Re: [PATCH v2 01/16] powerpc: Replace unreachable() with it's builtin variant in __WARN_FLAGS()

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 29/08/2022 à 07:52, Sathvika Vasireddy a écrit :
> objtool is throwing *unannotated intra-function call* warnings in
> .c files with a few instructions that are marked unreachable. The
> problem comes from the annotate_unreachable() macro that is
> called by unreachable(). This annotation is adding a call to a
> function with size 0, and objtool does not add such symbols
> to the rbtree. Due to this reason, find_call_destination() function
> is not able to find the destination symbol for that call.
> 
> With the annotation (annotate_unreachable()), gcc seems to
> generate a 'bl' to unreachable symbol with size 0. But with
> the builtin variant of unreachable (__builtin_unreachable()),
> gcc does not emit calls to such symbols and the warnings
> go away. Given that the codegen remains same, and that
> there are no 'bl' instructions to such symbols emitted, fix
> these warnings by replacing unreachable() with it's builtin
> variant in __WARN_FLAGS().

How can you say that the codegen remains the same if with the original 
you get stale 'bl' instructions and with the alternative you don't ?

> 
> Also, add barrier_before_unreachable() before __builtin_unreachable()
> to work around a gcc bug [1], for the problem reported at [2].

Here my comment was not related to the gcc bug [1] but to gcc bug 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365 , which was worked 
around by commit 173a3efd3edb ("bug.h: work around GCC PR82365 in BUG()")

By chance it also solve the problem [1] as you mention.

> 
> [1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106751
> [2]: https://lkml.org/lkml/2022/8/25/418
> 
> Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>

Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>

> ---
>   arch/powerpc/include/asm/bug.h | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
> index 61a4736355c2..ef42adb44aa3 100644
> --- a/arch/powerpc/include/asm/bug.h
> +++ b/arch/powerpc/include/asm/bug.h
> @@ -99,7 +99,8 @@
>   	__label__ __label_warn_on;				\
>   								\
>   	WARN_ENTRY("twi 31, 0, 0", BUGFLAG_WARNING | (flags), __label_warn_on); \
> -	unreachable();						\
> +	barrier_before_unreachable();				\
> +	__builtin_unreachable();				\
>   								\
>   __label_warn_on:						\
>   	break;							\