这段代码如何计算经过的 CPU 周期数？
这段代码取自这个SO线程，计算行之间运行代码所消耗的CPU周期数// 1 和 //2。
$ cat cyc.c
#include<stdio.h>
static __inline__ unsigned long long rdtsc(void)
{
unsigned long long int x;
__asm__ volatile (".byte 0x0f, 0x31" : "=A" (x));
return x;
}
int main() {
unsigned long long cycles = rdtsc(); //1
cycles = rdtsc() - cycles; //2
printf("Time is %d\n", (unsigned)cycles);
return 0;
}
$ gcc cyc.c -o cyc
$ ./cyc
Time is 73
$ ./cyc
Time is 74
$ ./cyc
Time is 63
$ ./cyc
Time is 73
$
rdtsc() 函数如何工作？
Taken from this SO thread, this piece of code calculates the number of CPU cycles elapsed running code between lines //1 and //2.
$ cat cyc.c
#include<stdio.h>
static __inline__ unsigned long long rdtsc(void)
{
unsigned long long int x;
__asm__ volatile (".byte 0x0f, 0x31" : "=A" (x));
return x;
}
int main() {
unsigned long long cycles = rdtsc(); //1
cycles = rdtsc() - cycles; //2
printf("Time is %d\n", (unsigned)cycles);
return 0;
}
$ gcc cyc.c -o cyc
$ ./cyc
Time is 73
$ ./cyc
Time is 74
$ ./cyc
Time is 63
$ ./cyc
Time is 73
$
How does the rdtsc() function work?
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该函数执行 x86 指令 RTDSC，该指令的操作码恰好为
0x0f, 0x31。处理器在内部跟踪时钟周期，并读取该数字。当然，这仅适用于 x86 处理器，其他处理器将需要不同的指令。
http://en.wikipedia.org/wiki/Time_Stamp_Counter
The function executes the x86 instruction RTDSC, which happens to have an opcode of
0x0f, 0x31. The processor keeps track of clock cycles internally, and this reads that number.Of course, this only works on x86 procs, other processors will need different instructions.
http://en.wikipedia.org/wiki/Time_Stamp_Counter