Subject: [PATCH 03/17] csky: Add __attribute_const__ to ffs()-family implementations

From: Kees Cook <[email-masked]>
Date: Mon, 4 Aug 2025 09:43:59 -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 C-SKY's implementations of
ffs(), __ffs(), fls(), and __fls() 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=csky defconfig with GCC csky-linux 15.1.0.

Link: https://github.com/KSPP/linux/issues/364 [1]
Signed-off-by: Kees Cook <[email-masked]>
---
 arch/csky/include/asm/bitops.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/csky/include/asm/bitops.h b/arch/csky/include/asm/bitops.h
index 72e1b2aa29a0..80d67eee6e86 100644
--- a/arch/csky/include/asm/bitops.h
+++ b/arch/csky/include/asm/bitops.h
@@ -9,7 +9,7 @@
 /*
  * asm-generic/bitops/ffs.h
  */
-static inline int ffs(int x)
+static inline __attribute_const__ int ffs(int x)
 {
 	if (!x)
 		return 0;
@@ -26,7 +26,7 @@ static inline int ffs(int x)
 /*
  * asm-generic/bitops/__ffs.h
  */
-static __always_inline unsigned long __ffs(unsigned long x)
+static __always_inline __attribute_const__ unsigned long __ffs(unsigned long x)
 {
 	asm volatile (
 		"brev %0\n"
@@ -39,7 +39,7 @@ static __always_inline unsigned long __ffs(unsigned long x)
 /*
  * asm-generic/bitops/fls.h
  */
-static __always_inline int fls(unsigned int x)
+static __always_inline __attribute_const__ int fls(unsigned int x)
 {
 	asm volatile(
 		"ff1 %0\n"
@@ -52,7 +52,7 @@ static __always_inline int fls(unsigned int x)
 /*
  * asm-generic/bitops/__fls.h
  */
-static __always_inline unsigned long __fls(unsigned long x)
+static __always_inline __attribute_const__ unsigned long __fls(unsigned long x)
 {
 	return fls(x) - 1;
 }
-- 
2.34.1