
> *To:* Gabriel Dos Reis <gdr_at_[hidden]>
> *Cc:* sg15_at_[hidden]; David Blaikie <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]>
> 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
>



SG15 list run by sg15-owner@lists.isocpp.org

Older archives