Messages in this thread Patch in this message | | | From | Vincent Mailhol <> | Subject | [PATCH 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions | Date | Tue, 10 May 2022 23:25:49 +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 simplify 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: b8 ff ff ff ff mov $0xffffffff,%eax | 5: 0f bc c7 bsf %edi,%eax | 8: 83 c0 01 add $0x1,%eax | b: c3 ret | c: 0f 1f 40 00 nopl 0x0(%rax) | | 0000000000000010 <bar>: | 10: b8 19 00 00 00 mov $0x19,%eax | 15: c3 ret
And clang would produce:
| 0000000000000000 <foo>: | 0: 55 push %rbp | 1: 48 89 e5 mov %rsp,%rbp | 4: b8 ff ff ff ff mov $0xffffffff,%eax | 9: 0f bc 05 00 00 00 00 bsf 0x0(%rip),%eax # 10 <foo+0x10> | 10: ff c0 inc %eax | 12: 5d pop %rbp | 13: c3 ret | 14: 66 2e 0f 1f 84 00 00 cs nopw 0x0(%rax,%rax,1) | 1b: 00 00 00 | 1e: 66 90 xchg %ax,%ax | | 0000000000000020 <bar>: | 20: 55 push %rbp | 21: 48 89 e5 mov %rsp,%rbp | 24: b8 19 00 00 00 mov $0x19,%eax | 29: 5d pop %rbp | 2a: c3 ret
For both examples, we clearly see 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).
This patch uses the __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, this patch 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)
[1] commit ca3d30cc02f7 ("x86_64, asm: Optimise fls(), ffs() and fls64()") http://lkml.kernel.org/r/20111213145654.14362.39868.stgit@warthog.procyon.org.uk
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr> --- arch/x86/include/asm/bitops.h | 29 +++++++++++++++++------------ 1 file changed, 17 insertions(+), 12 deletions(-)
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index a288ecd230ab..535a7a358c14 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 __ffs_asm(int x) { int r; @@ -310,6 +299,22 @@ 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) : \ + __ffs_asm(x)) + /** * fls - find last set bit in word * @x: the word to search -- 2.35.1
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