Function __asm__ __volatile__("rdtsc");

Question

I don't know what exactly does this code:

int rdtsc(){
    __asm__ __volatile__("rdtsc");

Please, someone can explain me? why "rdtsc"?

Answer 1

Actually, that's not very good code at all.

RDTSC is the x86 instruction "ReaD TimeStamp Counter" - it reads a 64-bit counter that counts up at every clock cycle of your processor.

But since it's a 64-bit number, it's stored in EAX (low part) and EDX (high part), and if this code is ever used in a case where it is inlined, the compiler doesn't know that EDX is being clobbered. Or that the inline assembly sets EAX before falling off the end of a non-void function.

The compiler doesn't "understand" the assembler code, it's a black box which you must describe with input/output operands so it knows there's an output in EDX:EAX. (Or an output in EAX with EDX being clobbered). I would do this:

uint64_t rdtsc()
{
   uint32_t hi, lo;
   __asm__ __volatile__ ("rdtsc" : "=a"(lo), "=d"(hi));
   return ( (uint64_t)lo)|( ((uint64_t)hi)<<32 );
}

thus giving a time-count that doesn't wrap around every second or two on a modern machine, and which tells the compiler which registers your asm statement modifies.

Or use the __rdtsc() intrinsic to get the compiler to emit the rdtsc instruction itself, and know where the outputs are. See Get CPU cycle count?.