[PATCH v2 2/4] compiler_types: Add __assume macro

Heiko Carstens
2025-09-16T13:48:13Z

Make the statement attribute "assume" with a new __assume macro available.

The assume attribute is used to indicate that a certain condition is
assumed to be true. Compilers may or may not use this indication to
generate optimized code. If this condition is violated at runtime, the
behavior is undefined.

Note that the clang documentation states that optimizers may react
differently to this attribute, and this may even have a negative
performance impact. Therefore this attribute should be used with care.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
---
include/linux/compiler_types.h | 23 +++++++++++++++++++++++
init/Kconfig | 10 ++++++++++
2 files changed, 33 insertions(+)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index a910f9fa5341..41c16fb8eb40 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -329,6 +329,29 @@ struct ftrace_likely_data {
#define __no_sanitize_or_inline __always_inline
#endif

+/*
+ * The assume attribute is used to indicate that a certain condition is
+ * assumed to be true. If this condition is violated at runtime, the behavior
+ * is undefined. Compilers may or may not use this indication to generate
+ * optimized code.
+ *
+ * Note that the clang documentation states that optimizers may react
+ * differently to this attribute, and this may even have a negative
+ * performance impact. Therefore this attribute should be used with care.
+ *
+ * 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 59ae2b967195..935eff59af97 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