Re: [PATCH 03/17] csky: Add __attribute_const__ to ffs()-family implementations
From: Guo Ren <guoren@kernel.org>
Date: 2025-10-19 06:50:30
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linux-alpha, linux-arch, linux-hardening, linux-m68k, linux-mips, linux-riscv, linux-s390, linux-sh, lkml, llvm, sparclinux
On Mon, Aug 04, 2025 at 09:43:59AM -0700, Kees Cook wrote:
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.
LGTM. Acked-by: Guo Ren <guoren@kernel.org>
quoted hunk ↗ jump to hunk
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 <kees@kernel.org> --- 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