doubt about 'bool is_executing'

[Gedare Bloom]

On Thu, Jul 29, 2021 at 12:56 AM Sebastian Huber
<sebastian.huber at embedded-brains.de> wrote:
>
> On 28/07/2021 21:54, Gedare Bloom wrote:
> > Hello all,
> >
> > In most (all?) the SMP ports, we have in the CPU Context Control:
> >    bool is_executing;
>
> Yes, all SMP ports need this. It is used to implement a TTAS lock in the
> context switch:
>
> https://docs.rtems.org/branches/master/c-user/symmetric_multiprocessing_services.html#thread-dispatch-details
>
> >
> > This variable often is accessed through assembly language (i.e.,
> > during the context switch), using something like
> > ARCH_CONTEXT_CONTROL_IS_EXECUTING_OFFSET. Unfortunately, the 'bool'
> > type is not standard, or rather its size is implementation-defined.  I
> > think it is generally an 8-bit value though for us. I see that the
> > riscv port uses explicitly a uint32_t so that's nice.
> >
> > I think we should replace 'bool' with uint8_t in the other ports that
> > are using byte-size accesses from assembly, just to be well-defined.
> > Most likely, this doesn't actually affect anything and I'm just a
> > little paranoid.
>
> Is the bool size and alignment not defined by the ABI? We don't have to
> use this type, we could also use an uint32_t or whatever. The convention
> is that a zero value is false, everything else is true.
>
I'm no expert, but I believe bool is defined by the compiler. There is
only a minimum size requirement of 1 byte, but no maximum size. So, we
do get consistent size, but we rely on gcc (and clang) to do the same
thing with bool and to not change their behavior in the future. This
is probably safe, but making assumptions about what compiler
implementers do can lead us into trouble.

I would switch to an explicitly sized type, either uint8_t or uint32_t
depending whether alignment may be more useful than potential 3B space
savings. Since we only deal with 32b and larger architectures with
data caches, the use of smaller types doesn't offer any great time
advantages.

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