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

[Nathan Chancellor <nathan@kernel.org>]

On Thu, Sep 11, 2025 at 04:56:59PM +0200, Heiko Carstens wrote:
> On Wed, Sep 10, 2025 at 06:32:43PM -0700, Nathan Chancellor wrote:
> > > + *
> > > + * 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
> 
> Thank you for having look. This is quite surprising. So after looking into the
> various header files it might be acceptable to add this to compiler_types.h,
> since there seem to be a few similar constructs.
> 
> Maybe something like this(?):

Ah, yeah, that would work too. I had not considered compiler_types.h
since most of those tend to involve dynamic checks via cc-option but I
do like keeping the documentation attached to the attribute in a single
location, rather than duplicating it in the individual compiler files.
This will also make it easy to move this into compiler_attributes.h in
the (likely distant) future when both compilers support this
unconditionally (or clang 19.1.0 is the minimum supported version so
__has_attribute can be used).

> From d9d67807e6854666507e55d9ac0c7b4ec659aa99 Mon Sep 17 00:00:00 2001
> From: Heiko Carstens <hca@linux.ibm.com>
> Date: Wed, 10 Sep 2025 14:18:07 +0200
> Subject: [PATCH] compiler_types: Add __assume macro
> 
> 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>

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

I do not think anyone really owns compiler_types.h so unless Miguel has
any objections from the compiler attributes perspective, I think you can
just take this via the s390 tree with the other two changes.

> ---
>  include/linux/compiler_types.h | 20 ++++++++++++++++++++
>  init/Kconfig                   | 10 ++++++++++
>  2 files changed, 30 insertions(+)
> 
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 16755431fc11..38a52a792e48 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -329,6 +329,26 @@ struct ftrace_likely_data {
>  #define __no_sanitize_or_inline __always_inline
>  #endif
>  
> +/*
> + * 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.
> + * Using this attribute requires careful analysis, since in some cases it may
> + * generate worse code (see clang documentation).
> + *
> + * Optional: only supported since gcc >= 13
> + * Optional: only supported since clang >= 19
> + *
> + *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#index-assume-statement-attribute
> + * clang: https://clang.llvm.org/docs/AttributeReference.html#id13
> + *
> + */
> +#ifdef CONFIG_CC_HAS_ASSUME
> +# define __assume(expr)			__attribute__((__assume__(expr)))
> +#else
> +# define __assume(expr)
> +#endif
> +
>  /*
>   * Optional: only supported since gcc >= 15
>   * Optional: only supported since clang >= 18
> diff --git a/init/Kconfig b/init/Kconfig
> index e3eb63eadc87..5882c5e74047 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -112,6 +112,16 @@ config TOOLS_SUPPORT_RELR
>  config CC_HAS_ASM_INLINE
>  	def_bool $(success,echo 'void foo(void) { asm inline (""); }' | $(CC) -x c - -c -o /dev/null)
>  
> +config CC_HAS_ASSUME
> +	bool
> +	# clang needs to be at least 19.1.0 since the meaning of the assume
> +	# attribute changed:
> +	# https://github.com/llvm/llvm-project/commit/c44fa3e8a9a44c2e9a575768a3c185354b9f6c17
> +	default y if CC_IS_CLANG && CLANG_VERSION >= 190100
> +	# supported since gcc 13.1.0
> +	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106654
> +	default y if CC_IS_GCC && GCC_VERSION >= 130100
> +
>  config CC_HAS_NO_PROFILE_FN_ATTR
>  	def_bool $(success,echo '__attribute__((no_profile_instrument_function)) int x();' | $(CC) -x c - -c -o /dev/null -Werror)
>  
> -- 
> 2.48.1