This issue is related to the recently stabilised x86 intrinsic function _rdtsc

The issue is that the function is defined to return a signed i64. This is in accordance with Intel's documentation. However, that is not how most people define it, including C/C++ compilers.

This is how GCC defines it:

extern __inline unsigned long long // < note the return type
__attribute__((__gnu_inline__, __always_inline__, __artificial__))
__rdtsc (void)
{
  return __builtin_ia32_rdtsc ();
}

This Clang example proves it also defines it as unsigned long long.

Now, I'm confused. Since the timestamp counter is a monotonically increasing timer, it wouldn't make sense to return negative values. In fact, I think it is allowed to return values with the most significant bit set (which would make the returned value negative, unless transmuted). Is the mismatch intended?