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

Peter Zijlstra
2019-06-26T10:43:40Z

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.