| Issue |
133955
|
| Summary |
[libclang] Theoretical ABI break in `libclang.so` 20.1.1
|
| Labels |
clang:as-a-library
|
| Assignees |
|
| Reporter |
isuckatcs
|
In `libclang.so.20.1.0` we have the following `Sema` class.
```c++
class Sema final : public SemaBase {
...
private:
/// Function or variable declarations to be checked for whether the deferred
/// diagnostics should be emitted.
llvm::SmallSetVector<Decl *, 4> DeclsToCheckForDeferredDiags;
/// Map of current shadowing declarations to shadowed declarations. Warn if
/// it looks like the user is trying to modify the shadowing declaration.
llvm::DenseMap<const NamedDecl *, const NamedDecl *> ShadowingDecls;
...
};
```
In `libclang.so.20.1.1` this definition is changed the way below.
```c++
class Sema final : public SemaBase {
...
public:
...
/// Function or variable declarations to be checked for whether the deferred
/// diagnostics should be emitted.
llvm::SmallSetVector<Decl *, 4> DeclsToCheckForDeferredDiags;
private:
/// Map of current shadowing declarations to shadowed declarations. Warn if
/// it looks like the user is trying to modify the shadowing declaration.
llvm::DenseMap<const NamedDecl *, const NamedDecl *> ShadowingDecls;
...
};
```
In other words, `DeclsToCheckForDeferredDiags` is made `public` in 20.1.1, when it was `private` in 20.1.0.
Until C++23 the C++ standard said the following about this.
> [Note: Non-static data members of a (non-union) class with the same access control (11.9) and non-zero
> size (6.7.2) are allocated so that later members have higher addresses within a class object. **The order of
> allocation of non-static data members with different access control is unspecified**.
>
> N4849 § 11.4 19
With C++23 however this restriction was removed though.
> [Note 8 : Non-variant non-static data members of non-zero size (6.7.2) are allocated so that later members
> have higher addresses within a class object (7.6.9).
>
> N4950 § 11.4.1 20
LLVM is still compiled targeting the C++17 standard, which means, the first quote of the standard applies and compilers have the right to change the field order of `Sema` IIUC. I'm not sure if there are compilers taking advantage of this, but if they do, the resulting `libclang.so` will not be ABI compatible with it's previous release.
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