Re: [for-next][PATCH 02/11] tracing: Add __cpumask to denote a trace event field that is a cpumask_t

Douglas Raillard
2022-12-13T19:50:33Z

On 13-12-2022 14:20, Douglas Raillard wrote:
>
> The only case I can think of where parsing would not follow regular C abstract declaration syntax is a type like that:
>
>     __data_loc int [3][]

After some experimentation, I came to the conclusion that "__data_loc <type> [] <id>" can indeed support any C type if they
need to be added in the future. I would heavily suggest that any future extension works as the following, to avoid messing up
the grammar in a subtle way that would prevent some types to be expressible, or make it a nightmare to implement.

The recipe to parse that with stock C parser is:
* consume "__data_loc"
* parse "<type> []" as a func prototype parameter declaration
(declaration using an abstract declarator, i.e. not introducing any identifier)
* parse "<id>" as an identifier.

> The outer-most array is by definition the dynamic one, so "[]". In normal C, [3] and [] would be swapped as
> the outer-most array comes first. That's not too bad though as it is not ambiguous and easy to fixup directly
> in the parse tree.

Simply swapping is wrong in the general case. The correct modification of the "<type> []" parse tree is doing a
"barrel shift" on nested array sizes. If the type is "int [1][2][]", it needs to be turned into "int [][1][2]".
The now-top-level sizeless array is the dynamic array. Note that pointers level are transparent,
so "int (*[1])[2][]" needs to be turned into "int (*[])[1][2]"

-- Douglas