| Issue |
113950
|
| Summary |
Reduce `Decl`'s dependency on `DeclContext`/other Decls.
|
| Labels |
clang
|
| Assignees |
erichkeane
|
| Reporter |
erichkeane
|
One of the outcomes of the llvm-dev talk by @zygoloid is that it got me thinking about ways to improve template instantiation time. One such problem is that we rebuild every `Decl` every time, particularly `VarDecl` thanks to its link to its `DeclContext` and other `Decl`s, instead of being able ot use them like we do with some `Stmt` nodes.
There are three steps to this:
1- Remove the dependency on `DeclContext` for `Decl` types where this doesn't matter. From a quick look, it seems that various `FunctionDecl` types probably still need the link, as does `VarDecl`s at the global state, so moving this out of `Decl` and reproducing the `DeclContext` references in each place it is needed (plus conditionally NOT referencing it in the `VarDecl` case) is likely a great first step.
2- Replace the `DeclContext::decls` implementation/storage. Currently this is done by using a linked-list as a part of `Decl`, which obviously causes a dependency on the "next" declaration, so we could only re-use the 'last' declarations. Instead, we could perhaps have some sort of storage on `DeclContext` itself to do this relationship.
3- Update instantiation of various `Decl` types to re-use the node if none of its children change. `VarDecl` in the "parent is a function decl" case is probably a great first step that ends up saving us a bit, but other types likely assume they always need to be rebuilt. This part is likely a long-tail, but we can enable it via 1 and 2.
FILING this as an issue so that someone someday will work on this. I don't really have time short-term, so if someone is wanting to take this one, please feel free!
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