void added inline comments.
================
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
----------------
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?
Repository:
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