bitops: Add __attribute_const__ to generic ffs()-family implementations

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 generic 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 with x86_64 defconfig using GCC 14.2.0, which should validate
the implementations when used by ARM, ARM64, LoongArch, Microblaze,
NIOS2, and SPARC32 architectures.

Link: https://github.com/KSPP/linux/issues/364 [1]
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250804164417.1612371-2-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

include/asm-generic/bitops/__ffs.h

@@ -10,7 +10,7 @@
  *
  * Undefined if no bit exists, so code should check against 0 first.
  */
-static __always_inline unsigned int generic___ffs(unsigned long word)
+static __always_inline __attribute_const__ unsigned int generic___ffs(unsigned long word)
 {
 	unsigned int num = 0;
 

include/asm-generic/bitops/__fls.h

@@ -10,7 +10,7 @@
  *
  * Undefined if no set bit exists, so code should check against 0 first.
  */
-static __always_inline unsigned int generic___fls(unsigned long word)
+static __always_inline __attribute_const__ unsigned int generic___fls(unsigned long word)
 {
 	unsigned int num = BITS_PER_LONG - 1;
 

include/asm-generic/bitops/builtin-__ffs.h

@@ -8,7 +8,7 @@
  *
  * Undefined if no bit exists, so code should check against 0 first.
  */
-static __always_inline unsigned int __ffs(unsigned long word)
+static __always_inline __attribute_const__ unsigned int __ffs(unsigned long word)
 {
 	return __builtin_ctzl(word);
 }

include/asm-generic/bitops/builtin-__fls.h

@@ -8,7 +8,7 @@
  *
  * Undefined if no set bit exists, so code should check against 0 first.
  */
-static __always_inline unsigned int __fls(unsigned long word)
+static __always_inline __attribute_const__ unsigned int __fls(unsigned long word)
 {
 	return (sizeof(word) * 8) - 1 - __builtin_clzl(word);
 }

include/asm-generic/bitops/builtin-fls.h

@@ -9,7 +9,7 @@
  * This is defined the same way as ffs.
  * Note fls(0) = 0, fls(1) = 1, fls(0x80000000) = 32.
  */
-static __always_inline int fls(unsigned int x)
+static __always_inline __attribute_const__ int fls(unsigned int x)
 {
 	return x ? sizeof(x) * 8 - __builtin_clz(x) : 0;
 }

include/asm-generic/bitops/ffs.h

@@ -10,7 +10,7 @@
  * the libc and compiler builtin ffs routines, therefore
  * differs in spirit from ffz (man ffs).
  */
-static inline int generic_ffs(int x)
+static inline __attribute_const__ int generic_ffs(int x)
 {
 	int r = 1;
 

include/asm-generic/bitops/fls.h

@@ -10,7 +10,7 @@
  * Note fls(0) = 0, fls(1) = 1, fls(0x80000000) = 32.
  */
 
-static __always_inline int generic_fls(unsigned int x)
+static __always_inline __attribute_const__ int generic_fls(unsigned int x)
 {
 	int r = 32;
 

include/asm-generic/bitops/fls64.h

@@ -16,7 +16,7 @@
  * at position 64.
  */
 #if BITS_PER_LONG == 32
-static __always_inline int fls64(__u64 x)
+static __always_inline __attribute_const__ int fls64(__u64 x)
 {
 	__u32 h = x >> 32;
 	if (h)
@@ -24,7 +24,7 @@ static __always_inline int fls64(__u64 x)
 	return fls(x);
 }
 #elif BITS_PER_LONG == 64
-static __always_inline int fls64(__u64 x)
+static __always_inline __attribute_const__ int fls64(__u64 x)
 {
 	if (x == 0)
 		return 0;

include/linux/bitops.h

@@ -267,7 +267,7 @@ static inline int parity8(u8 val)
  * The result is not defined if no bits are set, so check that @word
  * is non-zero before calling this.
  */
-static inline unsigned int __ffs64(u64 word)
+static inline __attribute_const__ unsigned int __ffs64(u64 word)
 {
 #if BITS_PER_LONG == 32
 	if (((u32)word) == 0UL)

lib/clz_ctz.c

@@ -15,28 +15,28 @@
 #include <linux/kernel.h>
 
 int __weak __ctzsi2(int val);
-int __weak __ctzsi2(int val)
+int __weak __attribute_const__ __ctzsi2(int val)
 {
 	return __ffs(val);
 }
 EXPORT_SYMBOL(__ctzsi2);
 
 int __weak __clzsi2(int val);
-int __weak __clzsi2(int val)
+int __weak __attribute_const__ __clzsi2(int val)
 {
 	return 32 - fls(val);
 }
 EXPORT_SYMBOL(__clzsi2);
 
 int __weak __clzdi2(u64 val);
-int __weak __clzdi2(u64 val)
+int __weak __attribute_const__ __clzdi2(u64 val)
 {
 	return 64 - fls64(val);
 }
 EXPORT_SYMBOL(__clzdi2);
 
 int __weak __ctzdi2(u64 val);
-int __weak __ctzdi2(u64 val)
+int __weak __attribute_const__ __ctzdi2(u64 val)
 {
 	return __ffs64(val);
 }