Hi Tom,
> I agree with this conception of a small project. Both of the above examples generate an executable, but I'm assuming you don't intend that to be a requirement. I would expect the std module to be built if needed for an invocation that generates a static or shared library or a single object file as well.
I intended to make that a requirement. It looks incorrect to me to extend this for an invocation that generates single objects. Since the output of build systems are basically a list of invocations that generating single objects. Then we're stepping on the toes of build systems. I don't like to do that since it adds the complexity in the compiler side which should be done in the build systems side.
But now I think we can do that if the output of the invocation is in text form to read (e.g., `-S`, `-S -emit-llvm`) so that the output won't be used to produce other things further more. The motivation comes from supporting std module in compiler-explorer: https://github.com/compiler-explorer/compiler-explorer/issues/5404 <https://github.com/compiler-explorer/compiler-explorer/issues/5404 >.
Thanks,
Chuanqi
Thanks,
Chuanqi
------------------------------------------------------------------
From:Tom Honermann <tom_at_[hidden]>
Send Time:2023 Dec. 6 (Wed.) 10:45
To: SG15 <sg15_at_[hidden]pp.org>; Ran Regev <regev.ran_at_[hidden]>
Cc:Chuanqi <chuanqi.xcq_at_[hidden]>
Subject:Re: [SG15] [Modules] Should the compiler try to build std module implicitly?
On 12/4/23 9:43 PM, Chuanqi Xu via SG15 wrote:
Hi,
 It looks like we're interested in the direction to provide a simpler command line interfaces for "small" projects. I'll try to summarize this in a paper later.
A key point here is definition of "small" projects. I'll state my idea in the later section of the mail.
> 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 -stdliboption)?
If you were asking me (instead of the ideas of *wrapper*), I think it should be easy to find the locations of std module units according to the flags and platforms. Then it is an open question that if clang understands how to compile the std module from libstdc++ and MSVC. Since we don't know that with build systems yet. But from my experience in implementing a fake std module in libstdc++, it won't be a problem for the std module in libstdc++ if they follows the same style as libc++ did.
 I guess it depends on how complicated the standard library implementations get. They can become arbitrarily complicated of course. But if the metadata we've been discussing is provided, then a minimal built in build system should be able to identify the source files to be compiled, scan them, and then build the BMIs.
> I think a *wrapper* around the compiler that generates the dependencies and builds the needed BMI might be a more promising and better route.
Good idea. It reduces the work to implement the strategy I described. It should be easier to implement this within a yet-another clang tools. It may be problematic to implement this in a shell scripts or python scripts. Since we have to implement a tiny driver that way and it is also problematic to find the compiler and clang-scan-deps.

---
Hi Wyatt, I feel your reply may beyond the scope of the motivation of the post a little bit. I don't talk about building modules implicitly nor standardizing the GCC's module mapper. All I want to talk about is the std module. I don't want to generalize it too much. Otherwise the compiler are trying to do the jobs of build systems.
For the definition of "small" project mentioned above. In my mind, the "small" project should be able to be compiled within a single command line.
e.g.,
```
clang++ -std=c++20 hello.cpp -o hello
```
or if there are several input:
```
clang++ -std=c++20 a.cpp b.cpp c.cpp -o a.out
```
Then if a project is not suitable to be compiled in this way, please reach out build systems. This is the model in my mind.
I agree with this conception of a small project. Both of the above examples generate an executable, but I'm assuming you don't intend that to be a requirement. I would expect the std module to be built if needed for an invocation that generates a static or shared library or a single object file as well.
Tom.
Thanks,
Chuanqi
Thanks,
Chuanqi
------------------------------------------------------------------
From: SG15 <sg15_at_[hidden]> <mailto:sg15_at_lists.isocpp.org >
Send Time:2023 Dec. 5 (Tue.) 03:01
To:Ran Regev <regev.ran_at_[hidden]> <mailto:regev.ran_at_[hidden] >; SG15 <sg15_at_[hidden]> <mailto:sg15_at_[hidden] >
Cc:Tom Honermann <tom_at_[hidden]> <mailto:tom_at_[hidden] >
Subject:Re: [SG15] [Modules] Should the compiler try to build std module implicitly?
On 12/4/23 12:43 PM, Ran Regev wrote:
On Mon, Dec 4, 2023, 19:31 Tom Honermann via SG15 <sg15_at_[hidden] <mailto: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).
Tom.
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 <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 <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 <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
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