llvm/llvm-project

[C++20] [Modules] Not proper module initializer

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#174065 opened on Dec 31, 2025

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ABIclang:moduleshelp wantedplatform:windows

Description

In my local linux machine, for

export module a;

When I generate the code, I can see:

define void @_ZGIW1a() #0 section ".text.startup" {
entry:
  ret void
}

this is used for initializing entities for the module unit. Itanium C++ ABI asks us to generate this even if we don't need to initialize any thing.

But if I turned MSVC ABI, I can say nothing for the initializer. We need input from MSVC, either formal docs or developers, how should we do here. And if we missed anything, we need to implement the module initializer for Windows ABI.

Estimated difficulty: medium

Contributor guide

Tech stack
cpp
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
C++20 modules knowledgeLLVM compiler internalsABI understanding
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Examine the current module initializer code generation in clang (likely in CodeGenModule.cpp or similar). Understand the Itanium C++ ABI's requirement for emitting a module initializer even when empty. Research MSVC ABI documentation or check Microsoft's open source compiler (e.g., MSVC STL or clang cl) for how they handle module initializers. Check the issue comments for any additional context or pointers from maintainers. The goal is to determine whether MSVC ABI requires a module initializer and, if so, implement it in LLVM's MSVC ABI path.