[cfe-dev] [ Builtins ] Returning a custom struct

[ThePhD via cfe-dev]

Dear John,

     I didn't quite realize you could do that! Thank you for that. I
managed to make a lot of progress from your tip, and I'm almost done with
the builtin!

     I hope I can get something done to put up for basic discussion to the
clang mailing list soon. As a slight side question, if I'm looking for
"review" (but not necessarily with the goal of upstreaming to Clang just
yet), would it be appropriate to send a diff to the mailing list here?

Sincerely,
ThePhD


On Sat, May 26, 2018 at 4:39 PM, John McCall <rjmccall at apple.com> wrote:

> On May 26, 2018, at 1:51 PM, ThePhD via cfe-dev <cfe-dev at lists.llvm.org>
> wrote:
> Dear cfe-dev,
>
>      I am new at hacking on Clang and I've made a decent amount of forward
> progress in defining my own Constant Expression builtin for C++. It takes
> arguments which must be compile-time evaluatable. This worked out just fine
> so far, and has allowed me to get started on implementing std::embed
> <https://rawgit.com/ThePhD/embed/master/papers/P1040%20-%20embed.html>.
>
>      I wanted to wrap my Constant Expression builtin in a C++-defined
> `constexpr` function. The problem with this is that any argument passed to
> my wrapping function that used my builtin ended up becoming non-evaluatable
> in *Visit calls during the Sema::ActOnFinish* phase.
>
>      This has a few consequences. My wrapper function uses 2 less
> arguments than my builtin currently, passing 2 variables to the builtin to
> be filled in as output parameters, and then placed into a nice struct the
> user can use (unsigned char* and a size_t). I lose the interface if I use
> the builtin directly and someone has to provide the output parameters.
>
>      I tried to use the "t" specifier on the Builtin in Builtin.def to let
> me define my own type checking, but so far it seems like even with "t" I
> can't make my builtin have a signature that returns a custom structure.
>
>
> Are you trying to cast the result of the CallExpr?  You should just set it
> directly; that's allowed for custom-typechecked builtins.
>
> John.
>
>
>      Does anyone know how I would begin to work around these problems? My
> first interest is to make the builtin I wrote match the struct-returning
> interface I need. That would allow me to drop the 2 output parameters and
> instead return a structure. I do not know how to do this.
>
>      My second idea is to abandon defining it through Builtins.def
> altogether, but I'm not sure how to go about defining a builtin that's not
> part of that list.
>
>      Does anyone have any insight into this?
>
> Sincerely and Hopefully,
> ThePhD
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>
>
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