TypeInfo in struct ABI

[Basile B.]

On Monday, 17 January 2022 at 15:26:46 UTC, Teodor Dutu wrote:
> Hi,
>
> In order to allocate a `struct` type, two DRuntime hooks are 
> used: `_d_newitemT` for uninitialised structs and 
> `_d_newitemiT` for initialised ones. Both hooks call 
> `_d_newitemU`, which allocates memory for the struct and 
> [writes the struct’s 
> `TypeInfo`](https://github.com/dlang/druntime/blob/9d99b1ccd90d1e66c31e27cc71b8a266612aa14a/src/rt/lifetime.d#L1122) at the end of the allocated chunk.
>
> We now attempt to convert these hooks to templates, with the 
> goal of not using `TypeInfo` at all. However, the fact that the 
> ABI of a struct contains its `TypeInfo` prevents us from 
> achieving this goal. Is this `TypeInfo` object still necessary 
> for anything, or are we free to remove it from the ABI 
> altogether so that we have cleaner implementations for 
> `_d_newitem{U,iT,T}`?
>
> Thanks,
> Teodor

That trailing info seems to be unused.

TypeInfo is retrieved using `typeid`. So let's took a closer look 
at the code generated [for 
that](https://github.com/dlang/dmd/blob/e4d6fe8ee5af972125624230ded5a81e55b40711/src/dmd/e2ir.d#L818-L838), in dmd.e2ir.d, which gives the best idea of the code executed by the program.

For structs it's clear that the static type is *always* used and 
not that trailing stuff that's written passed the actual struct 
instance space. For structs that uses the code generated in the 
`if (Type t = isType(e.obj))` branch.