[clang] [llvm] [Clang][AST][NFCI] Make Decl reference attributes directly (PR #219138)

[Erich Keane via cfe-commits]

erichkeane wrote:

> FYI, Erich and I dumped the layout.
> 
> ```
> *** Dumping AST Record Layout
>          0 | class clang::Decl
>          0 |   (Decl vtable pointer)
>          8 |   class llvm::PointerIntPair<class clang::Decl *, 3, enum clang::Decl::ModuleOwnershipKind> NextInContextAndBits
>          8 |     struct llvm::detail::PunnedPointer<class clang::Decl *> Value
>          8 |       unsigned char[8] Data
>         16 |   class llvm::PointerUnion<class clang::DeclContext *, struct clang::Decl::MultipleDC *> DeclCtx
>         16 |     class llvm::pointer_union_detail::PointerUnionMembers<class llvm::PointerUnion<class clang::DeclContext *, struct clang::Decl::MultipleDC *>, 0, class clang::DeclContext *, struct clang::Decl::MultipleDC *> (base)
>         16 |       class llvm::pointer_union_detail::PointerUnionMembers<class llvm::PointerUnion<class clang::DeclContext *, struct clang::Decl::MultipleDC *>, 1, struct clang::Decl::MultipleDC *> (base)
>         16 |         class llvm::pointer_union_detail::PointerUnionMembers<class llvm::PointerUnion<class clang::DeclContext *, struct clang::Decl::MultipleDC *>, 2> (base)
>         16 |           struct llvm::detail::PunnedPointer<void *> Val
>         16 |             unsigned char[8] Data
>         24 |   class clang::SourceLocation Loc
>         24 |     UIntTy ID
>     28:0-6 |   unsigned int DeclKind
>     28:7-7 |   unsigned int InvalidDecl
>     29:0-0 |   unsigned int HasAttrs
>     29:1-1 |   unsigned int Implicit
>     29:2-2 |   unsigned int Used
>     29:3-3 |   unsigned int Referenced
>     29:4-4 |   unsigned int TopLevelDeclInObjCContainer
>     29:5-6 |   unsigned int Access
>     29:7-7 |   unsigned int FromASTFile
>    30:0-13 |   unsigned int IdentifierNamespace
>     32:0-2 |   unsigned int CacheValidAndLinkage
>            | [sizeof=40, dsize=33, align=8,
>            |  nvsize=33, nvalign=8]
> ```
> 
> Based on that, the change probably makes sense. We should note that we are gonna be pretty bit-starved afterwards, the new pointer could have its alignment bits stolen and we probably can reduce IdentifierNamespace by a couple of bits, but that's it

IMO, we are LONG overdue for removing the `NextInContext` and replace it with a collection of some sort in `DeclContext`.  The linked-list functionality has some distinct problems (costing us a pointer in EVERY decl is just the beginning, we end up having to instantiate stuff we otherwise don't need to as a result), but it is a pretty big task.  AFTER this patch, it doesn't actually save us anything anymore I think (since we still have the `ModuleOwnershipKind` in there that we would need to add to the bitfield), but it would free up a TON of bits.



https://github.com/llvm/llvm-project/pull/219138