Re: [PATCH 1/3] Compiler Attributes: Add __assume macro

[Nathan Chancellor <nathan@kernel.org>]

Hi Heiko,

On Wed, Sep 10, 2025 at 05:12:14PM +0200, Heiko Carstens wrote:
> Make the statement attribute "assume" with a new __assume macro available.
> 
> This allows compilers to generate better code, however code which makes use
> of __assume must be written as if the compiler ignores the hint. Otherwise
> this may lead to subtle bugs if code is compiled with compilers which do
> not support the attribute.
> 
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
> ---
>  include/linux/compiler_attributes.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> index c16d4199bf92..16c3d4a865e2 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -54,6 +54,22 @@
>   */
>  #define __always_inline                 inline __attribute__((__always_inline__))
>  
> +/*
> + * Beware: Code which makes use of __assume must be written as if the compiler
> + * ignores the hint. Otherwise this may lead to subtle bugs if code is compiled
> + * with compilers which do not support the attribute.

It may be worth noting that careful analysis should be performed when
adding this attribute since clang's documentation [1] (more on that
below...) notes that it could hurt optimizations just as much as it
could help it.

> + *
> + * Optional: only supported since GCC >= 13.1, clang >= 12.0
> + *
> + *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#index-assume-statement-attribute
> + * clang: https://clang.llvm.org/docs/AttributeReference.html#assume

Looking at this link sent me down a bit of a rabbit hole :) Prior to
Clang 19.1.0 [2], assume was an OpenMP attribute, which has completely
different semantics and errors out when used in the way the series does:

  In file included from kernel/bounds.c:13:
  In file included from include/linux/log2.h:12:
  In file included from include/linux/bitops.h:67:
  arch/s390/include/asm/bitops.h:173:12: error: expected string literal as argument of '__assume__' attribute
    173 |                 __assume(bit <= 64);
        |                          ^

Unfortunately, I think __assume will need to be handled in the compiler
specific headers :/

[1]: https://clang.llvm.org/docs/AttributeReference.html#id13
[2]: https://github.com/llvm/llvm-project/commit/c44fa3e8a9a44c2e9a575768a3c185354b9f6c17

Cheers,
Nathan

> + */
> +#if __has_attribute(assume)
> +# define __assume(expr)                 __attribute__((__assume__(expr)))
> +#else
> +# define __assume(expr)
> +#endif
> +
>  /*
>   * The second argument is optional (default 0), so we use a variadic macro
>   * to make the shorthand.
> -- 
> 2.48.1
>