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@xxxxxxxxxxxxx> > --- > 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 >
