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

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Message-ID: <3a11ed33-0a3d-2668-ab2b-c44eea7eddbe@arm.com>
Date:   Tue, 13 Dec 2022 19:50:18 +0000
From:   Douglas Raillard <douglas.raillard@....com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Valentin Schneider <vschneid@...hat.com>
Subject: Re: [for-next][PATCH 02/11] tracing: Add __cpumask to denote a trace
 event field that is a cpumask_t

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