Subject: [PATCH 13/17] parisc: Add __attribute_const__ to ffs()-family implementations

From: Kees Cook <[email-masked]>
Date: Mon, 4 Aug 2025 09:44:09 -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 PARISC's implementations of
ffs(), __ffs(), 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=parisc defconfig with GCC hppa-linux-gnu 14.2.0.

Link: https://github.com/KSPP/linux/issues/364 [1]
Signed-off-by: Kees Cook <[email-masked]>
---
 arch/parisc/include/asm/bitops.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/parisc/include/asm/bitops.h b/arch/parisc/include/asm/bitops.h
index 0ec9cfc5131f..bd1280a8a5ec 100644
--- a/arch/parisc/include/asm/bitops.h
+++ b/arch/parisc/include/asm/bitops.h
@@ -123,7 +123,7 @@ static __inline__ int test_and_change_bit(int nr, volatile unsigned long * addr)
  * cycles for each mispredicted branch.
  */
 
-static __inline__ unsigned long __ffs(unsigned long x)
+static __inline__ __attribute_const__ unsigned long __ffs(unsigned long x)
 {
 	unsigned long ret;
 
@@ -161,7 +161,7 @@ static __inline__ unsigned long __ffs(unsigned long x)
  * This is defined the same way as the libc and compiler builtin
  * ffs routines, therefore differs in spirit from the above ffz (man ffs).
  */
-static __inline__ int ffs(int x)
+static __inline__ __attribute_const__ int ffs(int x)
 {
 	return x ? (__ffs((unsigned long)x) + 1) : 0;
 }
@@ -171,7 +171,7 @@ static __inline__ int ffs(int x)
  * fls(0) = 0, fls(1) = 1, fls(0x80000000) = 32.
  */
 
-static __inline__ int fls(unsigned int x)
+static __inline__ __attribute_const__ int fls(unsigned int x)
 {
 	int ret;
 	if (!x)
-- 
2.34.1