[libcxx-commits] [PATCH] D140584: [Clang] Refactor "Designators" into a unified implementation [NFC]

[Bill Wendling via Phabricator via libcxx-commits]

void added inline comments.


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Comment at: clang/include/clang/AST/Designator.h:88
+  /// An array designator, e.g., "[42] = 0" and "[42 ... 50] = 1".
+  template <typename Ty> struct ArrayDesignatorInfo {
+    /// Location of the first and last index expression within the designated
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rsmith wrote:
> void wrote:
> > rsmith wrote:
> > > void wrote:
> > > > rsmith wrote:
> > > > > void wrote:
> > > > > > void wrote:
> > > > > > > rsmith wrote:
> > > > > > > > Can we move the templating out from here to the whole `Designator` and `Designation` classes? It shouldn't be possible to mix the two kinds in the same `Designation`.
> > > > > > > Grr...My previous comment was eaten.
> > > > > > > 
> > > > > > > I'll give it a shot.
> > > > > > > 
> > > > > > > However, I'm a bit surprised at how designators are handled by Clang. I expected that a `Designation` would be an `Expr` with the `Designator`s being L-values (e.g. `MemberExpr`s / `ArraySubscriptExpr`s), but instead the `Designation` exists just long enough to be turned into an explicit initialization list. Is there a reason to do it that way instead of using expressions?
> > > > > > So it looks like moving the template outside of the class won't work. The ability to switch between `Expr` and `unsigned` while retaining the same overall type is hardwired into things like the `ASTImporter`.
> > > > > > 
> > > > > > This is kind of a massive mess. Maybe we shouldn't even allow them to use both `Expr` and `unsigned` but instead require them to use one or the other? Maybe we could require `unsigned` with the understanding that the `Expr` can be converted into a constant?
> > > > > I'm not understanding something. Currently the `ASTImporter` only deals with `DesignatedInitExpr::Designator`s , which only ever store integer indexes.
> > > > > 
> > > > > Basically, today, we have two different classes:
> > > > > - A class that's specific to `DesignatedInitExpr`, and tracks array index expressions by storing the index of the expression within the `DesignatedInitExpr`'s list of children; this is also what `ASTImporter` can import, because it's the one that's used in the AST's representation.
> > > > > - A class that's specific to `Sema`'s processing that tracks array index expressions as `Expr*` instead.
> > > > > 
> > > > > You want to refactor them to share code, which makes sense, because they are basically the same other than how they refer to expressions. (Not quite: `DesignatedInitExpr` can apparently refer to a field either as an `IdentifierInfo*` or as a `FieldDecl*`, whereas the `Sema` version always uses the `IdentifierInfo*` representation.)
> > > > > 
> > > > > Each current user of one of these two classes uses only one of the two, which means they're either exclusively using integers to refer to expressions or exclusively using `Expr*`. So it seems to me that you should be able to update each user to use either `Designator<unsigned>` or `Designator<Expr*>`, depending on which class they used before.
> > > > > 
> > > > > What am I missing?
> > > > I'm still allowing them to use a `Designator<unsigned>` / `Designator<Expr*>` as they see fit, only it's hidden from them via the `Create` methods. I personally find the use of two different versions (one using `unsigned` and one using `Expr*`) completely baffling. Why can't they all use `Expr*`? Also the `ASTImporter` only outputs the start of an array init range, which is at the very least counter-intuitive. That's one of the issues I'd like to tackle with follow-up patches, hopefully getting rid of the need for this template all together. This does mean that in the interim a non-array range designator will have extra `End` & `EllisisLoc` fields that aren't used, but that shouldn't be too horrible, given that they'd be there anyway because of the union.
> > > The reason that's jumping out at me for having separate integer / `Expr*` implementations here is space-efficiency -- we get to make array range designators (and hence designators overall) be only 16 bytes rather than the 32 bytes they occupy in this patch (assuming 64-bit pointers) by storing indexes instead of pointers.
> > > 
> > > If your eventual plan is to remove the children list from `DesignatedInitExpr`, and store the pointers only in the designators, that seems to cost 8 bytes per designator in the two common cases:
> > > 
> > > - For a field designator: 32 bytes (with 16 bytes of padding) versus 16 bytes + 8 bytes for the child pointer today
> > > - For an array designator: 32 bytes (with 16 bytes of padding) versus 16 bytes + 8 bytes for the child pointer today
> > > - For an array range designator: 32 bytes (4 bytes of padding) versus 16 bytes + 16 bytes for two child pointers today
> > > 
> > > ... plus it'll presumably be painful to make the `Stmt` child iterator be able to handle this.
> > > 
> > > If you don't remove the separate children list from `DesignatedInitExpr`, then it seems like this approach will cost 16 bytes per designator in all cases, and we'll need to be careful in AST serialization / deserialization that we don't accidentally duplicate the `Expr`s that now have two pointers pointing to them instead of one, and likewise anywhere else that assumes each `Expr` is only reachable by one path through the AST (eg, `TreeTransform`, the recursive AST visitor).
> > > 
> > > I think some more visibility into the eventual plan would help.
> > The plan isn't detailed, but I basically want to address several of the points you mentioned here. In particular, I think the structure of `DesignatedInitExpr` is backwards from how every other `Expr` is handled in Clang. For instance, the `Expr` for something like `s.t.u` is a `MemberExpr` with a `MemberExpr` as its sub-expression and so on. `DesignatedInitExpr` on the other hand basically has a list of maybe expressions, maybe integers that refer to parts of the structure / array. It seems cleaner to me to use the `MemberExpr` / `ArraySubscriptExpr` way of referring to the member being initialized rather than using a specialized list that has to be handled differently from other `Expr`'s.
> > 
> > The first step in my evil plot is to do this simple refactoring, so that there's no initial functionality change, before I do the more invasive changes that may break things.
> > 
> > I'm doing this because I'm working on a feature that uses the `DIE` syntax, and it would be much simpler to have it be a `MemberExpr`.
> > 
> > Am I completely off base here?
> Thanks, that's really helpful.
> 
> OK, so the end design would be something like (just building out some details here so I can think about this better; I'm not expecting you would necessarily do exactly this!):
> - `DesignatedInitExpr` becomes a base class with derived classes for member designators, array designators, and array range designators (And maybe also there's a different representation for unresolved versus resolved member designators.)
> - These work much like `MemberExpr` / `ArraySubscriptExpr`, except that they don't have a "base" object (and instead store an initializer for the nominated field / array element(s)), and are "inside-out": for `.t.u = x` we want the top-level expression to be a `DesignatedInitMemberExpr` that names `t` and whose initializer is a `DesignatedInitMemberExpr` that names `u` and whose initializer is `x`, whereas for `s.t.u` the top-level expression is a `MemberExpr` that names `u` and its base subexpression is a `MemberExpr` that names `t`. That is, while we model `s.t.u = x` as `((s.t).u) = x`, we model `.t.u = x` more like `.t = (.u = x)`
> - We use the same representation both in `DesignatedInitExpr` and in `Sema` (and everywhere else that looks at the syntactic form of an initializer list).
> - `Designation` and `Designator` are removed entirely.
> 
> If something like that is the plan, then yes, I think that's completely reasonable, and seems like a nice improvement -- and I think it's fine if the intermediate state increases the AST size for designated initializers, as this patch does, especially if you're aiming for this to be completed within a single release cycle.
> 
> (Just quickly checking the size of the representation: a single-field `DesignatedInitExpr` is currently 40 bytes + 16 bytes for the `Designator` array, and I think could easily be 40 bytes *total* with the new representation; with two fields, it's currently 40 + 32 and could be 40 + 40 with the new representation. I think that's going to be a win essentially always.)
Thanks! I'm going to dig into the DIE stuff more and will have a much more detailed plan that I can give you to see what you think before I go too far down the wrong path. :-)


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