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

From: Peter C++ <peter.cpp_at_[hidden]>
Date: Mon, 4 Dec 2023 10:56:27 +0100
+100 from me FWIW. 

sent from a mobile device so please excuse strange words due to autocorrection. 
Peter Sommerlad
+41-79-432 23 32

On 4 Dec 2023, at 10:31, Chuanqi Xu via SG15 <sg15_at_[hidden]> wrote:

Hi Ran,

    Before we jump into the details and other topics, let's sure if we are interested in the raised topic (should the compiler treat std module specially to make it easier for beginners and small projects). And if I read correctly, you're agree with the direction?

    Then there are some details about modules. 

I am not sure we must start with the source code of every library we want to import (including the standard). 

     Currently, the consensus are, we need to compile the modules we're importing locally since we agree that the BMIs are not ready to be distributed at least for now. (Let's ignore distributed build temporarily).

    Then in the same machine, can we reuse the BMIs across different projects if possible? I think it is possible but I am not sure which build systems implemented this.

   So generally, it'd better to treat BMIs as by-product and assume we need to build modules from source.

In the BMI-well-known-place (like headers). 

  But we don't have one. And as far as I know, no one is proposing this in detail.

Sure. Without this ability we are doing wrong. Std shouldn't be special.

   But stl (without modules) is already special to me. The compiler knows the position of headers. The compiler are able to link the standard libraries automatically. Also the standard libraries use some special intrinsics and special pragma to implement themselves. So I think std can be special, while it may  not be necessary.



From: SG15 <sg15_at_[hidden]>
Send Time:2023 Dec. 4 (Mon.) 15:34
To: SG15 <sg15_at_[hidden]>
Cc:Ran Regev <regev.ran_at_[hidden]>
Subject:Re: [SG15] [Modules] Should the compiler try to build std module implicitly?

On Mon, Dec 4, 2023, 08:00 Chuanqi Xu via SG15 <sg15_at_[hidden]> 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

If the code of hello.cpp is similar to the example below (just include instead of import) then even in this example the compiler did a "magic" for you by finding this include without your specific direction. It simply "knew" where to search.

    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.

The same mechanism that is used by compilers to find includes may be used to find modules. 

    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).

I am not sure we must start with the source code of every library we want to import (including the standard). It feels like that if we had a place for BMIs then we are good to go using them. There is a question what to do when they are missing. Maybe an error, not trying to build from source. 

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?
In the BMI-well-known-place (like headers). 

Should we try to reuse the BMIs?

Can we extend the process to other modules?
Sure. Without this ability we are doing wrong. Std shouldn't be special.

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.
I don't agree. BMI should be reused as far as I understand. 

- The compiler shouldn't try to reuse the BMIs. That is the job of the build systems.
I think that as source files are the input of #incluse, BMI should be the input of import. Therefore the compiler should try and find ONLY BMIs when it sees import directive. It is the build system to make sure they exists. 

- 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.
I disagree. Std and other modules should behave the same. 

The motivation is primarily for beginners and educations. How do you feel about the idea?


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Received on 2023-12-04 09:56:31