Remote query for structure layout

[David Blaikie]

(let me know if I'm just being an unhelpful bystander here - I clearly
don't have a lot of context)

On Tue, Mar 30, 2021 at 5:16 PM Simon Marchi <simon.marchi@polymtl.ca> wrote:
>
> On 2021-03-30 6:44 p.m., David Blaikie wrote:
> > If it's "just" some user-code, is there a variable of the desired type being declared around the function call?
>
> AFAIK, this type wouldn't be used by the FreeRTOS code at all, so no.
> Again, here's my understanding, hopefully it's close enough to the
> reality.
>
> When a trap occurs, FreeRTOS saves the current task's register values on
> the task's stack, using some arch-specific assembly code.  For example,
> for RISC-V:
>
>     https://github.com/FreeRTOS/FreeRTOS-Kernel/blob/534eba66ce4a5bda45d5edeeb81ac5a3cf6d0df8/portable/GCC/RISC-V/portASM.S#L121

Ah, OK. So this is part of signal handling - so it's not part of the
code that's compiled into the user's program, for instance... not even
in some system library linked in, necessarily, I guess?

> The way these registers are pushed is an implementation detail of
> FreeRTOS.  And we can imagine that it can vary depending on the
> compile-time FreeRTOS configuration,

I guess in the worst case it could be totally dynamic - it could pick
a different layout each time.

(guess a side question: How's this different from other systems? I
don't know how other/more common systems handle registers during
signals)

> which OpenOCD doesn't know about.
>
> To get register values of scheduled out tasks, OpenOCD needs to
> interpret these register values from the tasks' stacks.
>
> So Tim's suggestion is: have FreeRTOS declare a structure that has the
> exact layout as the saved registers on the stack:
>
> struct freertos_saved_regs {
>   int x1;
>   int x5;
>   int x6;
>   ...
> };
>
> Consumers could read that structure's layout from the DWARF info, and
> read the register values based on that.  That would be a lot more robust
> than hard-coding in the consumers how FreeRTOS stores things.

If practical experience has shown the hard-coding is not
stable/reliable (than FreeRTOS does change its strategy from time to
time - but it's always constant for any given build of the FreeRTOS) I
guess.

But I'm not sure where FreeRTOS would expose this structure to user
code - it's not like there's a system library header that user code
must include...

> However, since that struct would never actually be used by FreeRTOS'
> code, the compiler won't emit it.  Hence the need to find a way to force
> the compiler to include it in the DWARF.
>
> Does that clarify the situation?

Somewhat - any lack of understanding is just my ignorance in this
field/area in general, to be clear. (I'm also not a core gdb
developer, so I'm not the sort of person you have to convince of
anything - just a curious bystander trying to understand/maybe offer
some insightful suggestions (I predominantly work on LLVM's debug info
emission, so that's my background/connection))

- Dave