Re: `likely` not applied with Clang and `CONFIG_TRACE_BRANCH_PROFILING`

[Steven Rostedt]

On Fri, 16 Oct 2026 19:12:42 +0200
Miguel Ojeda <ojeda@xxxxxxxxxx> wrote:

> Hi Nathan, Steve, Masami, Arnd, all,

Hi Miguel,

> 
> I was updating my old patch to add `likely()` and friends to the Rust
> side of the kernel [1] and I noticed something odd when comparing with
> the C side with Clang:
> 
>   https://godbolt.org/z/6Kendxzzd
> 
> Under `CONFIG_TRACE_BRANCH_PROFILING`, the way we use `__builtin_expect`
> with a temporary variable makes Clang ignore the hint (GCC is fine).
> 
> So essentially:
> 
>     #define likely(x)   __builtin_expect(!!(x), 1)
>     #define unlikely(x) __builtin_expect(!!(x), 0)
> 
> is fine for both compilers, but:
> 
>     #define __branch_check__(x, expect, is_constant) ({ \
>         long ______r; \
>         ...
>         ______r = __builtin_expect(!!(x), expect); \
>         ...
>         ______r; \
>     })
> 
>     #define likely(x)   (__branch_check__(x, 1, __builtin_constant_p(x)))
>     #define unlikely(x) (__branch_check__(x, 0, __builtin_constant_p(x)))
> 
> is not. Now, if we do move the `__builtin_expect` outside like:
> 
>     #define __branch_check__(x, expect, is_constant) ({ \
>         long ______r; \
>         ...
>         ______r = !!(x); \
>         ...
>         ______r; \
>     })
> 
>     #define likely(x)   __builtin_expect(__branch_check__(x, 1, __builtin_constant_p(x)), 1)
>     #define unlikely(x) __builtin_expect(__branch_check__(x, 0, __builtin_constant_p(x)), 0)
> 
> then it works with Clang again.
> 
> Asking an LLM to sample a few object files to see the effect, it does
> seem like it works as expected, e.g. `kernel/signal.o` goes from 1 to
> 125 branches carrying weights and 39 of its 355 functions change codegen
> (675 of ~3800 across the dozen objects sampled).
> 
> It doesn't mean it is better of course -- I didn't benchmark it, but it
> means it is passing the information to the compiler that we expected.
> 

If it works, I'm fine with changing it. The branch profile is *not* optimized ;-)

Hmm, which reminds me, I'll be enabling it again on my server to see where
things are wrong soon.

-- Steve