How to get the CPU cycle count in x86_64 from C++?

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Starting from GCC 4.5 and later, the __rdtsc() intrinsic is now supported by both MSVC and GCC.

But the include that’s needed is different:

#ifdef _WIN32
#include <intrin.h>
#else
#include <x86intrin.h>
#endif

Here’s the original answer before GCC 4.5.

Pulled directly out of one of my projects:

#include <stdint.h>

//  Windows
#ifdef _WIN32

#include <intrin.h>
uint64_t rdtsc(){
    return __rdtsc();
}

//  Linux/GCC
#else

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

#endif

This GNU C Extended asm tells the compiler:

  • volatile: the outputs aren’t a pure function of the inputs (so it has to re-run every time, not reuse an old result).
  • "=a"(lo) and "=d"(hi) : the output operands are fixed registers: EAX and EDX. (x86 machine constraints). The x86 rdtsc instruction puts its 64-bit result in EDX:EAX, so letting the compiler pick an output with "=r" wouldn’t work: there’s no way to ask the CPU for the result to go anywhere else.
  • ((uint64_t)hi << 32) | lo – zero-extend both 32-bit halves to 64-bit (because lo and hi are unsigned), and logically shift + OR them together into a single 64-bit C variable. In 32-bit code, this is just a reinterpretation; the values still just stay in a pair of 32-bit registers. In 64-bit code you typically get an actual shift + OR asm instructions, unless the high half optimizes away.

(editor’s note: this could probably be more efficient if you used unsigned long instead of unsigned int. Then the compiler would know that lo was already zero-extended into RAX. It wouldn’t know that the upper half was zero, so | and + are equivalent if it wanted to merge a different way. The intrinsic should in theory give you the best of both worlds as far as letting the optimizer do a good job.)

https://gcc.gnu.org/wiki/DontUseInlineAsm if you can avoid it. But hopefully this section is useful if you need to understand old code that uses inline asm so you can rewrite it with intrinsics. See also https://stackoverflow.com/tags/inline-assembly/info

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