Initialising Context Pointer during Semantic Analysis

[Johan]

On Friday, 25 November 2022 at 16:10:36 UTC, Teodor Dutu wrote:
> Hi,
>
> For SAoC 2022, I want to translate DRuntime hooks to templates. 
> I am working on `_d_newitem{U,T,iT}`. The compiler uses them to 
> lower `new S(args)` expressions, where `S` is a `struct`.
>
> To instantiate the new template hooks, the lowering has to be 
> moved from the glue layer to the semantic analysis. However, 
> when the constructed `struct` is nested, the lowering logic 
> also needs to copy the `struct`'s context pointer(s). This is 
> currently [handled by the glue layer in 
> `e2ir.d`](https://github.com/dlang/dmd/blob/6bf60ea0eb174631ede0074a77d3898d943e0b30/compiler/src/dmd/e2ir.d#L1191-L1209). Now the same logic needs to be replicated in the semantic analysis, where there is no machinery to find the right frame pointer, such as [`getEthis()` in `toir.d`](https://github.com/dlang/dmd/blob/6bf60ea0eb174631ede0074a77d3898d943e0b30/compiler/src/dmd/toir.d#L212).
>
> After discussing with my mentors, we have come up with 2 
> possible approaches:
>
> 1. Add a new AST node for copying context pointers to 
> `expression.d`: `ContextPointerExp`. The new lowering of `new 
> S(args)` in the semantic phase will insert a 
> `ContextPointerExp` node and then the glue layer will visit it 
> and copy the correct context pointer to the newly created 
> `struct`. This approach is efficient from a runtime 
> perspective, as it doesn't add any unnecessary computation, but 
> has the disadvantage of requiring changes beyond the frontend. 
> Thus it won't be transparent for LDC and GDC, who will have to 
> handle `ContextPointerExp` in their own glue layers.

This is not a deal breaker, as long as it is clearly 
communicated. So add a special section to the release notes for 
frontend-dependent projects what has changed / what was added. 
Adding AST nodes impacts more than just GDC and LDC.

Why not extend the information attached to a NewExpression node?
AST rewriting makes it much harder (or impossible) to generate 
good user diagnostics and debug information.

-Johan