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

[John McCall via cfe-dev]

> 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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