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Subject: Re: Draft: Requirements for Usage of C++ Modules at Bloomberg
From: Steve Downey (sdowney_at_[hidden])
Date: 2021-06-18 16:25:56


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of a build DAG, just out-of-date information.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]> wrote:

>
>
> 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
>>
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