Re: [XEN PATCH 1/3] xen: introduce static_assert_unreachable()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.01.2024 14:48, Federico Serafini wrote:
> Introduce macro static_asser_unreachable() to check that a program
> point is considered unreachable by the static analysis performed by the
> compiler, even at optimization level -O0.

Is it really intended to limit use of this macro to cases where even
at -O0 the compiler would eliminate respective code? Note that right
now even debug builds are done with some optimization, and some of
the DCE we're relying depends on that (iirc).

> --- a/xen/include/xen/compiler.h
> +++ b/xen/include/xen/compiler.h
> @@ -64,6 +64,14 @@
>  # define fallthrough        do {} while (0)  /* fallthrough */
>  #endif
>  
> +/*
> + * Add the following macro to check that a program point is considered
> + * unreachable by the static analysis performed by the compiler,
> + * even at optimization level -O0.
> + */
> +#define static_assert_unreachable() \
> +    asm(".error \"unreachable program point reached\"");

Did you check the diagnostic that results when this check actually
triggers? I expect it will be not really obvious from the message
you introduce where the issue actually is. I expect we will want
to use some of __FILE__ / __LINE__ / __FUNCTION__ to actually
supply such context.

Also: Stray semicolon and (nit) missing blanks.

Finally I wonder about case: We have ASSERT_UNREACHABLE() and it
may be indicated to use all uppercase her as well.

Jan