Subject: [PATCH 06/17] sh: Add __attribute_const__ to ffs()-family implementations

From: Kees Cook <[email-masked]>
Date: Mon, 4 Aug 2025 09:44:02 -0700

While tracking down a problem where constant expressions used by
BUILD_BUG_ON() suddenly stopped working[1], we found that an added static
initializer was convincing the compiler that it couldn't track the state
of the prior statically initialized value. Tracing this down found that
ffs() was used in the initializer macro, but since it wasn't marked with
__attribute__const__, the compiler had to assume the function might
change variable states as a side-effect (which is not true for ffs(),
which provides deterministic math results).

Add missing __attribute_const__ annotations to SH's implementations of
__ffs() and ffz() functions. These are pure mathematical functions that
always return the same result for the same input with no side effects,
making them eligible for compiler optimization.

Build tested ARCH=sh defconfig with GCC sh4-linux-gnu 14.2.0.

Link: https://github.com/KSPP/linux/issues/364 [1]
Signed-off-by: Kees Cook <[email-masked]>
---
 arch/sh/include/asm/bitops.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/sh/include/asm/bitops.h b/arch/sh/include/asm/bitops.h
index 10ceb0d6b5a9..aba3aa96a50e 100644
--- a/arch/sh/include/asm/bitops.h
+++ b/arch/sh/include/asm/bitops.h
@@ -24,7 +24,7 @@
 #include <asm-generic/bitops/non-atomic.h>
 #endif
 
-static inline unsigned long ffz(unsigned long word)
+static inline unsigned long __attribute_const__ ffz(unsigned long word)
 {
 	unsigned long result;
 
@@ -44,7 +44,7 @@ static inline unsigned long ffz(unsigned long word)
  *
  * Undefined if no bit exists, so code should check against 0 first.
  */
-static inline unsigned long __ffs(unsigned long word)
+static inline __attribute_const__ unsigned long __ffs(unsigned long word)
 {
 	unsigned long result;
 
-- 
2.34.1