Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jun 26, 2019 at 11:24:32AM +0200, Peter Zijlstra wrote:
> That is, would something like this:
> 
> diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
> index 06c3cc22a058..1761b1e76ddc 100644
> --- a/arch/x86/include/asm/jump_label.h
> +++ b/arch/x86/include/asm/jump_label.h
> @@ -32,7 +32,7 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
>  		: :  "i" (key), "i" (branch) : : l_yes);
>  
>  	return false;
> -l_yes:
> +l_yes: __attribute__((cold));
>  	return true;
>  }
>  
> @@ -49,7 +49,7 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
>  		: :  "i" (key), "i" (branch) : : l_yes);
>  
>  	return false;
> -l_yes:
> +l_yes: __attribute__((hot));
>  	return true;
>  }
>  
> Help LLVM?

No, that's broken. What we do is the likely() and unlikely() hints in
static_branch_{,un}likely():

#define static_branch_likely(x)							\
({										\
	bool branch;								\
	if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))	\
		branch = !arch_static_branch(&(x)->key, true);			\
	else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
		branch = !arch_static_branch_jump(&(x)->key, true);		\
	else									\
		branch = ____wrong_branch_error();				\
	likely(branch);								\
})

#define static_branch_unlikely(x)						\
({										\
	bool branch;								\
	if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))	\
		branch = arch_static_branch_jump(&(x)->key, false);		\
	else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
		branch = arch_static_branch(&(x)->key, false);			\
	else									\
		branch = ____wrong_branch_error();				\
	unlikely(branch);							\
})

That should convey out-of-line vs in-line 'hint'. And clearly that's
broken for LLVM.