C++ Logo


Advanced search

Re: [Modules] Should the compiler try to build std module implicitly?

From: Tom Honermann <tom_at_[hidden]>
Date: Mon, 4 Dec 2023 14:01:41 -0500
On 12/4/23 12:43 PM, Ran Regev wrote:
> On Mon, Dec 4, 2023, 19:31 Tom Honermann via SG15
> <sg15_at_[hidden]> wrote:
> If this can be done without too much difficulty, great.
> However, would you expect this to work for both -stdlib=libc++ and
> -stdlib=libstdc++? How about when Clang is used with Microsoft's
> standard library (for which there is no corresponding -stdlib option)?
> I think defaults should be used when no value is supplied. Like, -O0
> is the default in all compilers unless otherwise specified.
> The same for default std lib and any other parameter.
> The target is to enable portable:
> # std-tool -o hello hello.cpp
> Even when hello.cpp has import std; directive inside it.

Sure, but the default for Clang differs based on platform. So support
for Clang in general would require support for -std=libc++ (the default
on Apple), -stdlib=libstdc++ (the default on Linux), and the MSVC
standard library (the default for the *-windows-msvc target triples).
Thus, Clang (or a wrapper) would presumably need to be taught how to
build the BMIs for each (supported) version of each implementation. A
wrapper tool would need to know how to identify which standard library
implementation (and version) the compiler expects (preferably via some
form of compiler introspection).


> Ran.
> Tom.
> On 12/4/23 1:00 AM, Chuanqi Xu via SG15 wrote:
>> Hi Guys,
>> This post is mainly about some random thoughts about
>> teachability of std modules. Also [P2412R0: Minimal module
>> support for the standard library]
>> (https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2412r0.pdf)
>> mentions that the std module is important for educations and
>> beginners. If there are more interests, I can try to summarize
>> them into a paper.
>> When I start to learn C++, I only need to copy a hello world
>> example and run:
>> ```
>> clang++ hello.cpp -o hello
>> ```
>> However, with the current direction of std modules, the
>> beginner need to install the build systems and try to copy the
>> build scripts that them can hardly understand. Possibly some
>> simpler form of https://libcxx.llvm.org/Modules.html. But it
>> still seems scaring to beginners.
>> And I am wondering if we can simplify the process. For
>> example, the beginners can compile a hello world example with std
>> module:
>> ```
>> import std;
>> int main() { std::cout << "Hello modular world\n"; }
>> ```
>> with a single command line:
>> ```
>> clang++ -std=c++23 hello.cpp -o hello
>> ```
>> The compiler may achieve that when:
>> - the std module is required but not provided.
>> - the path to std module's BMI is not specified. (implied that
>> the invocation doesn't from build systems)
>> - the final output is an executable.
>> - the std module's source exists in the installed path (being
>> discussed in https://github.com/llvm/llvm-project/issues/73089).
>> then the compiler can try to compile the std module's source to
>> BMI and object file. Then we can import the BMI and link the
>> object file. There are some details. e.g., where should we put
>> the BMI? Should we try to reuse the BMIs? Can we extend the
>> process to other modules? They are open questions and my answers are:
>> - By default, they should be in `/tmp` and if `-save-temps` is
>> specified, they'll be in the same directory with the temporaries.
>> - The compiler shouldn't try to reuse the BMIs. That is the job
>> of the build systems.
>> - No, we can't. It is possible for std module since we'll try to
>> standardize the locations of std modules sources. So that the
>> tools are able to find the source of std modules. But it is not
>> the case for other generalized modules. Also I think this is the
>> job of build systems too.
>> The motivation is primarily for beginners and educations. How do
>> you feel about the idea?
>> Thanks,
>> Chuanqi
>> _______________________________________________
>> SG15 mailing list
>> SG15_at_[hidden]
>> https://lists.isocpp.org/mailman/listinfo.cgi/sg15
> _______________________________________________
> SG15 mailing list
> SG15_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/sg15

Received on 2023-12-04 19:01:44