关于bit操作(bit_set test_bit)

 

   今天发现一个奇怪的现象，用linux内核的bitops.h里面的bit_set和test_bit居然没有不用汇编的代码快。记录下来，备忘。

  

  下面是可执行程序，THUIRDB暂时没有bitops里面的实现，改用我自己写的实现了。当然bitops里的实现，nr可以很大，而我的代码nr只能是0-63之间的一个数，对THUIRDB的应用来说，足够了。

#include "stdint.h"

#include <iostream>

using namespace std;

#define BITOP_ADDR(x) "=m" (*(volatile long *) (x))

#define ADDR                            BITOP_ADDR(addr)

#ifdef CONFIG_SMP

#define LOCK_PREFIX /

                ".section .smp_locks,/"a/"/n"   /

                _ASM_ALIGN "/n"                 /

                _ASM_PTR "661f/n" /

                ".previous/n"                   /

                "661:/n/tlock; "

#else

#define LOCK_PREFIX ""

#endif

static inline void

set_bit(unsigned int nr, volatile unsigned long *addr)

{

        __asm__ volatile(LOCK_PREFIX "bts %1,%0"

                   : BITOP_ADDR(addr) : "Ir" (nr) : "memory");

}

static inline bool test_bit(int nr, volatile const unsigned long *addr)

{

        int oldbit;

        asm volatile("bt %2,%1/n/t"

                     "sbb %0,%0"

                     : "=r" (oldbit)

                     : "m" (*(unsigned long *)addr), "Ir" (nr));

        return oldbit!=0;

}

static  void _set_bit(int nr, uint64_t* addr)

{

        addr[0] |= ( 1UL << nr);

};

static bool _test_bit(int nr,uint64_t* addr)

{

        return  (0 < (addr[0] & (1UL<<nr)));

};

int main(void)

{

        uint64_t bit_arr=0;

        #ifdef T1

        for(int i = 0; i<1000000;++i)

        {

                _set_bit(id,&bit_arr);

        }

        #endif

        #ifdef T2

        for(int i = 0; i<1000000;++i)

        {

                set_bit(id,&bit_arr);

        }

        #endif

        #ifdef T3

        for(int i = 0; i<1000000;++i)

        {

                _test_bit(id,&bit_arr);

        }

        #endif

        #ifdef T4

        for(int i = 0; i<1000000;++i)

        {

                test_bit(id,&bit_arr);

        }

        #endif

        return 0;

}

以下是反汇编的结果，可以看出用内核的方法，代码短小，但效率较低，用移位与或的方法代码多，但效率高。

000000000040067c <_ZL7set_bitjPVm>: //set_bit

  40067c:       55                      push   %rbp

  40067d:       48 89 e5                mov    %rsp,%rbp

  400680:       89 7d fc                mov    % edi,-0x4(%rbp)

  400683:       48 89 75 f0             mov    %rsi,-0x10(%rbp)

  400687:       48 8b 45 f0             mov    -0x10(%rbp),%rax

  40068b:       8b 55 fc                mov    -0x4(%rbp),% edx

  40068e:       0f ab 10                bts    % edx,(%rax)

  400691:       c9                      leaveq 

  400692:       c3                      retq   

0000000000400693 <_ZL8test_bitiPVKm>://test_bit

  400693:       55                      push   %rbp

  400694:       48 89 e5                mov    %rsp,%rbp

  400697:       89 7d ec                mov    % edi,-0x14(%rbp)

  40069a:       48 89 75 e0             mov    %rsi,-0x20(%rbp)

  40069e:       48 8b 45 e0             mov    -0x20(%rbp),%rax

  4006a2:       8b 55 ec                mov    -0x14(%rbp),% edx

  4006a5:       0f a3 10                bt     % edx,(%rax)

  4006a8:       19 c0                   sbb    % eax,% eax

  4006aa:       89 45 fc                mov    % eax,-0x4(%rbp)

  4006ad:       83 7d fc 00             cmpl   $0x0,-0x4(%rbp)

  4006b1:       0f 95 c0                setne  %al

  4006b4:       c9                      leaveq 

  4006b5:       c3                      retq   

00000000004006b6 <_ZL8_set_bitiPm>://_set_bit

  4006b6:       55                      push   %rbp

  4006b7:       48 89 e5                mov    %rsp,%rbp

  4006ba:       53                      push   %rbx

  4006bb:       89 7d f4                mov    % edi,-0xc(%rbp)

  4006be:       48 89 75 e8             mov    %rsi,-0x18(%rbp)

  4006c2:       48 8b 45 e8             mov    -0x18(%rbp),%rax

  4006c6:       48 8b 10                mov    (%rax),%rdx

  4006c9:       8b 45 f4                mov    -0xc(%rbp),% eax

  4006cc:       bb 01 00 00 00          mov    $0x1,% ebx

  4006d1:       48 89 de                mov    %rbx,%rsi

  4006d4:       89 c1                   mov    % eax,% ecx

  4006d6:       48 d3 e6                shl    %cl,%rsi

  4006d9:       48 89 f0                mov    %rsi,%rax

  4006dc:       48 09 c2                or     %rax,%rdx

  4006df:       48 8b 45 e8             mov    -0x18(%rbp),%rax

  4006e3:       48 89 10                mov    %rdx,(%rax)

  4006e6:       5b                      pop    %rbx

  4006e7:       c9                      leaveq 

  4006e8:       c3                      retq   

00000000004006e9 <_ZL9_test_bitiPm>://_test_bit

  4006e9:       55                      push   %rbp

  4006ea:       48 89 e5                mov    %rsp,%rbp

  4006ed:       53                      push   %rbx

  4006ee:       89 7d f4                mov    % edi,-0xc(%rbp)

  4006f1:       48 89 75 e8             mov    %rsi,-0x18(%rbp)

  4006f5:       48 8b 45 e8             mov    -0x18(%rbp),%rax

  4006f9:       48 8b 10                mov    (%rax),%rdx

  4006fc:       8b 45 f4                mov    -0xc(%rbp),% eax

  4006ff:       48 89 d3                mov    %rdx,%rbx

  400702:       89 c1                   mov    % eax,% ecx

  400704:       48 d3 eb                shr    %cl,%rbx

  400707:       48 89 d8                mov    %rbx,%rax

  40070a:       83 e0 01                and    $0x1,% eax

  40070d:       5b                      pop    %rbx

  40070e:       c9                      leaveq 

  40070f:       c3                      retq   

T2用-O2优化后的代码，内联到main里面从400910到40093b

00000000004008f0 <main>: 4008f0: 55 push %rbp 4008f1: 31 c0 xor %eax,%eax 4008f3: 53 push %rbx 4008f4: 48 83 ec 18 sub $0x18,%rsp 4008f8: 48 8d 4c 24 08 lea 0x8(%rsp),%rcx 4008fd: 48 c7 44 24 08 00 00 movq $0x0,0x8(%rsp) 400904: 00 00 400906: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 40090d: 00 00 00 400910: 89 c2 mov %eax,%edx 400912: 83 e2 3f and $0x3f,%edx 400915: 0f ab 11 bts %edx,(%rcx) 400918: 83 c0 01 add $0x1,%eax 40091b: 3d 40 42 0f 00 cmp $0xf4240,%eax 400920: 75 ee jne 400910 <main+0x20> 400922: 48 8b 74 24 08 mov 0x8(%rsp),%rsi 400927: bf e0 0d 60 00 mov $0x600de0,%edi 40092c: e8 67 fe ff ff callq 400798 <_ZNSo9_M_insertImEERSoT_@plt> 400931: 48 89 c3 mov %rax,%rbx 400934: 48 8b 00 mov (%rax),%rax 400937: 48 8b 40 e8 mov -0x18(%rax),%rax 40093b: 48 8b ac 03 f0 00 00 mov 0xf0(%rbx,%rax,1),%rbp 400942: 00 400943: 48 85 ed test %rbp,%rbp 400946: 74 3f je 400987 <main+0x97> 400948: 80 7d 38 00 cmpb $0x0,0x38(%rbp) 40094c: 74 20 je 40096e <main+0x7e> 40094e: 0f b6 45 43 movzbl 0x43(%rbp),%eax 400952: 48 89 df mov %rbx,%rdi 400955: 0f be f0 movsbl %al,%esi 400958: e8 5b fe ff ff callq 4007b8 <_ZNSo3putEc@plt> 40095d: 48 89 c7 mov %rax,%rdi 400960: e8 23 fe ff ff callq 400788 <_ZNSo5flushEv@plt> 400965: 48 83 c4 18 add $0x18,%rsp 400969: 31 c0 xor %eax,%eax 40096b: 5b pop %rbx 40096c: 5d pop %rbp 40096d: c3 retq 40096e: 48 89 ef mov %rbp,%rdi 400971: e8 c2 fd ff ff callq 400738 <_ZNKSt5ctypeIcE13_M_widen_initEv@plt> 400976: 48 8b 45 00 mov 0x0(%rbp),%rax 40097a: be 0a 00 00 00 mov $0xa,%esi 40097f: 48 89 ef mov %rbp,%rdi 400982: ff 50 30 callq *0x30(%rax) 400985: eb cb jmp 400952 <main+0x62> 400987: e8 9c fd ff ff callq 400728 <_ZSt16__throw_bad_castv@plt> 40098c: 90 nop 40098d: 90 nop 40098e: 90 nop 40098f: 90 nop