[cfe-dev] Will Clang use PCH format to implement C++20 module system?

[Tom Honermann via cfe-dev]

These questions are more apt for the WG21 SG15 tooling study group (https://lists.isocpp.org/mailman/listinfo.cgi/sg15).

But briefly, module artifact files (.bmi, .pcm, ...) are not intended for distribution; they should be treated as transient build artifacts.  At present, for some compilers, these files are not usable between different versions of the same compiler (this is the case for gcc).  Module artifact files are not a substitute for source code.

Tom.

From: cfe-dev <cfe-dev-bounces at lists.llvm.org> On Behalf Of Yafei Liu via cfe-dev
Sent: Wednesday, January 8, 2020 1:30 AM
To: David Blaikie <dblaikie at gmail.com>
Cc: Clang Dev <cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] Will Clang use PCH format to implement C++20 module system?

So what will happen to package management platforms like apt?
For example, I wrote a module and wanna distribute it on apt, I have three possible ways of doing this:
1. since precompiled modules to be portable between compilers, I have to compile my code with different compilers then upload different modules.
2. I upload the source and let the user do the compile.
3. I don't use module at all, use the "header+library" old way.

which should be the right way?


On Wed, Jan 8, 2020 at 2:38 AM David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>> wrote:
It won't - there are no plans for precompiled modules to be portable between compilers. (someone at Microsoft has been having some discussions about maybe making some of that possible - but there's a fair bit of resistance, and so far as I know from Clang and GCC there's no work in that direction)

On Tue, Jan 7, 2020 at 4:10 AM Yafei Liu <yfliu at mobvoi.com<mailto:yfliu at mobvoi.com>> wrote:
Hey David, I got another question, do you know how will Clang handle modules which is compiled by other compiler? For example GCC? The AST serialization is not the same.

On Tue, Jan 7, 2020 at 2:30 AM David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>> wrote:


On Mon, Jan 6, 2020 at 2:28 AM Yafei Liu via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
Hi all, I'm curious if the PCH format will be used to implement C++20 module system? Any Clang community discussions or documents on this?

More or less. The basic AST serialization system is already shared between PCH, Clang Header Modules, and preliminary support for C++20 Modules. Not exactly the same, because the semantics of each are slightly different - but the core AST serialization is common across all 3.

What's more, I'm inventing a language using LLVM, and I want to generate some middle layer to communicate with c++ module, so is there any tools or apis to generate a PCH?

None that I know of. The only cross-language binding like that that I know of is Swift, which is one-way to the best of my knowledge (so Swift doesn't have to generate Clang-compatible AST files (PCH/PCM/whatever) - it only has to be able to read them). I imagine Swift's binary module representation might be similar for ease of interoperability, but I don't actually know.

- Dave
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