Messages in this thread Patch in this message | | | From | Vincent Mailhol <> | Subject | [PATCH v5 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions | Date | Fri, 12 Aug 2022 20:44:37 +0900 |
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For x86_64, the current ffs() implementation does not produce optimized code when called with a constant expression. On the contrary, the __builtin_ffs() function of both GCC and clang is able to fold the expression into a single instruction.
** Example **
Let's consider two dummy functions foo() and bar() as below:
#include <linux/bitops.h> #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 <foo>: 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 <Instructions after ret and before next function were redacted>
0000000000000020 <bar>: 20: b8 19 00 00 00 mov $0x19,%eax 25: c3 ret
And clang would produce:
0000000000000000 <foo>: 0: b8 ff ff ff ff mov $0xffffffff,%eax 5: 0f bc 05 00 00 00 00 bsf 0x0(%rip),%eax # c <foo+0xc> c: 83 c0 01 add $0x1,%eax f: c3 ret
0000000000000010 <bar>: 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 ffs() asm version of the kernel 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()") Link: http://lkml.kernel.org/r/20111213145654.14362.39868.stgit@warthog.procyon.org.uk
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> Signed-off-by: Vincent Mailhol <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 a288ecd230ab..6ed979547086 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -269,18 +269,7 @@ 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; @@ -310,6 +299,19 @@ 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 -- 2.35.1
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