[ Windows] [C++20] [Modules] [ABI] Linker errors relating to vftable with C++20 modules

[kamrann]

The following simple repro will generate multiple definition linker errors on Windows (regardless of link.exe or lld-link.exe, so appears the issue is with clang emitting symbols for every TU importing the module in question).

// mod.mxx
export module mod;
export struct poly
{
  virtual ~poly() = default;
};

// a.cxx
import mod;
poly p1;

// b.cxx
import mod;
poly p2;

int main() { return 0; }

Tested with multiple build systems (CMake and build2), on

• trunk (c44e9ff)

Edit: appears to be a regression, problem does not show up on 18.1.8 (3b5b5c1).

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Cameron Angus (kamrann)

The following simple repro will generate multiple definition linker errors on Windows (regardless of link.exe or lld-link.exe, so appears the issue is with clang emitting symbols for every TU importing the module in question).

// mod.mxx
export module mod;
export struct poly
{
  virtual ~poly() = default;
};

// a.cxx
import mod;
poly p1;

// b.cxx
import mod;
poly p2;

int main() { return 0; }

Tested with multiple build systems (CMake and build2), on

• trunk (c44e9ff)

Edit: appears to be a regression, problem does not show up on 18.1.8 (3b5b5c1).

[ChuanqiXu9]

Can you try the latest trunk?

[kamrann]

Tried again today (1eec81a), issue remains.

[ChuanqiXu9]

It looks like at least it works fine on linux: https://godbolt.org/z/zErjTqazf

[ChuanqiXu9]

I just realized that we changed the ABI spec for vtable in modules in ItaniumCXX ABI. But we didn't take care of it for Windows ABI..

I'll try to get connected with MSVC devs to make sure the behavior of vtables in modules in Windows ABI.

[kamrann]

Yep, sorry should have been clearer - I'm building my project on both Windows and Linux, and this issue only showed up on Windows (and even there, only at some point after 18.1.8).

I'll try to get connected with MSVC devs to make sure the behavior of vtables in modules in Windows ABI.

Sounds good, thanks!

[Ivan171]

I just realized that we changed the ABI spec for vtable in modules in ItaniumCXX ABI. But we didn't take care of it for Windows ABI.

I tried to build one of my projects with an updated clang (af784a5) and got errors related to vtable as well. My old build of clang (faf675c) builds without any error.

It's probably related to this issue.

[ChuanqiXu9]

I reverted that change for Itanium C++ ABI in #75912. So I think we can close this. Feel free to ask to reopen this if it is not actually fixed.