From 146034fed6ee75ec09cf8f996165e2296ceae0bb Mon Sep 17 00:00:00 2001 From: Vincent Mailhol Date: Wed, 7 Sep 2022 18:09:34 +0900 Subject: [PATCH] x86/asm/bitops: Use __builtin_ffs() to evaluate constant expressions For x86_64, the current ffs() implementation does not produce optimized code when called with a constant expression. On the contrary, the __builtin_ffs() functions of both GCC and clang are able to fold the expression into a single instruction. ** Example ** Consider two dummy functions foo() and bar() as below: #include #define CONST 0x01000000 unsigned int foo(void) { return ffs(CONST); } unsigned int bar(void) { return __builtin_ffs(CONST); } GCC would produce below assembly code: 0000000000000000 : 0: ba 00 00 00 01 mov $0x1000000,%edx 5: b8 ff ff ff ff mov $0xffffffff,%eax a: 0f bc c2 bsf %edx,%eax d: 83 c0 01 add $0x1,%eax 10: c3 ret 0000000000000020 : 20: b8 19 00 00 00 mov $0x19,%eax 25: c3 ret And clang would produce: 0000000000000000 : 0: b8 ff ff ff ff mov $0xffffffff,%eax 5: 0f bc 05 00 00 00 00 bsf 0x0(%rip),%eax # c c: 83 c0 01 add $0x1,%eax f: c3 ret 0000000000000010 : 10: b8 19 00 00 00 mov $0x19,%eax 15: c3 ret Both examples clearly demonstrate the benefit of using __builtin_ffs() instead of the kernel's asm implementation for constant expressions. However, for non constant expressions, the kernel's ffs() asm version remains better for x86_64 because, contrary to GCC, it doesn't emit the CMOV assembly instruction, c.f. [1] (noticeably, clang is able optimize out the CMOV call). Use __builtin_constant_p() to select between the kernel's ffs() and the __builtin_ffs() depending on whether the argument is constant or not. As a side benefit, replacing the ffs() function declaration by a macro also removes below -Wshadow warning: ./arch/x86/include/asm/bitops.h:283:28: warning: declaration of 'ffs' shadows a built-in function [-Wshadow] 283 | static __always_inline int ffs(int x) ** Statistics ** On a allyesconfig, before...: $ objdump -d vmlinux.o | grep bsf | wc -l 1081 ...and after: $ objdump -d vmlinux.o | grep bsf | wc -l 792 So, roughly 26.7% of the calls to ffs() were using constant expressions and could be optimized out. (tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1) [1] commit ca3d30cc02f7 ("x86_64, asm: Optimise fls(), ffs() and fls64()") [ bp: Massage commit message. ] Signed-off-by: Vincent Mailhol Signed-off-by: Borislav Petkov Reviewed-by: Nick Desaulniers Reviewed-by: Yury Norov Link: https://lore.kernel.org/r/20220511160319.1045812-1-mailhol.vincent@wanadoo.fr --- arch/x86/include/asm/bitops.h | 26 ++++++++++++++------------ 1 file changed, 14 insertions(+), 12 deletions(-) diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index 0fe9de58af313..879238e5a6a03 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -292,18 +292,7 @@ static __always_inline unsigned long __fls(unsigned long word) #undef ADDR #ifdef __KERNEL__ -/** - * ffs - find first set bit in word - * @x: the word to search - * - * This is defined the same way as the libc and compiler builtin ffs - * routines, therefore differs in spirit from the other bitops. - * - * ffs(value) returns 0 if value is 0 or the position of the first - * set bit if value is nonzero. The first (least significant) bit - * is at position 1. - */ -static __always_inline int ffs(int x) +static __always_inline int variable_ffs(int x) { int r; @@ -333,6 +322,19 @@ static __always_inline int ffs(int x) return r + 1; } +/** + * ffs - find first set bit in word + * @x: the word to search + * + * This is defined the same way as the libc and compiler builtin ffs + * routines, therefore differs in spirit from the other bitops. + * + * ffs(value) returns 0 if value is 0 or the position of the first + * set bit if value is nonzero. The first (least significant) bit + * is at position 1. + */ +#define ffs(x) (__builtin_constant_p(x) ? __builtin_ffs(x) : variable_ffs(x)) + /** * fls - find last set bit in word * @x: the word to search -- 2.30.2