[all-commits] [llvm/llvm-project] dfa7ff: [C++20] [Modules] [Reduced BMI] Combine the signat...

[Chuanqi Xu via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: dfa7ff97b24dc5a3dd714b45af288812c13d0110
      https://github.com/llvm/llvm-project/commit/dfa7ff97b24dc5a3dd714b45af288812c13d0110
  Author: Chuanqi Xu <yedeng.yd at linux.alibaba.com>
  Date:   2024-05-07 (Tue, 07 May 2024)

  Changed paths:
    M clang/include/clang/Serialization/ASTWriter.h
    M clang/lib/Serialization/ASTWriter.cpp
    A clang/test/Modules/function-transitive-change.cppm
    M clang/test/Modules/no-transitive-source-location-change.cppm

  Log Message:
  -----------
  [C++20] [Modules] [Reduced BMI] Combine the signature of used modules
into the current module

Following of https://github.com/llvm/llvm-project/pull/86912. After
https://github.com/llvm/llvm-project/pull/86912, with reduced BMI, the
BMI can keep unchange if the dependent modules only changes the
implementation (without introduing new decls). However, this is not
strictly correct.

For example:

```
// a.cppm
export module a;
export inline int a() { ... }

// b.cppm
export module b;
import a;
export inline int b() { return a(); }
```

Since both `a()` and `b()` are inline, we need to make sure the BMI of
`b.pcm` will change after the implementation of `a()` changes.

We can't get that naturally since we won't record the body of `a()`
during the writing process. We can't reuse ODRHash here since ODRHash
won't calculate the called function recursively. So ODRHash will be
problematic if `a()` calls other inline functions.

Probably we can solve this by a new hash mechanism. But the safety and
efficiency may a problem too. Here we just combine the hash value of the
used modules conservatively.



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