Subject: [PATCH 10/17] openrisc: Add __attribute_const__ to ffs()-family implementations

From: Kees Cook <[email-masked]>
Date: Mon, 4 Aug 2025 09:44:06 -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 OpenRISC'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=openrisc defconfig with GCC or1k-linux 15.1.0.

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

diff --git a/arch/openrisc/include/asm/bitops/__ffs.h b/arch/openrisc/include/asm/bitops/__ffs.h
index 1e224b616fdf..4827b66530b2 100644
--- a/arch/openrisc/include/asm/bitops/__ffs.h
+++ b/arch/openrisc/include/asm/bitops/__ffs.h
@@ -11,7 +11,7 @@
 
 #ifdef CONFIG_OPENRISC_HAVE_INST_FF1
 
-static inline unsigned long __ffs(unsigned long x)
+static inline __attribute_const__ unsigned long __ffs(unsigned long x)
 {
 	int ret;
 
diff --git a/arch/openrisc/include/asm/bitops/__fls.h b/arch/openrisc/include/asm/bitops/__fls.h
index 9658446ad141..637cc76fe4b7 100644
--- a/arch/openrisc/include/asm/bitops/__fls.h
+++ b/arch/openrisc/include/asm/bitops/__fls.h
@@ -11,7 +11,7 @@
 
 #ifdef CONFIG_OPENRISC_HAVE_INST_FL1
 
-static inline unsigned long __fls(unsigned long x)
+static inline __attribute_const__ unsigned long __fls(unsigned long x)
 {
 	int ret;
 
diff --git a/arch/openrisc/include/asm/bitops/ffs.h b/arch/openrisc/include/asm/bitops/ffs.h
index b4c835d6bc84..536a60ab9cc3 100644
--- a/arch/openrisc/include/asm/bitops/ffs.h
+++ b/arch/openrisc/include/asm/bitops/ffs.h
@@ -10,7 +10,7 @@
 
 #ifdef CONFIG_OPENRISC_HAVE_INST_FF1
 
-static inline int ffs(int x)
+static inline __attribute_const__ int ffs(int x)
 {
 	int ret;
 
diff --git a/arch/openrisc/include/asm/bitops/fls.h b/arch/openrisc/include/asm/bitops/fls.h
index 6b77f6556fb9..77da7639bb3e 100644
--- a/arch/openrisc/include/asm/bitops/fls.h
+++ b/arch/openrisc/include/asm/bitops/fls.h
@@ -11,7 +11,7 @@
 
 #ifdef CONFIG_OPENRISC_HAVE_INST_FL1
 
-static inline int fls(unsigned int x)
+static inline __attribute_const__ int fls(unsigned int x)
 {
 	int ret;
 
-- 
2.34.1