c - 使用 RDTSC 测量时差 - 结果太大

Question

我正在尝试计算运行单个 ASM 指令所需的 CPU 周期数。为此，我创建了这个函数:

measure_register_op:
    # Calculate time of required for movl operation

    # function setup
    pushl %ebp
    movl %esp, %ebp
    pushl %ebx
    pushl %edi

    xor %edi, %edi

    # first time measurement
    xorl %eax, %eax
    cpuid               # sync of threads
    rdtsc               # result in edx:eax

    # we are measuring instuction below
    movl %eax, %edi     

    # second time measurement
    cpuid               # sync of threads
    rdtsc               # result in edx:eax

    # time difference
    sub %eax, %edi

    # move to EAX. Value of EAX is what function returns
    movl %edi, %eax

    # End of function
    popl %edi
    popl %ebx
    mov %ebp, %esp
    popl %ebp

    ret

我在 *.c 文件中使用它:

extern unsigned int measure_register_op();

int main(void)
{

    for (int a = 0; a < 10; a++)
    {
        printf("Instruction took %u cycles \n", measure_register_op());
    }

    return 0;
}

问题是:我看到的值太大了。我现在收到 3684414156。这里会出什么问题？

编辑:从 EBX 更改为 EDI，但结果仍然相似。它必须与 rdtsc 本身有关。在调试器中，我可以看到第二个测量结果为 0x7f61e078，第一个为 0x42999940，减法后仍然给出 1019758392

编辑:这是我的生成文件。也许我编译不正确:

compile: measurement.s measurement.c
    gcc -g measurement.s measurement.c -o ./build/measurement -m32

编辑:这是我看到的确切结果:

Instruction took 4294966680 cycles 
Instruction took 4294966696 cycles 
Instruction took 4294966688 cycles 
Instruction took 4294966672 cycles 
Instruction took 4294966680 cycles 
Instruction took 4294966688 cycles 
Instruction took 4294966688 cycles 
Instruction took 4294966696 cycles 
Instruction took 4294966688 cycles 
Instruction took 4294966680 cycles 

Answer 1

cpuid 破坏了 ebx 和许多其他寄存器。您需要避免在此处使用 cpuid 或将值保存在不会被破坏的地方。